The Soil Potassium Cycle

1. Potassium: Nature and Ecological Roles: Potassium is an essential plant nutrient that sometimes limits plant growth. Potassium is only present in soil as a positively charged cation (K⁺). It’s entire lifecycle in the soil is actually linked mostly with cation exchange and mineral weathering. Potassium doesn’t cause any nutrient pollution problems and is generally pretty inert.

 

2. Potassium in Plant and Animal Nutrition: Potassiums main function in plants and animals is actually not to be synthesized into organic compounds, rather, it activates enzymes. It is the activator for over 80 enzymes responsible for living functions. Potassium always remains in the ionic form as well. Potassium is linked to plants having a really good response to environmental stress as well. It improves a plants hardiness, drought tolerance, and disease resistance. This is an essential nutrient to help plants flower or produce fruit. Potassium is necessary for animals including humans and helps regulate the central nervous system and maintain healthy blood vessels.

Deficiency Symptoms in Plants: Potassium deficiency in plants creates a foliar yellowing on the tips and edges of the oldest leaves first due to it’s mobility in plants. Certain plants, mainly legumes, will get while spots towards the tips of their leaves. To tell it apart from salinity damage look for these symptoms in old vs new growth.

 

3. The Potassium Cycle: Potassium comes from primary minerals in soil like micas and potassium feldspar. These minerals slowly weather and eventually the potassium becomes more and more available to be held in soil solution or taken up by plant roots. Once plants take up a ton of potassium it is leached from leaves to soil by rainfall. This and animal urine is how it gets returned to the soil. Some potassium is lost to soil losses, runoff, and leaching to groundwater. When plants are harvested and taken away from the soil entirely, the nutrients including potassium goes to wherever the plant matter goes thus ag. soils need potassium amendments. Soils amendments high in potassium include poultry manure and wood ash.

 

4. The Potassium Problem in Soil Fertility:

Availability of Potassium: Most mineral soils hold a larger volume of potassium than phosphorus and actually the total quantity of soil potassium is larger than any other major nutrient. However, available potassium is found in small amounts in the soil.

Leaching Losses: Potassium is mostly lost from the soil by leaching however its positive structure is great for binding to negative cation exchange sites on clay or humus. Liming can help keep potassium in the soil due to complementary ion effect (K­⁺ ions can more easily replace Ca²⁺ ions in cation exchange).

Plant Uptake and Removal: Plants take up about as much potassium as they do nitrogen. The removal mostly occurs when plants are harvested and most of the plant matter is removed from the soil it was grown.

Luxury Consumption: Plants will actually take up more potassium than they need when available quantities are large. This means that anything a plant takes up above it’s required potassium will be wasted if the plant is removed from where it grew. High potassium levels in plants could actually inhibit calcium and magnesium uptake causing nutritional imbalance.

 

5. Forms and Availability of Potassium in Soils: There are four forms of potassium involved in the soil potassium cycle: 1. K in mineral structures (unavailable), 2. K in nonexchangeable mineral forms (slowly available), 3. Exchangeable K on colloid surfaces (readily available), and 4. Water soluble K ions (readily available). The exchange of potassium between these four forms is a function of the types of clay minerals. Soils with 2:1 clays have the most potassium. Some plants can actually obtain potassium from generally unavailable forms on primary mineral structures.

Relatively Unavailable Forms: This includes about 90-98% of all soil potassium. This is the potassium held up in feldspar or mica. This form can release potassium slowly over the course of many years. This weathering of the primary potassium minerals is generally assisted by organic/inorganic acids and acidic clays and humus.

Readily Available Forms: Only about 1-2% of total soil potassium. This form of K exists in two forms: 1. In the soil solution, 2. Exchangeable on colloid surfaces. Most of this is in the exchangeable form on charged colloid sites. The free K in soil solution is mostly used by higher plants. There is a continued equilibrium in soils which keeps potassium evenly in solution and on exchange sites.

Slowly Available Forms: Sometimes in type 2:1 mineral clay soils, potassium as well as the similarly sized ammonium molecule will get permanently affixed between layers of growing soil colloids. These aren’t held on exchange sites and so are “nonexchangeable ions”.  This however, acts as an important reservoir for slowly released ions.

Release of Fixed Potassium: There is a lot of nonexchangeable/fixed potassium in soils. It is continually released to an exchangeable form in amounts large enough to be important. There is a good equilibrium between exchangeable K and nonexchangeable. This means some soils, especially sandy soils, have lower CEC and thus don’t maintain potassium well. More clayey soils have a better time maintaining enough potassium ions throughout a growing season.

 

6. Factors Affecting Potassium Fixation in Soils: There are four soil conditions that influence the amount of K that can be fixed: 1. Types of soil colloids, 2. Wet and dry cycles, 3. Freeze and thaw cycles, 4. The presence of lime.

Effects of Type of Clay and Moisture: Type 1:1 clays fix a small amount of potassium while 2:1 clays fix K in large quantities. Freeze/thaw and wet/dry cycles help to stimulate the exchange equilibrium of K.

Influence of pH: Lime application generally increases soil pH and potassium fixation. In acidic soils, colloids are holding H⁺ ions preventing K⁺ from getting close to the exchange sites thus keeping K⁺ from fixating. So as the pH increases, more K is able to be fixed to soil colloids. Another factor is root uptake, the more calcium and magnesium in soils the more competition these cations have for root uptake and thus less potassium could be absorbed.

 

7. Practical Aspects of Potassium Management: In most soils, the potassium fertility issue comes with the rate at which potassium can be converted to a plant available form. When plant material isn’t removed, cycling between plant matter and soil is usually adequate to supply the next seasons plants. A tricky thing about potassium management is that K uptake by plants is not consistent throughout the growing season. Many farmers rely on potassium additions with fertilizer however, excessive K depress Ca and Mg which can cause disruption in plant and animal health.

Frequency of Application: The author suggests that it’s best to apply small amounts of potassium to fields every so often in order to prevent excess leaching or over absorption of K by plants. Application would generally increase over many years until only maintenance levels were needed, soils would become built up enough to supply a good amount of potassium by themselves.

The Soil Phosphorus Cycle

1. Phosphorus in Plant Nutrition and Soil Fertility:

Both plants and animals require phosphorus to survive. It is an essential component in ATP or adenosine triphosphate which is the energy which cells use to complete basic biological functions.  Phosphorus is also essential in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) both of which are essential for creating proteins, expressing or maintaining genetic code. Another universally important P containing compound is the phospholipid which is essential for cellular membranes. Bones and teeth are also made from calcium-phosphate which is why bone meal is such a good phosphorus fertilizer.

Phosphorus and Plant Growth:

• Aspects of Plant Growth Enhanced: Phosphorus is one of the essential plant nutrients because it plays a role is vital plant operations such as photosynthesis, n-fixation, flowering, and fruiting. An adequate supply of P encourages roots to grow and bolsters structural tissues.
• Symptoms of Phosphorus Deficiency in Plants: A plant undergoing phosphorus deficiency will be generally non-productive. It will present with stunting, thin spindly stems, and dark bluish green foliage. Other than these things it’s often hard to tell a P deficient plant unless comparing it to one that is not deficient. It’s easier to tell during the plants later stages where it will show poor flowering and seed production and delayed maturity. Some plants with phosphorus deficiency will develop a purple hue. Phosphorus is very mobile in the plant so it will be moved from old to new growth and older leaves will show deficiency first.

The Phosphorus Problem in Soil Fertility: There are three main problems with phosphorus fertility. One: Natural phosphorus content in soils in pretty low (200-2000kg/ha), Two: Most P compounds in soils aren’t plant available due to insolubility, Three: When plant available soluble P is added to soils it generally gets fixed to an unavailable form soon after. Due to this fixation only a small percentage of applied phosphorus fertilizers will be of use to plants. Phosphorus just like any other nutrient in ag. systems then becomes necessary to purchase and spread on fields consistently. This then builds up the amount of fixed phosphorus soils can hold and leaves more to be taken up by plants. Farmers who can’t afford to purchase fertilizers struggle. In developing nations like areas in sub-Saharan Africa, phosphorus is often the limiting nutrient in their soils making the hunger and disease cycle worse.   

2. Effects of Phosphorus on Environmental Quality:

Land Degradation: Many heavily weather soils have little capacity for phosphorus to support plant growth. Low phosphorus means little vegetation which leads to more soil degradation. Undisturbed soils generally have enough phosphorus stored up in plant biomass and soil organic matter to sustain a crop of grasses or trees. Once soils are disturbed by either removing trees, planting and harvesting crops, etc. phosphorus is removed quickly. The losses are in eroded soil particles and in runoff water. In only a few years a system can be completely devoid of phosphorus for plants. Legumes used to restore a soil’s nitrogen are often very hard hit by lack of phosphorus due to its crucial role in nodulation and N-fixation. The author predicts 1-2 billion he of land in the world is phosphorus deficient.

Water Quality Degradation: Eutrophication in large bodies of water like lakes an oceans is caused primarily by phosphorus runoff. Point sources of pollution are easy to control since it’s an identifiable source which can be regulated. However, non-point source pollution such as from land runoff is less easily controlled and therefor is the primary cause of phosphorus pollution.

