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.

 

  1. 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.

 

  1. 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.

 

  1. 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.

 

  1. 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.

 

  1. 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.

 

  1. 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.

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