Plant-available potassium (K) in the soil only represents a small fraction of total soil K. Efforts to better predict crop response to K fertilization highlight the dynamic nature of processes regulating the supply of K from the soil to growing crops. The 4R nutrient steward-ship principles of the right K source applied at the right rate at the right time and in the right place interact with inherent soil properties to optimize K availability.
Discussing K availability for crops may be opening a can of worms, but … let’s go fishing! Potassium is a fascinating chemical constituent of soil that has integral roles in plant physiology that influence crop yield and quality. Predicting the availability of soil K and the yield response of crops to K fertilization is a complex subject that re-quires sufficient background. This article will address (1) the importance of K for crop growth and development, (2) the factors affecting soil K availability demonstrated in research, and (3) how 4R Nutrient Stewardship strategies can influence K management.
Potassium in crops and soils Unlike nitrogen (N) and phosphorus (P), K does not occur in soil and plants in an organic form but as a K+ ion instead. In soils, the common practice is to identify K as either being in the soil solution (solution K), located on cation exchange sites of minerals or organic matter (exchangeable K), located between the interlayers of phyllosilicate soil clays (interlayer K), or as a structural component of potassium-bearing minerals (structural K). The sum of these components reflects the total soil K content. Potassium does occur within soil microbes; however, this fraction is relatively small compared with others.
Potassium’s occurrence in ionic form means that it is crucial for physiological processes that require a balance of electric charge between anions and cations in both plants and animals. Potassium ion concentrations in the plant regulate osmotic potential and water influx, protein synthesis, enzyme activation, cell integrity, disease toler-ance, and stomatal opening. As such, crop deficiency of K can inhibit photosynthetic activity, reduce water use efficiency, and ultimately reduce yield and harvested crop quality (Pettigrew, 2008). As a function of the high mobil-ity of K in plant tissue, deficiency symptoms can vary by crop and time of year. Cereals, such as corn or wheat, will show chlorosis on older leaf margins. Soybean can show similar signs; however late-season deficiency symptoms can be seen in the upper canopy when early-season root growth was inhibited. Fruit tree crops will show smaller yellow leaves and reduced fruit size, shelf life, and overall quality (Drahorad, 1999; Zekri and Obreza, 2013). Cell integrity and photosynthesis rely on adequate K concen-trations, and chlorosis or necrosis of vegetative tissue and impairment of reproductive organs across many crops will be telltale signs of K inadequacies.
Potassium contends with N for sheer amount taken up by the plant. Grain crops like corn, soybean, and wheat can uptake 1.4, 2.3, and 2.0 lb K2O/bu harvested, respectively. In general, when greater amounts of biomass are harvested, more K is removed from a field and leads to a decline in soil-test K. Forage crops, such as alfalfa, have been recently reported to remove from 149 lb K/ac/year to 239 lb K/ac/year across multiple locations (Jungers et al., 2019). Russet potatoes have been shown to remove 240 lb K2O/ac (Stark et al., 2004) and can take up 5 to 14 lb K2O/ac/day through approximately 80 days. Corn silage at 65% moisture can remove 9 lb K2O/ ton of silage while corn and soybean grain at 15% moisture can remove 0.22 and 1.2 lb of K2O/ac, respectively (Mallarino et al., 2013). If crop residues remain in the field, tillage operations and type of crop may influence the rate of K release back to the soil (Oltmans and Mallarino, 2015).
Potassium uptake and residue management contrib-ute to potential availability of soil K for the following crop. However, soil properties such as clay content (soil texture), mineralogy (soil parent material), porosity (soil structure), cation exchange capacity (CEC), and organic matter content define the quantity and intensity of K availability. These properties determine the amount of aeration in the root zone, water-holding capacity, negative exchange sites available for cations, and ultimately, the soil-test K. As K travels to crop roots primarily by diffusion, any soil property that facilitates soil solution movement will affect K diffusion. Potassium diffusion requires opti-mum moisture, temperature, and a gradient wherein the K moves from areas of high concentration to low concen-trations, generally found near active roots (Sparks, 1987). Soil management that influences any of these properties can limit K availability and inhibit optimum use of soil K, and vigilant monitoring for K deficiencies when crops are drought stressed is worthwhile.
Check Out The Full Article: Jones-2019-Crops_&_Soils Dec 2019
By John D. Jones, Director of The Foundation for Agronomic Research, Washington, DC