• Phosphorus Enrichment of Soils: Several impacts of agriculture which lead to increases of phosphorus runoff include: timber harvest, overgrazing, soil tillage, and soil fertilizer or manure applications. Due to the intensive management of agricultural land, this is often the main source of phosphorus runoff. A big problem with animal operations is the amount of phosphorus in manure due to the amount in feed. Often times farm animals are fed too much phosphorus and it comes out in their manure which is then spread on fields making P runoff more excessive.
• Phosphorus Losses in Runoff: Agriculture which involves tilling generally increases the amount of runoff particulate phosphorus whereas with manure that isn’t tilled in, losses are primarily dissolved phosphorus. Timber harvest and forest fires will also often increase the amount of eroded phosphorus sediment.
• Livestock Concentration: One of the worst contributors to phosphorus runoff is from concentrated animal feeding operations of poultry, cattle, or swine. Even if that manure is eventually spread onto fields, rain will eventually force a good amount of it into waterways causing increased eutrophication. This along with suburban septic drain fields contributes to the growth of the toxic algae pfiesteria. These toxic algae’s will release deadly toxins into the water as well as take up all of the oxygen and nutrients in lakes, streams, oceans, etc. and leave none for inhabitants of these water ecosystems leading to massive fish die offs.

3. The Phosphorus Cycle:

• Phosphorus in Soil Solution: Compared to other macronutrients the amount of phosphorus in soil is much lower. Plant roots absorb dissolved phosphorus in soil solution mainly as phosphate ions (HPO₄^2- or H₂PO₄^–). Although plants may also take up organic phosphorus. The type of phosphorus in soils is heavily dependent on soil pH. Dihydrogen phosphate (H₂PO_4­^–) is prevalent more in acidic soils and hydrogen phosphate (HPO₄^2-) is present in more alkaline soils.
• Uptake by Roots and Mycorrhizae: In order for plants to take up phosphorus, these ions much be moved to the roots which must infiltrate into places where the ions are held. Mycorrhizae also helps in this process by absorbing phosphorus with their hyphae and could also possibly access more strongly bound phosphorus. Once phosphate is within the fungal cells it is safe from being washed away or bound up. However, mycorrhizae are usually only helpful in undisturbed soils.
• Decomposition of Plant Residues: Once in the plant, some phosphorus is moved into the shoots where it becomes part of plant tissues. When the plants reach the end of the season they will shed their leaves and their roots will start to die off or animal will eat them. This is how the phosphorus is returned to the soil either through organic matter decomposition or animal wastes. Microorganisms will then temporarily hold some phosphorus up in their own cells then releasing some via mineralization. Once in soil organic matter, phosphorus will either be stored and some will be mineralized for plant uptake.
• Chemical Forms in Soils: In most soils, there is only 0.01% phosphorus which exists in three compounds. 1. Organic phosphorus, 2. Calcium bound inorganic phosphorus, 3. Iron aluminum bound inorganic phosphorus. Organic phosphorus is often distributed between active, passive, and slow fraction of soil organic matter. Calcium bound phosphorus is found mostly in alkaline soils while iron aluminum bound P is in acidic soils. Most of this phosphorus has low solubility meaning it’s unavailable for plant uptake. Unlike other nutrients, phosphorus is not lost from the soil in a gaseous form. Usually inorganic forms of phosphorus are strongly adsorbed by soil minerals.
• Gains and Losses: Phosphorus is lost from the soil system by plant removal, erosion of soil particles, dissolved P in runoff water, and leaching to groundwater. Phosphorus is added to the soil by adsorption and dry deposition from atmospheric particles, application of manure or fertilizer, and organic matter additions.

4. Organic Phosphorus in Soils:

Organic Phosphorus Compounds: Three broad groups of organic phosphorus: 1. Inositol phosphates or phosphate esters of a sugar like compound inositol (C₆H₆(OH)₆), 2. Nucleic acids, 3. Phospholipids. Of these three categories of organic phosphorus, inositol phosphates are the most abundant making up 10 to 50% total organic phosphorus. One of the most common inositols is phytic acid, a store of plant phosphorus found in seeds. Interestingly, non-ruminants that are fed grain can’t digest phytic acid so they need a different source supplemented making their manure even higher in phosphorus. Nucleic acids are absorbed by humic compounds and silicate clays. Nucleic acids and phospholipids generally make up 1-2% of organic phosphorus in soils. More work is still being done on the other sources of organic phosphorus in soils. Most of these organic forms of phosphorus aren’t available until mineralized by microorganisms.

The Soil Sulfur Cycle

1. Importance of Sulfur:

Sulfur is an important element for many biological reactions and plays a vital role in plant and animal nutrition. Sulfur can also be an environmental pollutant causing acid rain, forest decline, acid mine drainage, acid sulfate soils, and water toxicity.

Roles in Plants and Animals: Sulfur is part of what makes up several amino acids, vitamins, and protein enzymes. Without sulfur containing amino acids or vitamins humans show malnourishment. Sulfur is close with Nitrogen in creating protein enzyme synthesis. Certain plant species require a large amount of sulfur compared to others.

 Deficiencies of Sulfur: Healthy plants generally have about 0.15-0.45% sulfur. Plants which are not getting enough of the element are spindly, with thin stems, slow growth and late maturity. These plants have foliage that is light green or yellow. Unlike nitrogen, sulfur is immobile in the plant so the newest growth will show deficiency as the stores of sulfur are used up. Another distinguishing factor for sulfur deficiency is the interveinal striping on leaves. In agriculture this particular deficiency is becoming more common for 3 reasons: 1. Sulfur emissions have decreased with enforcement of clean air standards, 2. NPK fertilizers consistently lack sulfur now compared to 20 years ago, 3. As more crops are being harvested and taken out of the soil so is their sulfur content.

• Areas of deficiency: These deficiencies have been reported in most of the world but are worse where the soil parent material is already lacking sulfur, or there’s a lot of runoff and leaching, or where the atmosphere doesn’t replenish it to the soil. This is most common in tropical areas. Slash and burn agriculture also increases the loss of sulfur in soils effected by this, the sulfur is converted into gas sulfur oxide and blown away to replenish soils elsewhere. Certain types of soils will contain more sulfur than others for example oxisols contain a lot of inorganic sulfur and some organic sulfur while spodosols contain almost no inorganic sulfur and a lot of organic sulfur concentrated towards the top of the profile.

2. Natural Sources of Sulfur:

The three major sources of soil available to plants are in the form of 1. Organic matter, 2. Soil minerals, 3. Sulfur gasses in the atmosphere. In natural systems sulfur is taken up by plants then returned to the same soil once the plant completes it’s life cycle, that is enough for more plants to grow.

      Organic Matter: In surface soils of temperate humid regions 90-98% of sulfur is present in organic forms. The exact forms of sulfur in the organic matter aren’t yet known completely. However, it is known that three main sulfur compounds exist in organic matter, 1. Ester sulfate (glucose sulfate), 2. Sulfoxide compound, 3. Carbon-bonded sulfur (cysteine). Soil organisms break down these sulfur compounds into soluble forms. Soils which have low organic matter naturally will generally get their sulfur from mineral forms such as gypsum.

       Soil Minerals: inorganic forms of sulfur aren’t as plentiful as organic forms however they include soluble and available compounds for plants and microorganisms. The two most common inorganic sulfur compounds are sulfates and sulfides. Sulfates are easily soluble and assimilated by plants, it’s common in regions with little precipitation in soils like mollisols and aridisols. Sulfides are found in humid region soils with restricted drainage and are often oxidized into plant available sulfate when soils are drained. Sometimes oxidization of a lot of sulfide will lead to soil acidification. Sometimes clays will bond with sulfate and slowly release it via anion exchange at a low pH.

      Atmospheric Sulfur: The atmosphere contains varying quantities of carbonyl sulfide (COS), hydrogen sulfide (H₂S), sulfur dioxide (SO₂) and other sulfur gasses. In nature, these forms of sulfur come from volcanic activity, soil volatilization, ocean spray, and biomass burning. Humans have added the element of industry which also emits sulfides. When sulfur returns to the earth in dry particles is it called dry deposition, and wet deposition is when these particles are brought by rain, each type is about half of the deposited sulfur.  Atmospheric sulfur becomes part of the soil-plant system though deposition high in H₂SO­₄ which is absorbed by soils or plant foliage. Acid rain can harm the health of lakes and forests ecosystems via sulfur toxicity and usually effects areas downwind of infrastructure.

3. The Sulfur Cycle:

The major transformations sulfur goes through:   https://www.wur.nl/en/show/The-use-of-the-sulfur-cycle-for-the-removal-of-metals-and-Scompounds.htm

The relationships among the four major forms of sulfur are connected via redox reactions. Four forms: 1. Sulfides 2. Sulfates 3. Organic sulfur 4. Elemental sulfur. The sulfur cycle is incredibly like the nitrogen cycle. The atmosphere is an important source of the element in both systems as well as several other similarities.

4. Behavior of Sulfur Compounds in Soils:

Mineralization: Sulfur behaves similar to N as it is absorbed by plants and microbes. Organic forms must be mineralized before being immobilized or used by plants. The rate at which mineralization occurs depends on soil moisture, aeration, temperature, and pH. When conditions favor microbial activity sulfur can be more readily mineralized. In a lot of soil OM sulfur is in the reduced state bonded to carbon in protein or amino acid compounds.

Immobilization: Immobilization of inorganic or mineralized forms of sulfur occur when low-sulfur energy rich organic materials are added to the soil. Immobilization is thought to be the same as Nitrogen, energy rich material stimulates microbe growth and inorganic sulfate is assimilated into microbial tissue. A C:S ration greater than 400:1 generally leads to such immobilization. This sulfur immobilization pattern could point to sulfur and carbon being closely associated in a constant ratio. Tests done all over the world show that the C:N:S ratio is consistently 100:8:1.

