Potassium Availability

Certain plant nutrients must go through biological, chemical and physical cycles to become plant available.  The potassium cycle is much simpler.  Many mineral soils contain a surplus of potassium.  However, much of this is not in the soil solution but held tightly in parent materials like micas and feldspars.  For these to become plant accessible, they must first weather.  As they naturally break down, potassium on the outside edges goes from nonexchangeable to available forms in the soil solution for plants to utilize.

Potassium, being a macronutrient, is taken into the plant in rather large amounts.  This amount can vary from 5-10 times the amount of phosphorus.  As vegetation begins to wither and die, also known as necrosis, potassium is released from the plant back to the soil.  Unlike other residue cycling, K is readily available through this process.

There are four key locations for K in the soil.  In primary mineral structure (unavailable), nonexchangeable K in secondary minerals (slowly available), Exchangeable K on soil colloids and K soluble in water (readily available).  This makes 90-98% of all soil K to be unavailable to plants.  As mentioned above, weathering is the solvent action of carbonic, organic and inorganic acids as well as acid clays and humus.  Weathering must occur to make exchangeable K from parent materials.  Some plants with finer root structures can access this nutrient between clay layers, but many row crops cannot.  The amount of K fixed depends on the nature of the soil colloids, wetting and drying, freezing and thawing and the presence of excess lime.

There are soil types that are difficult to change soil test K values.  A few clay types are Illite (mica-type clay), vermiculite, smectite and kaolinite.  Illite (Drummer, Flanagan, Sable) has the highest K content of common clays. K sits between the layers and is slowly released and has a good long-term K supply.  It can also fix K if soils become dry or compacted. Vermiculite (Graymont, Alida, Iva) has a high CEC and can hold lots of potassium.  It also has a higher fixing capacity than Smectite. Smectite (Patton, Wabash, Toledo) has a high CEC but less inherent K.  This type is a great reservoir for soil K but not a great direct source.  Kaolinite (Bluford, Berks, Gilpin) is a 1:1 clay.  It has a low CEC, poor K retention and needs frequent K fertilization to meet crop demand. 

Potassium cycling is a dynamic equilibrium.  As soon as the plant takes in K, more is released back into the soil solution from exchangeable K.  A plant may take up more K than necessary, called luxury consumption, and this does not directly increase yield.  Potassium applications vary greatly on crop type, soil type, region and residue management.  If the soil test K levels cannot be achieved because of fixation, pH and other soil influences then applications may need to be more frequent to keep up with crop demand.

Source: Brady, N. C., & Weil, R. R. (2016). The nature and properties of soils (13th ed.). Pearson Education.