By George Silva, Senior Educator

Although potassium (K) is listed among the top three macronutrients (N-P-K) needed for crop production, nitrogen (N) and phosphorus (P) receive most of our attention. This is partly because N and P have potential to cause long-term environmental implications. Also, N and P get incorporated into key complex molecules within the cell, such as DNA, proteins, enzymes, etc. In contrast, K rarely poses a threat to the environment. It remains in the plant and animal tissues in its ionic form K+. When crop and animal residues decompose on the soil surface, the soluble K will seep into the soil.

Soil minerals such as feldspars and micas are high in K and will release their K into the soil upon weathering. Some K is bonded in the interlayer position of clay particles. Many global K deposits for mining purposes were formed millions of years ago when early seas evaporated and marine salt was concentrated and covered with deep layers of sediment. Large reserves of buried K deposits occur around the world, with the largest reserves and fertilizer production coming from Canada, Belarus and Russia. Currently, the global supply of K remains stable.

Recently, K has received much needed awareness with the realization of its critical role in plant and human nutrition. In January 2017, the International Plant Nutrition Institute held a 3-day conference devoted to K, titled “Frontiers of Potassium.” Some of the recent advances on sustainable intensification, 4R stewardship practices and linking science and technology to frontier practices were highlighted. The current literature indicates that K, at a global level, is as limiting as N and P for plant productivity.

Role of K in Plant Nutrition

Potassium is associated with many metabolic processes and functions within the plant. It is known that K activates as many as 60 enzymatic and plant hormonal reactions. It is vital to photosynthesis and protein synthesis. It has a fundamental role in regulating leaf stomata openings and controlling water use, particularly under dry conditions (Read the Michigan State University Extension article “Drought and potassium deficiency in corn and soybeans” for more information.)

Foliar K deficiency symptoms were observed on many Michigan corn and soybean fields during the drought of 2016. When the K supply is limited, plants have reduced yields, poor quality and are more susceptible to pest damage. A local example would be the increased soybean aphid damage in K deficient fields. Potassium also helps in winter hardiness and overcoming stress situations, which may be vital in a changing climate.

Vegetables, fruits and legumes remove large quantities of K from soils. Harvesting 9 tons of alfalfa per acre will remove over 450 pounds K2O; a potato crop of 450 cwt per acre removes 500 pounds K2O; and harvesting 40 tons per acre of tomatoes will take over 450 pounds K2O per acre. Read MSU Extension bulletin E2934, “Nutrient Recommendations for Vegetable Crops in Michigan,” for vegetable K removal rates.

Maintenance of an adequate K supply has become essential for sustaining food production. Even though K is widespread in soil minerals and rocks, many soils need additional K fertilizer to meet the plant removal rates. In Michigan, about 65-70% of the soil samples tested in the past would indicate a need for supplemental K as fertilizer, whereas only about 35% of the soils would require supplemental P.