5. Sulfur Oxidization and Reduction:

The Oxidation Process: During microbial decomposition of organic C-bonded sulfur, sulfides are formed along with other incompletely oxidized substances, such as elemental sulfur (S­⁰), thiosulfates (S­₂O₃^2-), and polythionates (S_2xO­_3x^2-). These reduced compounds are subject to oxidization. The oxidation of some sulfur compounds can occur by only chemical reactions but most are biochemical carried out by autotrophic bacteria. Sulfur oxidation is actually easier to do than nitrification due to the flexibility of S-oxidizing bacteria.

The Reduction Process: Sulfate ions are unstable in anaerobic environments. They are reduced to sulfide ions via bacteria. In poorly drained soils the sulfide ion reacts immediately with iron or manganese, this helps form iron sulfides which help prevent iron toxicity. Sulfide ions will also undergo hydrolysis to form gaseous hydrogen sulfide (origin of the marshy rotten egg smell). The redox reactions of inorganic sulfur compounds play an important role in determining the quality of sulfate and the state of sulfur oxidation is important in the acidity of soil and water draining from soils.

Acidity from Sulfur Oxidation: Sulfur oxidation is an acidifying process. This explains why elemental sulfur can be applied to lower soil pH if it’s too basic. Sulfur in the atmosphere can form strong acids that make rainwater drop to a pH of ~4 or lower (rain is usually 5.6 or more).

• Extreme Soil Acidity: The acidifying effect of sulfur oxidation can bring about very acidic soil conditions that cause pollution in the form of runoff. Sulfidic soils contain high levels of reduced sulfur from having been under ocean water in the past. These sulfides are usually stable so long as conditions remain anaerobic. But if they are exposed to oxygen they can quickly oxidize to sulfuric acid driving pH levels to 1.5, these conditions are almost impossible to remediate. These acids can run into streams causing serious water pollution.

6. Sulfur Retention and Exchange:

The sulfate ion is plant available and since many sulfate compounds are soluble the sulfate would be leached from soils especially in humid regions if it weren’t for adsorption by soil colloids. Sulfate ions are attracted to the positive charges that are found in acid soils containing iron and aluminum oxide clays. Sulfates react with hydroxyl groups exposed on the surface of the clays. Adsorption increases at lower pH.

Sulfate Adsorption and Leaching or Nonacid Cations: When sulfate leaches from the soil it usually is accompanied by cations (Ca and Mg). In soils with high sulfate adsorption ability, leaching is low and the loss of companion cations is as well. Sulfur is seen as an indirect conserver of Ca and Mg cations in soil.

7. Sulfur and Soil Fertility Maintenance:

The problem of maintaining adequate quantities of sulfur for mineral nutrition of plants is becoming increasingly important. Farmers must become attentive at preventing deficiencies of soil nutrients including sulfur. Crop residue and manure can help replenish the sulfur that has been removed. However, in regions where there was already a sulfur deficiency dependence on fertilizers is the main solution. The author predicts sulfur deficiency will become more and more of a problem in the future.

Soil Nitrogen Cycle

1. Influence of Nitrogen on Plant Growth and Development:
   • Roles in the plant: Nitrogen is essential in plants and is a major part of all amino acids, nucleic acids and chlorophyll. It’s also an essential carbohydrate needed for growth and development and synthesis of other nutrients. Plants need nitrogen to become the best producers and nitrogen availability is necessary for all plant growth. Plant foliage contains 2.5-4% nitrogen.
   • Deficiency: Plant nitrogen deficiency leads to chlorosis causing leaves to turn yellow and pale. In these plants, protein content is low and sugar is high due to lack of nitrogen to form proteins. Within the plant, nitrogen is mobile and when experiencing a shortage, a plant will route nitrogen to new growth forgoing upkeep of old growth.
   • Oversupply: Too much nitrogen leads to plant vegetation to enlarge while stem structures remain weak due to fast growth. These plants become prone to toppling over from rain or wind. Too much nitrogen (often the result of fertilizer over application) causes crops to degrade in color and low sugar fruits. Plants will favor vegetative production over flower, fruit, or root production. Over nitrification can also lead to environmental degradation in the form of water pollution and cyanobacteria blooms.
   • Forms of nitrogen taken up by plants: Plant roots take up nitrogen as dissolved nitrate (NO­­­­­­­­­_3­­^–) and ammonium (NH₄⁺) ions. Uptake of ammonium lowers rhizosphere pH and uptake of nitrate raises pH. These pH changes then influence the uptake of other nutrients. Nitrite (NO₂^–) can also be taken up by plants but not nearly as much as the other two. Plants can also take up soluble organic forms of nitrogen such as amino acids and proteins but when mineral N is present in more abundance plants prefer that.

 

2. Distribution of Nitrogen and the Nitrogen Cycle:

The atmosphere above each hectare of soil contains 75,000mg of nitrogen, it makes up 78% of air as di-nitrogen (N₂). The problem with accessing all of this nitrogen is the triple bond between the two atoms. Nitrogen is only made available in nature for use by plants and animals by soil microbial nitrogen fixation and lightning. This creates a reactive nitrogen which is usually bound to a hydrogen, oxygen, or carbon. The next highest concentration of nitrogen is in the soil. The A horizons generally range from 0.02 to 0.5% nitrogen. In forest soils the O horizon might contain another 1 to 2 mg of nitrogen per hectare. Most soil nitrogen is found in various organic molecules. Soil organic matter contains about 5% nitrogen.

• The nitrogen cycle: The nitrogen cycle follows an atom of nitrogen through its many different forms and explains how plants can keep using up soil nitrogen and not deplete this resource. The same nitrogen is cycled through the system and captured by microbes and used by plants in the soil. Nitrogen in ammonium goes to one of five possible places: 1. Immobilization by microorganisms, 2. Removal by plant uptake, 3. Fixation un the interlayers of some 2:1 clay minerals, 4. Volatilization after being transformed to ammonia gas, or 5. Oxidation to nitrite and subsequently to nitrate by a microbial process known as nitrification. The nitrogen in nitrate can have 4 possible routs including 1. Immobilization by microorganisms, 2. Removal by plant uptake, 3. Loss to ground-water by leaching, or 4. Volatilization to the atmosphere as several nitrogen-containing gasses in a process known as denitrification.

 

3. Immobilization and Mineralization:

Most of soil nitrogen is in organic compounds that protect it from loss but leave it largely unavailable to other plants. A lot of this is in amine groups as proteins or humic compounds. When microbes break down these compounds simple amino compounds are formed and the amine groups are hydrolyzed, the nitrogen in then released as ammonium ions. Decomposition begins by breaking down large, insoluble, nitrogen containing organic molecules into smaller units until it becomes ammonium. Enzymes which control these processes are created by microorganisms and operate either within the organism or freely in the soil. This process is known as mineralization because organic compounds are broken down into mineral constituents.

Only about 1.5-3.5% of organic nitrogen in soil mineralizes annually, this still is enough for most soils to sustain proper plant growth unless soils are deficient in organic matter. In fact, in organic matter percentage is know, it can be estimated what percent of nitrogen will be mineralized in a growing season.

The opposite of mineralization is the formation of complex organic molecules from minerals is known as immobilization. This can either be done biologically or non-biologically with chemical reactions. Biological immobilization occurs when microbes need more nitrogen than they’re able to eat so they scavenge for ammonium or nitrate and incorporate these minerals as proteins. When these organisms die some of the N in them makes up the humus complex. These processes or nitrogen immobilization or mineralization occur at the same time and the resulting net nitrogen in soil depends on the carbon nitrogen ration and the amount of organic matter decomposing.

4. Soluble Organic Nitrogen (SON):

Up until very recently studies on nitrogen uptake and leaching were focused on mineral nitrogen, now that we know soluble organic nitrogen can also be taken up by plants or leached out of soils. These SON compounds account for 0.3-1.5% of total organic N in soils. In some soils SON is found at a higher concentration than mineral N.

• Plant absorption of SON: In acidic or infertile soils without a lot of mineral nitrogen, SON is the primary source of N for plants. This form of N may be taken up directly with plant roots or by assimilated via mycorrhizae. This only occurs after the mineralized N is used up.
• Microbial utilization: The organic nitrogen with low molecular weight is used in microbial cells for direct assimilation. Once in a microbial cell breakdown occurs with enzymes and the N is used to make proteins and other complex components in a microbe. If they eat more N than they can use they place the mineralized N back into the soil. This can cause competition between plants and microbes for N in certain systems.
• Leaching potential of SON: Soluble organic N is easily leached from soils. In certain forest ecosystems almost all N that was leached from the soil was SON. SON can be a contributing factor to environmental degradation via nitrogen pollution.
• Chemical makeup of SON: Not much is known of the chemical makeup of SON however about a third of it is in the form of amino compounds.

5. Ammonium Fixation by Clay Minerals:

Since ammonium is positively charged it will react with negatively charged surfaces such as clay and humus where they are available for plant uptake but still a little protected from leaching. Certain crystalline clay minerals will trap ammonium and potassium due to their structure. Ammonium fixation is generally greater in topsoil due to the higher clay content. In some forests ammonium is immobilized due to fixation or chemical reactions with humus. Rates of release of fixed ammonium are too slow to be practical.

6. Ammonia Volatilization:

Ammonia gas (NH­₃) is produced from the breakdown of OM, farm manure, and certain fertilizers. The following formula shows that ammonium and ammonia are produced in equilibrium: NH₄­⁺ + OH^–  =  H₂O + NH₃

From this equation we can assume ammonia volatilization will increase with pH levels and ammonia producing amendments will drive the reaction to the left raising the pH. Clay and humus hold onto ammonia so losses are greatest where there are fewest soil colloids.

