Potassium (K) is an essential nutrient for plant growth, but it generally receives less attention than nitrogen (N) and phosphorus (P) in many crop production systems. Many regions of the United States and all of the Canadian provinces remove more K from the soil during harvest than is returned to the soil in fertilizer and manure (Fig. 1). In the United States, an average of only 3 kg K is replaced as fertilizer and manure for every 4 kg K removed in crops [Potash and Phosphate Institute (PPI), 2002]. This net export of K in harvested crops ultimately results in a depletion of nutrients from the soil and increasing occurrences of deficiency.
Potassium is the soil cation required in the largest amount by plants, regardless of nutrient management philosophy. Large amounts of K are required to maintain plant health and vigor. Some specific roles of K in the plant include osmoregulation, internal cation/anion balance, enzyme activation, and proper water relations. Potassium plays a vital role in photosynthate translocation, especially to grains, tubers, and fruit. Protein synthesis is facilitated with an adequate supply of K. Tolerance of external stress such as frost, drought, heat, and high light intensity is enhanced with proper K nutrition. Stresses from disease and insect damage are also reduced with an adequate supply of K. Although there are no known harmful effects of K to the environment or to human health, the consequences of inadequate K can be severe for crop growth and efficient utilization of other nutrients such as N and P. Maintenance of adequate K is essential for both organic and conventional crop production.
Supplemental K is sometimes called “potash,” a term that comes from an early production technique in which K was leached from wood ashes and concentrated by evaporating the leachate in large iron pots (Mikkelsen and Bruulsema, 2005). Clearly, this practice is no longer practical and is not environmentally sustainable. This potash collection method depended on the tree roots to deplete the soil of K, which was then recovered from the wood after it was harvested and burned. Large amounts of wood were burned to collect relatively little potash. Today, most K fertilizer, whether used in organic or conventional agriculture, comes from ancient marine salts deposited as inland seas evaporated. This natural geological process is visible today in places such as the Great Salt Lake and the Dead Sea, as illustrated in Figure 2.
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