• Volatilization from wetlands: There can be excess ammonia gas loss from fertilizer applications to fishponds or flooded fields. The fertilizers stimulates algae growth. The algae increase the amount of CO­₂ which reduces carbonic acid formation. As a result the water pH increases a lot and ammonia is released into the atmosphere.
• Ammonia absorption: Soils and plants can absorb ammonia from the atmosphere and provide usable N for plants and soil microbes.

7. Nitrification:

Nitrification is when certain soil bacteria can oxidize ammonium ions creating nitrites then nitrates. These bacteria are autotrophic because they can oxidize the ammonium ions themselves rather than organic matter. The first step of nitrification involves converting ammonium to nitrite with a specific group of bacteria. Then nitrite is immediately acted upon by Nitrobacter bacteria to form nitrate. As long as the system’s conditions are favorable for both reactions it usually follows that nitrate immediately converts to nitrate. Nitrite is actually toxic in even very small amounts to plants. Nitrification significantly lowers soil pH by producing H­⁺ ions.

Nitrification can also be reversed via denitrification which is anaerobic and done by heterotrophic bacteria and some fungi will produce N₂O gas.

Soil Conditions Affecting Nitrification: Nitrifying bacteria is more sensitive than ammonificating organisms. Nitrification requires a supply of ammonium ions but excess can be toxic to nitrobacter. Nitrifying organisms require oxygen and are favored in well drained soils. They also like temperatures between 20-30 degrees Celsius. Nitrification proceeds most rapidly when there is a lot of Ca²⁺ and Mg²⁺ and nutrients are at optimal level for plants. Provided the appropriate conditions, nitrification happens rapidly ensuring a high supply of nitrate.

8. The Nitrate Leaching Problem:

Negatively charged nitrate ions aren’t adsorbed to clays or humus. Therefore, these mineral nitrogen ions are most commonly leached from the soil. There are three reasons why this is concerning; 1. The loss of this nutrient impoverishes the ecosystem, 2. Leaching nitrate anions stimulates soil acidification as well as leaching of cations like Ca²⁺, Mg²⁺, and K⁺, 3. This leaching causes water quality issues downstream.

• Water-Quality Impacts: these over nitrification problems are mainly associated with draining of nitrate from waters into the groundwater. This can contaminate ground water and cause health problems. The issue is mostly with concentration of nitrates and level of exposure. This can also damage aquatic ecosystems. Total N load may be both SON and ammonium but N leached through soil as nitrate is often the main issue.
• Volume of Leaching Water: the amount of leaching water is determined by precipitation, irrigation, and evapotranspiration as well as soil texture and structure of course.
• Concentration of Nitrogen: This is dependent on the amount of soil nitrate there is during periods of leaching. Most ecosystems maintain a close balance between nitrogen uptake/immobilization and nitrogen release/nitrification. The systems in which nitrogen concentration is usually too high are agricultural systems. CAFOs are the worst contributor.
• Timing of N Input: in humid-temperate climates N leaching is lowest when plant growth is at its peak in mid-summer and highest in spring and late fall. Soils which leach nitrogen the least are those where perennial plants grow rather than annuals.

9. Gaseous Losses by Denitrification:

Nitrogen is lost to the atmosphere when nitrate is converted to ammonia during denitrification. The organisms that make this possible are bacteria like those which are responsible for nitrification. These organisms are heterotrophs in that they derive what they need by oxidizing organic compounds. Some are also autotrophs which get what they need from oxidizing sulfide. For reactions to happen sources of OM must provide for the denitrifiers. They prefer low oxygen levels and temps from 25-35 degrees Celsius. When oxygen levels are low this favors production of dinitrogen (N₂) however NO and N₂O are also formed. The type of gas released depends on pH, temperature, oxygen availability, and concentrations of nitrate/nitrite.

Atmospheric Pollution: Dinitrogen is common in the atmosphere and generally very inert. The issues of N pollution come from nitrogen oxides which can cause environmental damage in four ways; 1. NO and N₂O can both contribute to the creation of nitric acid which is a key component of acid rain, 2. Nitrogen oxides can react with organic pollutants to create ozone at the surface level which contributes to urban smog, 3. NO contributes to climate change by increasing the ozone effect 300 times more than CO₂ by absorbing radiation, 4. N₂O can actually deplete the ozone layer. If this were the only thing that was depleting ozone it wouldn’t be much cause for concern however, along with CFCs, motor vehicle exhaust, and fertilizing in or as a result of agricultural systems the ozone layer has depleted greatly.

Quantity of Nitrogen Lost via Denitrification: The magnitude of denitrification loss is difficult to predict with certainty since it’s so dependent on soil conditions. In natural systems denitrification occurs slowly throughout the year and is dependent on the aeration of the soil and the amount of OM. Natural systems typically lose 5-15 kg N/ha/yr while intensively fertilized agricultural systems will put off 30-60 kg N/he/yr. The other source of denitrification is waterways. It’s been found that about 5-20% of stream N is lost to denitrification.

Denitrification in Flooded Soils: Soils in wetlands or rice paddies denitrification losses can be pretty high. In fully submerged soils which don’t dry out, nitrification occurs at water surface level in order to use oxygen and denitrification occurs at soil level. A management strategy to avoid excess nitrification is the incorporate fertilizer into soils rather than applying only on the surface. This keeps nitrogen in it’s ammonium form since nitrification needs oxygen. Natural wetlands experiencing an annual wet and dry period will produce much more denitrification than other systems and this is considered to protect estuaries and wet lands from eutrophying too much from excess nutrients.

Denitrification in Groundwater: Nitrates are removed from the soil as they pass over riparian zones on the way to streams/rivers due to lots of organic compounds and anaerobic conditions typically found in these areas.

10. Biological Nitrogen Fixation:

Biological nitrogen fixation is one of the most important biochemical reactions for life to sustain on Earth. Bacteria converts inert dinitrogen to more bio-available forms. It’s believed that terrestrial systems will fix as much as 139 million mg of N per year. The manufacture of fertilizers could possibly fix just as much N.

• The mechanism: The enzyme nitrogenase catalyzes the reaction by converting dinitrogen gas to ammonia: N₂ + 8H⁺ + 6e^– à 2NH₃ + H­₂

Ammonia will then be used to create amino acids and proteins.

Nitrogenase is a complex consisting of two proteins one containing only iron and the other containing molybdenum and iron. Breaking a dinitrogen triple bond requires a lot of energy so the process is enhanced by symbiosis with plants which supply energy with photosynthates.  Nitrogenase is destroyed by O­₂ so organisms which perform this function have to protect the enzyme from oxygenation. In root nodule nitrogen fixation, a compound called leghemoglobin is used to protect the enzyme by binding O­₂, this also makes oxygen available for other things if necessary. This reaction is dependent on the amount of ammonia already in the soil, if there’s too much the reaction doesn’t occur and if there’s too little, more root nodules will grow. The N fixing organisms need a lot of molybdenum, iron, phosphorus, and sulfur.

11. Symbiotic Fixation with Legumes:

Symbiosis of bacteria and legumes contributes to a lot of N fixation especially in agricultural systems. The two types of bacterium are rhizobium (fast growth acid producing) and bradyrhizobium (slow growth). These bacterium infect root and cortical cells of the plant and form root nodules.

• Organisms involved: Certain types of these bacteria genera go with certain plants and will not interact with others. Therefore it takes time or ecological specificity for the correct type of bacteria to go with the correct plant.
• Quantity of N fixed: like anything else, the rate of fixation is dependent on soils and climate. This legume-rhizobium relationship tends to occur the best in just slightly acidic soils high in nutrients. High levels of soil N depress the fixation done by legumes. The amount of nitrogen fixed can be pretty high of course systems with this symbiotic nodule relationship will produce much higher amounts of nitrogen.
• Effect on Soil N level: The N fixing plants and bacteria can eventually increase overall soil N content which helps other plants in the system benefit. Some direct transfer may occur via mycorrhizae connecting the two plants. Most of the added N benefit is due to mineralization of ammonium, N rich root exudates, or sloughed off root and nodule tissues. However, not all legumes will produce N for plant assimilation, the soils must already by deficient in N. Usually in ag. Systems the only way legume nitrogen stays in the soil is if it is turned in and used as green manure. Legumes that are simply harvested don’t actually leave a lot of N in the soils. Perennial legumes are also good at keeping N in the soil.

12. Symbiotic Fixation with Nonlegumes:

Nodule-Forming Nonlegumes: There are about 200 species of nonlegumes that produce symbiotic N-fixing root nodules. These are species of woody plants which are symbiotic with the bacteria actinomycetes. Some of these plant species can get all their required nitrogen from this symbiosis. These plants are generally used to repopulate extremely nutrient poor soils which wouldn’t support plant life, they then make it more inhabitable by dropping leaf litter OM and nitrogen, and releasing root exudates. There are two other species of green plants that also fix N with root nodules: Gunnera and Nostoc.

Symbiotic Nitrogen Fixation without Nodules: These systems involve cyanobacteria, spirillum, and azobacter all of which work in conjunction with plants the cyanobacteria populates plant leaves while the latter two work using root exudates in the rhizosphere.

13. Nonsymbiotic Nitrogen Fixation:

Free-living or non-symbiotic microorganisms populate certain soils and fix nitrogen however aren’t in conjunction with any specific plant species.

Soil Chemistry

Adsorption of Cations and Anions

Soil colloids have surface charges which attract certain ions, cations having a positive charge and anions having a negative charge. The adsorption (adhesion of ions to a surface) of ions to colloids in soil is an incredibly important part of soil fertility and nutrient availability. Some important soil ions include: ammonium (NH₄⁺), hydrogen (H⁺), sodium (Na⁺), nitrate (NO₃^–), etc. Due to the dominant charges found in soils, different amounts of cations vs. anions may be present. For example, in a soil with many negative charges, cations tend to be more abundant than anions since opposite charges attract. Cations and anions may be attracted to the same colloid due to it having negative and positive charges.

Outer and Inner Sphere Complexes: Water molecules surround cat- and anions in the soil solution. An outer sphere complex, molecules of water form a bridge from charged colloid to ion. In these cases the ions are only held to the colloid via loose electrostatic attraction, these can be easily replaced with other similar ions. Adsorption via and inner sphere complex directly links the colloid to the ion. This type of adsorption has strong bonds and ions aren’t easily removed, it usually occurs when ions fit snugly in a certain specific colloid.

Cation Exchange Reactions

Cations held in an outer sphere complex are only loosely held in bond with water molecules. If one of these more loosely held complex moves a little further away from the colloid it gives the opportunity for another to take its place then the original cation moves into solution. This is the bases for cation exchanges. Anion exchange is when a similarly hydrated or outer complex anion replaces another in the same way. Since outer sphere complexes are how nutrients move about in soils, the set of colloids (soil) which are able to exchange cations and anions in this manner are called the exchange complex.

Principles Governing Cation Exchange Reactions:

• Reversibility: Cation exchange reactions, the swapping of two cations from soil solution to being bound to a colloid is a reversible reaction.
• Charge Equivalence: Of course since no mass is being created or destroyed the cations that exchange with each other must be of equal charge.
• Ratio Law: Exchanges between more than one similarly charged cations between solution and colloid, will eventually even out creating an equilibrium. This means the ratio of cations in solution will be the same as those on colloids. This is also applied to cations of different charges but becomes a more complex version but still equalized.
• Anion Effects on Mass Action: anions always accompany cations in solution, so we must consider their effect. Though these reactions are technically reversible, they will most likely continue if the released ion can’t be reversed due to mass action (the rate of a chemical reaction must be directly proportional to the product concentrations of reactants). This is because, the released cation either precipitates, volatilizes, or strongly associates with an anion. Displaced cations or those in solution will be removed and thus won’t reverse exchange.
• Cation Selectivity: Some types of ions are held more tightly to the colloid than others and thus they won’t all replace each other equally, it will actually vary based on the strength of the charge holding ions to colloids. The general rule is that an ion with a larger charge and a small radius of water molecules will be held more tightly to the colloid. The ones which are held less tightly to the colloid are most likely to be exchanged. Certain colloids will have a “preference” for certain cations based on the properties of the cations. Colloids can influence adsorption favoring certain molecules based on the type of colloid and which impacts the availability of certain nutrients.
• Complementary Cations: The likelihood of an exchange is based on the strength of adsorption of other cations. An ion in solution is more likely to replace a loosely held neighboring ion than one similar to itself (complementary ions).

Cation Exchange Capacity

“The sum total of the exchangeable cations that soil can adsorb.”

Means of Expression: The CEC is the number of moles of positive charge adsorbed by unit mass. The easier way to work with CEC involves ceni-moles of charge per kilogram (cmol_c/kg). An example of a soil CEC would be 15 cmol_c/kg, meaning 1 kg of this soil can hold 15 cmol_c of H⁺ ions. Expression of CEC is based on charge not number of ions.

How to Determine CEC: In general, a concentrated solution of a particular exchanger cation is used to leach the soil sample, these completely replace exchangeable cations. CEC is then measured by the number of exchanger ions adsorbed or the amounts of cations originally held on the exchange complex.

• Buffer CEC Methods: The procedure to determine CEC usually requires a pH buffer. If the soil pH is less than the pH of the buffered solution, then these methods also measure pH-dependent exchange sites. These methods measure the maximum or potential CEC of a soil.
• Effective CEC (unbuffered): This allows the exchange to take place at the actual pH of the soil. This method only measured the effective CEC which can hold exchangeable cations at the pH of the soil sampled.

Cation Exchange Capacity of Soils: The CEC of a particular soil is determined by the amounts and types of soil colloids. Sandy soil has very low CEC due to lack of colloids whereas loamy soils or forest soils that have more humus and clay have a higher CEC. Contribution of organic matter and presence of humus are important factors because of the high cation exchange capacity of these colloids. If quantity of different colloids is known it’s possible to estimate the CEC of a soil.

pH and CEC: The CEC of most soils increases with pH, at low pH values, CEC is also low. As pH rises, the negative charges of clays, humus, allophane, etc. increase thus increasing CEC.

Exchangeable Cations in Field Soils

Specific cations vary based on climactic region. Cations that dominate the exchange complex have a big influence on soil properties. In soils, the proportion of CEC by a particular cation is called the “saturation percentage” for that cation. Thus is 50% of the CEC is accounted for by Ca²⁺, the soil is said to have a “calcium saturation of 50%”. This is useful for determining sources of acidity or alkalinity. So the percentage of acidic cations like H⁺ in solution gives an indication of the pH conditions but also increases the percentage of nonacid cation saturation or base saturation.

Cation Saturation and Nutrient Availability: Hydrogen ions from root hairs and microorganisms replace nutrient cations from the exchange complex these nutrients are then forced into soil solution where they are assimilated by roots and soil organisms or removed by water. The percent saturation of plant nutrients greatly influence the availability for uptake of these nutrients.

Influence of Complementary Cations: The effect of complementary ions held on colloids influences plant uptake of nutrients. The strength of adsorption of ions varies based on the charge and ring size. If complementary ions surrounding a loosely held K⁺ ion are held more tightly, then a H⁺ ion from a root is less likely to find a complementary ion and instead more likely to replace the K⁺ making the potassium more readily available for uptake or leaching. This particular example will occur more so in acidic soils. Nutrient antagonisms in certain soils cause inhibition of uptake of some cations by plants.

Effect of Type of Colloid: Depending on the type of colloid, ability to exchange cations based on charge density of the colloid will determine how high base saturation percentage must be before exchange of particular nutrients occurs.

Anion Exchange

Anions are held by soil colloids in two major ways. First, they are held by anion adsorption mechanisms similar to those of cation adsorption.  Second, they may actually react with surface oxides and hydroxides forming a more definitive inner-sphere complex. The basic principles of anion exchange are similar to those of cation exchange. Positive charges are associated with kaolinite, iron and aluminum oxides, and allophane types of clay soil. Same as cation exchange, equivalent quantities of anions are exchanged based on their charge. These reactions can also be reversed. The opposite of cation exchange happens in terms of pH, as pH increases anion exchange capacity decreases. This AEC an important soil function which makes anions available for plant uptake and prevents the leaching of anions from soil.

Inner Sphere Complexes:

Weathering and CEC/AEC Levels:

12. Sorption of Pesticides and Groundwater Contamination

Distribution Coefficients:

13. Binding of Biomolecules to Clay and Humus

Soil Classification

“Individual Soils”: Soil, like many other living biological things, is a group made up of individuals. Meaning that there are several types of heterogeneous individual soils that make up what is known as “the soil”. These distinctions are made based on specific soil characteristics present. The transition from one soil type/individual to another is a gradual changing gradient.

Pedon and Polypedon:

A “pendon” is a method of describing or illustrating what a certain soil individuals characteristics are, “the smallest sampling unit that displays the full range of properties of a particular soil”. They are usually samples of about 1-10 meters squared which display the full range of soil properties for a given soil type.

A group of similar pendons within close proximity are called a “polypendon” when a polypendon is identified, then the similar characteristics deem that particular soil as an individual.

Groupings:

Between the most broad category of “the soil” and the most specific identification of a particular soil “a pendon”, there are many different groupings of soils. Humans have been classifying soils for thousands of years based on what is most and least useful at a certain point in time. However, scientific soil classification only began in the 1800s.  There are several countries which still use their own national soil classification system. To create a more general idea of what to call a particular soil which is crucial for science, the Food and Ag. Organization of the UN developed a simple classification system called the “World Reference Base for Soils”. At the most general level, soils are classified into 32 “Soil Reference Groups” which are classified by how they were formed or their “pedogenic process” because this tells us what properties a certain soil group should have based on parent material or origin of soil formation. This system also includes prefix qualifiers which are slightly more specific and finally the last level of detail is in the suffix qualifiers which indicate specific soil qualifications.

Soil Taxonomy:

Taxonomy is the study of the principals of classification, soil taxonomy provides a hierarchical grouping of natural soil bodies based on soil properties which can be measured and agreed upon, less based on possible soil formation and more on what properties the soil currently has. Although, soils can exhibit similar basic qualities based on the original formation.

Diagnostic surface & subsurface horizons:

The horizons that occur at the soil surface are called “epipedons” (meaning over soil in Greek).  This includes the uppermost layers of soil that have been darkened due to organic matter content (O and A sometimes B horizons).

• Mollic epipedon: this is a mineral surface horizon with dark color and greater than 0.6% organic carbon throughout. It’s known to be tick and soft even when dry. Base saturation is greater than 50%. Moist for at least three months out of the year when soil temp is at or above 5 degrees Celsius at a depth up to 50cm. Generally found in grasslands.
• Umbric epipedon: Same general characteristics as mollic but a lower percentage of base saturation. Mineral horizon develops when there is higher rainfall and parent material has a lower content of Calcium and Magnesium.
• Ochric epipedon: a mineral horizon that is very thin or light in color or low in organic matter. Not as deep as the other two and usually hard when dry.
• Melanic epipedon: this mineral horizon is very dark or black in color due to its high organic matter content (organic C >6%). Generally found in soils with a high concentration of allophane, a mineral developed from volcanic ash. It’s very light weight and fluffy.
• Histic epipedon: this is a 20-60cm thick layer of organic materials (peat, muck, etc.) and is formed in wet areas, it’s characteristically dark colored and has a low density.

Under the soil surface, soil taxonomy goes further into classifying subsurface soil horizons which help to define a particular soil.

• Argillic horizon: A subsurface accumulation of silicate clay that have leached down from the upper horizons or originated there. The clay coatings are usually formed on soil pore walls and appear as shiny surfaces or bridges between sand grains.
• Natric horizon: This also has silicate clay but also has 15% exchangeable sodium on its aggregates. Generally found in arid to semiarid areas.
• Kandic horizon: This horizon is characterized by an accumulation of iron and aluminum oxides and low activity silicate clays. These clays have a particularly low cation holding capacity and the layers (or epipedon) above this horizon has generally lost a lot of clay to leaching.
• Oxic horizon: Highly weathered subsurface horizon which is very high in iron and aluminum oxides and low in silicate clays. The horizon is at least 30cm deep and has less than 10% weatherable minerals. It’s very crumbly and not sticky despite high clay content. Found mostly in humid and tropical/subtropical regions.
• Spodic horizon: An illuvial (meaning things washed into it) horizon with accumulation of colloidal organic matter and aluminum oxide. Common in highly leached forests soils of cool and humid climates. Typical of soils with sandy textured parent material.
• Sombric horizon: Another illuvial horizon characterized by a dark color due to accumulation of organic matter. Low base saturation found in cool moist soils in high plateaus and mountains in tropical/subtropical regions.
• Albic horizon: A light colored horizon which leaches minerals and clay particles out of it (eluvial). It is low in clay and iron and aluminum oxides.
• Calcic horizon: Contains illuviation’s of carbonates appearing as white and chalk-like nodules in the lower part of the horizon.
• Gypsic horizon: These have an accumulation of gypsum
• Salic horizon: Have an accumulation of water soluble salts found in mostly arid and semiarid regions.
• Duripan, fragipan, and placic horizons: Characterized by densely compacted or cemented materials called “pans”. They resist water movement and the penetration of plant roots. They may encourage runoff and erosion due to the lack of permeability.

Soil Moisture Regimes (SMR):

A SMR refers to the amount of water saturating the soil or that which is plant available measured by a control section throughout different seasons. The SMR control section works by seeing how far 2.5cm (upper) or 7.5cm (lower) of water will reach within 24 hours when added to a dry soil. There are several moisture regime classes used to characterize soils:

• Aquic: Soil which is completely saturated with water for most of the year, poor aeration.
• Udic: Soil moisture is high enough all year for most years to meet plant needs. Common in humid climates and true for 1/3 of the worlds land area.
• Perudic: When a soil is extremely saturated with excess moisture leaching out most of the year.
• Ustic: Intermediate moisture, there is moisture for plants during the growing season with some significant periods of drought.
• Aridic: This type of soil is dry for at least half of the growing season and moist for less than 90 consecutive days. These soils occur in arid climates. Torric can be used to describe soils similar to this in regions with cold winters and hot dry summers.
• Xeric: These soils experience drought in the summer and are characteristic to Mediterranean climates with cool moist winters and warm dry summers.

Soil Temperature Regimes:

This is a measure based on a soil’s annual soil temperature, mean summer temperature, and the different between average summer and winter temperatures (taken at 50cm depth). These regimes include: cryic, frigid, mesic, and thermic. 

Categories and Nomenclature:

Each part of a soil name conveys a concept of soil character or origin. The way the soil is named gives a clue to what it might look like. For example: aridisols (meaning dry soils) are probably found in arid regions. The names of suborders are generally representative of the order they are found in. The family names identify soils within a subgroup that have similar texture, minerals, and mean temperature, examples of family name: “fine, mixed, mesic” of an active Typic Argiaquolls (of the suborder Aquolls and order Mollisols). In this case, active, means clays are active in cation exchange. The most specific level of soil taxonomy, soil series, gets its name after a geographic feature near where they were first classified (ex.: Fort Collins, Miami, Cecil, etc.). There are approximately 23,000 soil series in the US alone. To get even more specific, soil series can be described based on surface texture, slope, erosion, etc. these are called “phases”. Soil phases are used more on a local rather than general taxonomic scale.

Soil Orders:

There are 12 soil orders for all of the soils on Earth’s land. Soils are sorted into these orders based primarily on the soil properties that reflect a major course of development concerning the present major diagnostic soil horizons. Weathering that advances soil formation can also differentiate soils, the more weathered the soil the more horizon development there is. For example, the least weathered soils are entisols and have little to no profile development whereas highly weathered soils found in humid tropical areas such as oxisols, have the most soil horizons/greatest “soil development”.

http://lmecol.evsc.virginia.edu/soils/handouts/KeySoilOrders.pdf

• Inceptisols:

These soils have few diagnostic features being one order more weathered than an entisol. They typically don’t have a B horizon. Inceptisols get their name because the soils origin of profile development is still present. Inceptisols commonly have a “cambic” horizon, meaning one with weakly developed mineral soil that has too little illuviation to be called a B horizon. There are subsurface horizons composed of duripans, fragipans, calcic, gypsic, and sulfuric mineral horizons. Can often have an epipedon that is either ochric or slightly mollic or umbric.

Inceptisols are found in most climates and bioregions around the world. The seem to occur more in mountainous areas and support lowland rice crops in Asia. There are 7 suborders ranging from human made anthrepts to xerepts, found in areas with dry summers and wet winters.

https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/class/maps/?cid=nrcs142p2_053602

• Aridisols:

These soils are characterized by dry climates, hence the name arid-isol (arid – soil). They occupy a larger global area than any other soil other than entisols. A major feature of these soils is their water deficiency, soil moisture is only high enough to support plant growth for less than 90 consecutive days every year, thus natural vegetation in these soils adapts to lack of moisture and small slow growing shrubs or short grasses and possibly succulents will grow on these soils. They have an ochric epipedon that is light in color with little to no organic matter accumulation. Soil formation processes redistributed soluble minerals but not enough water is present for leaching. Therefore, the typical soil horizon will by calcic, gypsic, salic, or natric.

The suborder argids (clay) aridisols have an argillic horizon (“subsurface accumulation of silicate clay that have leached down from the upper horizons or originated there”) and most formed under a wetter climate that prevailed thousands of years ago. As time passes and carbonates build up from dust and other soil forming processes these horizons become covered in carbonate creating calcid aridisols. Depending on the topography of the land, different suborders of aridisols will form. On erosion prone areas a typical cambid aridisol will form.

These types of soils occur mostly in the western United States with Argids (clay) being the dominant suborder. Aridisols are not ideal for agriculture and require lots of irrigation in order to cultivate crops. Humans use cattle and browser herbivores to harness energy from the plants here but the production is low. By using animals to graze on these lands, both vegetation and soil becomes more homogenous.

https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/class/maps/?cid=nrcs142p2_053595

• Mollisols:

The most important developmental addition for mollisols is calcium rich organic matter from dense grass root systems. This forms a thick, soft (moll = soft) mollic epipedon, rich in dark organic matter. The surface horizon is full of humus and can reach up to 80cm deep. It has a high cation exchange capacity. These soils are generally found in humid regions. This soil is highly aggregated and does not form hard when it dries out. They have either an argillic, natric, albic, or cambic subsurface horizons but not oxic or spodic.

Mollisols cover the most land area in the US and are dominant in the Great Plains region. These soils have really high fertility and account for most of the world’s crop production.

https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/class/maps/?cid=nrcs142p2_053604

 

 

 

Soil Orders

5 soil orders and soil profiles:

Entisols:

These soils were recently developed from unconsolidated parent material with little variation of horizons, generally they mostly have an A horizon. Soils that don’t fit into other soil orders are entisols and thus are very diverse in composition and setting. Many are found in steep rocky areas although entisols in flood plains provide fertile soils. These are the most extensive of soil orders covering about 18% of Earth’s non-ice land and about 12% of US soils. They are divided into 6 suborders:

1. Wassents: These are submerged in water for more than 21 hours each day.
2. Aquents: Water table at or near the surface for most of the year (wetlands or river beds).
3. Arents: These have been disturbed and contain horizons that aren’t arranged in an orderly manner.
4. Psamments: Sandy entisols
5. Fluvents: Alluvial Entisols found on floodplains
6. Orthents: Miscellaneous common Entisols fitting no other suborder.

 

Alfisols:

These are moderately leached, slightly acidic soils that have high fertility and mainly formed under forest. They have a subsurface horizon B which includes clays and are found in temperate humid and sub-humid regions. The occupy about 10% of land area. They’re pretty productive growth medium. There are five suborders:

1. Aqualfs: With a water table at/near surface for most of the year
2. Cryalfs: Alfisols of cold climates
3. Ustalfs: Semiarid and subhumid climates
4. Xeralfs: Temperate with dry summers and moist winters.
5. Udalfs: Humid climate

 

Ultisols:

Highly weathered and leached soil with clay rich B horizon. Acid forest soils with low fertility found primarily in humid temperate and tropical areas typical to older (more weathered) more stable landscapes. These characteristically have red and orange horizons due to oxidized iron leaching. They occupy most of the southeastern US and about 9% of total US land. They can support productive forests but high acidity and poor nutrient availability means agriculture would need excessive fertilizer and lime use. 5 suborders:

1. Aquults: Water table at or near surface for most of the year
2. Humults: well drained with high OM
3. Udults: Humid climate ultisols
4. Ustults: Semiarid and subhumid climates
5. Xerults: temperate with dry summer and moist winter

Aridisols:

Rich in calcium carbonate and found in arid (dry) regions with some horizon development. They are mostly dry and have limited leaching due to lack of precipitation. They contain horizons rich in clay, calcium carbonate, silica, salts, and gypsum. They make up about 8% of US land. Used mainly for range/pasture land and wildlife plains. Agriculture on these soils requires a lot of irrigation. They’re generally found in the western US. There are 7 suborders of aridisols:

1. Cryids: found in cold climates
2. Salids: Soluble salt accumulation
3. Durids: Silicon dioxide cemented subsurface horizon
4. Gypsids: Gypsum accumulation
5. Argids: Clay accumulation
6. Calcids: Calcium carbonate accumulation
7. Cambids: Weakly developed B horizon (more leaching than others)

 

Mollisols:

These are the soils of the grassland ecosystems they are characterized by thick dark surface horizons with lots of fertility and OM from long term accumulation of plant roots. They’re mostly prairie regions like the great plains of the US making up the most extensive US soil order accounting for about 21.5% of land area. They’re arguably the most important soils to agriculture. There are 8 suborders:

1. Albolls: Wet with light colored horizons formed via iron reduction
2. Aquolls: Water table at or near surface for most of the year
3. Rendolls: Shallow mollisols over calcareous (limey calcium carbonate rich) parent materials.
4. Gelolls: Found in very cold less than 0 degree celsius regions.
5. Cryolls: Cold climate mollisols
6. Xerolls: Temperate with dry summers and moist winters
7. Ustolls: Semiarid and subhumid climates
8. Udolls: Humid climate

 

The latin names given to soil orders generally tell you about them:

• Entisols: Enti – entire
• Alfisols: Al – aluminium, Fi – iron
• Ultisols: Ulti – ultimate (for the product of continuous weathering w/o new soil formation)
• Aridisols: Arid – dry
• Mollisols: Moll – soft (from OM)

 

Soil Formation

Soil Formation:

Russian soil scientists in the late 19th century discovered that soil profiles in places with similar topography, climate, and plants, are also similar even if two plots are hundreds of meters apart. This is due to 5 formation factors:

• Parent materials (precursors to the soil)
• Climate (precipitation and temperature)
• Biota (living things including plants, microbes, soil animals, etc)
• Topography (slope, aspect, and landscape position)
• Time (from when parent material began weathering to current)
• So the definition of soil can be refined based on formation information: “dynamic natural bodies having properties derived from the combined effects of climate and biotic activities, as modified by topography, acting on parent materials over a period of time.

Parent material:

Movement of tectonic plates and other geological processes brings parent material to the surface for weathering and soil formation. Parent material is the first indicator of a particular soils characteristics. Parent material make up will usually determine the texture of soils derived from it which in turn will determine soil porosity. Certain parent material mineral components can have a future effect on soil type, ex: limestone parent material makes more basic, slowly acidifying soils. Clay is a major component of soils and parent material can help determine what kinds of the abundance of certain soil clays.

Inorganic parent material may have been original rock to an area broken down or rock that was transported to a different area. Parent materials are often defined by how they got to where they are whether it was via water, gravity, glacier, or wind and thus the soils from certain deposits will be referred to by their method of transport:

“The Nature and Properties of Soil: Fourteenth Edition” by Nyle C. Brady and Ray R. Weil pg. 42

Soil formed from original exposed rock is called residual parent material which has gone through extensive weathering. Warm humid climates will have parent material that is leached and oxidized creating characteristic horizontal stripes or red and orange, on the drier colder climates the parent material will more resemble the solid rock it came from. Keep in mind, soils are not only a product of one of these formation factors, rather they are a geological map of a myriad of different influences from weathering of parent material to the type of animal wastes the soil encounters. Type of weathering of parent materials, whether chemical or physical will largely influence how the soil will turn out.

Climate:

One of the most important soil formation factors is climate because it determines how intensely weathered certain areas will be and what types of plants can grow there. Effective precipitation and temperature both affect the rate of chemical, physical, and biological processes.

Effective precipitation:

Water must be able to reach into the regolith to be effective in soil formation. Water which percolates through the soil will transport soluble materials to different levels and thus simulating reactions that differentiate the horizons. Certain soils which have little to now water percolation will salinify (become salty) which inhibits plant growth.

Temperature:

plays an incredibly important role, for every 10 degrees Celsius higher the rate of biochemical reactions doubles. Due to water and temperatures effect on decomposition, these factors influence the amount of OM in soils.

Soil Biota:

Soil biota influence soil formation to a great extent. Ex: vegetation cover can reduce soil erosion and decomposition of leaves and other plant material contribute minerals to the soil. Organismic effects on soil formation are studied by looking at ecotones or areas with the same soil formation factors other than vegetation, the difference between forest soils and grassland soils of the same climatic region with the same parent material etc. will show differences in soil horizons due to the effect of vegetation. One key difference is what the OM is formed from, in grasslands it’s from the grass root systems and in forests it comes from leaf litter. What makes grasslands stay grass is often fire which also impacts soil formation. Due to these different natural processes and different vegetation, soils will form their individual characteristics of everything from a deeper A horizon to a more acidic soil environment. Of course vegetation and its effect on soils has an effect on the type of microbiota living in the soils. For example: grasslands will be predominant in bacteria while forests will harbor more mycelium or fungi. Microbes affect soil formation by dictating the structures in the soil with aggregates.
The role of animals:

certain soil dwelling animals such as voles, prairie dogs, gophers, etc. affect the soil by burrowing and moving some materials to different layers in the soil. Tunnels can also help water infiltrate into the soil. They act as little tillers which turn the soil over over the course of hundreds of years. Animals can also expedite soil erosion. Smaller creatures like worms, ants, and other bugs will influence soil formation. Worms for example, burrow and eat the soil on lower levels then pooping it into other layers, digestion via worm also creates more plant available minerals. Their burrows also aerate the soil. Human manipulation of land also impacts soil formation (burning to maintain grassland, agriculture, landfills, etc).

Topography:

Topography, or the makeup of the land (slopes, rivers, plateus, etc.) determines how dramatically climatic forces can impact the land. Areas of steep slope will most likely have more water runoff and soil erosion and less vegetation than flatter areas. Areas where the land dips and water collects, the soil profile is more developed however, too much water can saturate the sols restricting aeration and decomposition. Soils with the same formation conditions but varying topography will generally influence each other creating a chain or sequence of formation due to drainage patterns and weatherability, this is called a toposequence catena (aka: a chain of development influenced primarily by topography).

Time:

Soil formation begins when new rock is exposed to weathering, flooding deposits sediment, glaciers dump minerals, or bulldozers remove tons of surface material. Weathering happens at different rates and soil age generally refers to the extent of weathering and profile formation. (The image from the textbook illustrates soil formation from unweathered bare rock in a humid climate over one hundred thousand years)

“The Nature and Properties of Soil: Fourteenth Edition” by Nyle C. Brady and Ray R. Weil pg. 63

–       First lichens and mosses begin breaking down exposed rock aided by wind and water erosion. Over a few hundred years a small layer of minerals and organic matter and dusts form. This forms the basis for larger plants.

–       Transportations of various means bring plant seeds to the area and small grasses and shrubs begin to grow and break down rock and add to the organic horizon.

–       The next ten thousand years larger plants and trees establish soil organisms become abundant and a more substantial O horizon forms on top of a growing A horizon of mostly granular minerals.

–       As water filters soluble particles deeper into the ground, a distinct B horizon forms. As silicates and clays form and become compressed the B horizon develops a blocky structure.

–       The soil begins to change and deepen over time.

The Four Basic Soil Formation Processes:

Soil forming, known as pedogenic is what defines different soil types. There are four primary ways in which soils can form from bedrock or deposited materials.

1. Transformations: these occur when mineralization of geologic materials creates primary minerals which are then reformed into various silicate clays and hydrous oxides of iron and aluminum. The decompositions of organic material and formation of humus and organic compounds are more examples of soil forming transformations. Anything which changes the size, composition, or formation of soil constituents is a transformation.
2. Translocations: This involves the movement of soil materials laterally though the soil horizons, usually this is caused by water moving up and down in the soil. The water will float fine particles and dissolve and relocate soluble minerals. Soil fauna anything from squirrels digging in the soil to earthworms making tunnels is also an example of soil translocation and these organisms play a large role in soil formation through this avenue.
3. Additions: Anything added to the soil is an addition including what dead plant matter collects and wind and water relocation of geological materials. When water evaporates from the ground, salts and silicates are left behind and also considered additions. As well as of course fertilizers and human additions.
4. Losses: There are several ways material can be lost from the soil. One is by leaching of minerals and fine soil particles by water. Surface erosion from exposed soils by wind and water will also cause losses. Microbial decomposition could also count as a loss of organic matter in soils. Grazing and agriculture cause major soil losses as well in nutrients and organic matter.

Development of Soil Horizons:

Soil formation from thick uniform sandy rock really begins once plant or fungi begin to add things to the broken down rock particles like organic matter and nutrient components. Soil organisms decompose and transform OM into humus and other organic compounds. More of these additions of organic matter and a rich water holding humus layers create better environments for plant roots thus expediting the cycle and making way for more mature plant species. Once nutrients and food is available, small organisms begin to make habitats thus contributing more to the formation of soils by adding things, breaking things down, and translocating soil materials.

• A Horizon: Usually the first to be developed from the compaction and collection of organic matter, humus, plants, etc. It presents as a darker color and its properties are very different from the soils parent material. This layer will develop soil aggregates making more habitats for microorganisms.
• B and C Horizons: Organic acids (carbonic etc) are washed through the soil where they can chemically break down other materials. Soluble minerals are also washed down. This creates a lack of minerals in the upper layer and a pool in lower layers. Over time, the carbonic acids is moved to the lowest depths of water penetration where only roots can retrieve the minerals. Minerals that are weathered and transformed into play particles will simultaneously begin to collect deeper in the soil profile. Sub horizons of B are differentiated by clay moving down the soil profile and certain types of clay like silicate clay creating layers. Once the deeper layers experience repeated wetting and drying cycles they begin to crack and form block like soil structures. Soil horizons generally change and continue to develop constantly and the more mature a soil the more likely it is to have many different soil horizons.

The Soil Profile: Soils are characterized by the sequence and formation of their horizons. As discussed previously, a soil profile is a vertical section of soil exposed to analyze the horizons.

Layers:

There are six master soil horizons commonly recognized. Subordinate horizons which occur in certain types of soils are also considered when classifying soils.

1. O Horizon: This layer is primarily composed on organic horizons that generally form above the mineral layer. This layer is formed from dead plant matter and animal wastes or carcasses. Generally, these aren’t found in grasslands and are mostly known as forest floors.
2. A Horizon: Generally the topmost mineral layer of soils given a darker color by the organic matter content. Usually has a coarse texture due to leaching of smaller particles.
3. E Horizon: Primary zones for eluviation (or washing out) of minerals such as clay, iron, aluminum oxide. This leaves a sandy soil concentrated in quartz. This horizon is generally found under the A horizon and has a lighter color. These are more common to forests and less so to grasslands as well.
4. B Horizon: Located under the previous three and has undergone changes so that it’s parent material is no longer identifiable. This horizon is formed via the illuviation (meaning washing into) of minerals from layers above it. Depending on the climate conditions the soil is in, the B horizon can be a sink for different types of minerals.
5. C Horizon: This is unconsolidated material under the solum (the most weathered upper horizons of the soil). The C horizon is characterized by being much less weathered than the horizons above it and may resemble parent materials. The structure of the C horizon is often similar to that of parent rock or original geological deposits. This horizon may continue to weather at the top and become part of the B horizon.
6. R Horizon: The hard unconsolidated rock with little to no weathering at all.

Subdivisions: Above are what is called master horizons. Within each one of those layers there are sometimes sublayers, referred to by the master horizon letter followed by a number.

Subordinate distinctions: Often in order to more specifically give information about the type of material or origin of the material that makes up a master horizon or its subdivisions, a lowercase letter will follow the horizon type. These lower case letters can tell a lot about the makeup of the soil horizon.

Ex: a – organic matter, highly decomposed

t – an accumulation of silicate clays

o – accumulation of Iron and Aluminum oxides

i – organic matter, slightly decomposed

e – organic matter, intermediate decomposition

Soil Origins in Nature: All soil wasn’t just created from bare bedrock or lava rock. Soil formation is a dynamic ever changing process in which soils are transported to and from certain sites by different means. A lot of parent material was once soil itself with its own layers. Formation of soils happens over thousands of years and different soil forming factors are occurring all at the same time creating what we see as a soil profile today.

What Is Soil?

What is soil?

Soil is a giant living, breathing, eating, organismic system which is teaming with symbiotic and parasitic life coupled with inorganic foundations. It is Soil is crucial to life on Earth, and plays a major role in climate and natural cycles. Human as well as other life depends on soils for survival. Soil is NOT DIRT. Dirt has no life, it is sterile, and not actually found in nature but rather created by humans and stuffed into bags sold at hardware stores.

–       Soil: refers to the type of material it is.

–       A soil: refers to a specific habitat or type of soil found in specific bio-regions.

–       The soil: The conglomeration of all soil types that make up the pedosphere.

–       Soils: everything that is included in a soil ecosystem, organic and inorganic.

–       Pedosphere: The ring on the Earth where soil is found. Between the atmosphere and tectonic plates/bedrock.

Soil functions in our ecosystem: 

Soil acts as a medium for plant growth and provides a place for nutrient and water storage and uptake. What grows in soils can determine a soil’s characteristics and soil characteristics can determine what can grow there. Soil is one determining factors of carrying capacity for other organisms in an ecosystem, whatever vegetation the soil can support determines what else can live off of that vegetation. Soil plays a crucial role in Earth water systems. It can act as water storage, purification, loss, and utilization.

Soil is the “garbage bin” for all of nature, or more accurately a recycling bin. Soil microbes can break down and digest dead and decaying plant, animal, and other organic materials as well as organic wastes. This process breaks down organic materials into inorganic nutrients for plant or organisms to use. Without this essential soil function, we’d drown in our own wastes. This also factors into carrying capacity of land, the land can only support as much waste as it can efficiently get rid of.

Soils provide habitats for flora and fauna of all sizes from the smallest bacteria to even people who will use it to build structures or dig into for storage. Soil is arguably the most biodiverse of all Earth habitats. As I mentioned before, soil plays a large role in natural systems. It sequesters numerous natural gasses such as nitrogen, carbon, methane, and oxygen to name a few.

Soil’s last function is as an engineering medium which provides foundation for buildings and infrastructure and has been used for building material, sunscreen, and storage space since the dawn of humanity.

Soil as a medium for plant growth:

Half of all of the plant world exists underground as roots. Some things that soils provide for these expansive root systems include: physical anchoring/support, air and water, temperature control, protection from toxins, and of course nutrients. An important function of soils in terms of plant roots is ventilation, which allows gasses to flow in and out of the soil, helping plant respiration. Water storage is an important soil function for plants as well since plants are in constant need of a water supply and it’s not always raining. Water provides plants with cooling, nutrient solution, cell strength, and photosynthesis. One important benefit of soils is providing a space for nutrient minerals (nutrients in basic and plant available forms) which can be sequestered in soil water solution and taken up by plants. This is important for plant growth and for getting certain minerals into animals including humans via consuming the plant.

The foundation:

The foundation of soil is the bedrock, possibly slightly weathered/broken down on the top and all that is resting on top of it is the regolith, the layers of non-solid rock. Often the regolith atop a certain bedrock was actually native to a different place and transported via wind, water, glacier, or other natural transport. The regolith can be either a few meters deep or hundreds of meters deep. The most biologically active section is towards the top, yet microbiotic life can be found all the way down to bedrock.  The solum is what is commonly thought of as soil, making up the upper layers of regolith. Soil horizons are different layers and compositions of the solum.

Soil Profile and it’s horizons:

http://www.timberpress.com/blog/2015/07/how-soils-form-and-age/

A soil profile is how the soil is layered from the surface down. Soil horizons are the different layers within a soil profile. Often the top horizon is called the organic (O) horizon since that’s where all the plants sprout and their roots bunch, as you go further down into the soil horizons will change and have different names. Soil horizons will change based on climate and weather conditions of a region. Weathering forms these layers from the surface down, as soil gets deeper, the layers tend to change less with surface conditions and will remain more like the original bedrock or “parent material”.

Horizon layers:

–       O Horizon: Forms the uppermost thin layer of plants and their roots, contains the most organic matter.

–       A Horizon: The next lowest, made mostly of mineral particles but is colored darker due to lots of organic matter (OM). This and O is referred to as “Topsoil”.

–       E Horizon: Under the A, only in some soils that are intensely weathered and leached, containing little to no organic matter. This layer and deeper is the “subsoil”, provides water and nutrients.

–       B Horizon: Mostly mineral based contains very little if any OM and consists of leached minerals, clays, and gasses. Plant roots can spread all the way down to this level and below.

–       C Horizon: The least weathered part of the soil where many

Soil texture:

Influences many soil properties including habitability, and foundation quality.

–       Sand: Larger particles in soil between 2.0mm and 0.05mm. Feels gritty, usually smoother microsurface.

–       Silt: Between 0.05mm and 0.002mm, cannot be felt individually, feels smooth.

–       Clay: These are the smallest particles less than 0.002mm large and feels sticky. These particles have the most surface area and have “colloidal” properties which can be seen via electron microscope and often have charges which attract certain ions.

http://www.jonesscience.com/unit-4.html

Primary minerals are those which haven’t been changed much since their inception from molten lava. They make up many sand and some silt particles. Clays and some other silts were formed by the breakdown or weathering of weaker minerals known as secondary minerals.

Soil Structure:

While soil texture is the building block of soils, the structure is how they fit together. These particles often form clumps of micro-environments called “aggregates”. Aggregate make up, structure, and form are all important to soil function and habitability due to their ability to sequester microorganisms and store water and gasses.

Soil Organic Matter:

Is a source of most of the world’s carbon. OM comes from dead animals, animal wastes, dead plants, plant litter, and other once alive things. This plays a crucial role in global climate and soil function. It’s small volume in soil is misleading because of the great benefits it provides. The OM binds aggregates together, provides food for biota, provides storage of future plant nutrients, and holds water. Humus is to organic matter as clay is to soil minerals. Meaning, humus is a charged particle that acts as a colloid for organic matter.

Soil water:

Is an important factor for the survival of plants, habitat for certain organisms, transportation for nutrients, and soil formation. Often this is referred to as the “soil solution” since once water goes into the soil it immediately dissolves nutrients into itself and it doesn’t flow like it would on something non-permeable, it soaks and stores in the soil like it’s in suspension.