The 4Rs and Potassium

| July 13, 2017

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Are we meeting crop K needs?

Using the 4R nutrient stewardship approach of selecting the right source at the right rate at the right time and in the right place is just as important when dealing with a nutrient supply challenges as it is for reducing risk of nutrient loss. Potassium (K) behavior in the soil is different from nitrogen (N) or phosphorus (P). While some of the same factors influence your 4R decisions, right source, right rate, right time, and right place; the variability of the soil’s ability to supply and store K is quite dynamic. Availability of K in the soil is dependent on the percent and type of clay present, cation exchange capacity (CEC, meq/100 g), organic matter (OM) content, soil moisture level, and soil temperature.

Plants require K in similar quantities to N. In the plant, K activates enzymes, is involved in protein synthesis, photosynthesis, water regulation, stomatal movement, and phloem transport. Potassium nutrition levels are related to dehydration and wilting responses as well as response to disease and pest pressure. In general, when crops are grown in soils with insufficient K they produce less than optimum yields, and they do not use water or N efficiently (Mikkelsen and Roberts, 2017).

In N and P management, beyond production, we are more often than not talking about concerns with cropping system loss to water and air. With K, the focus is on supply and availability. In 2015, the International Plant Nutrition Institute (IPNI) summarized soil test results from private and public soil testing laboratories in the United States and Canada to provide an indicator resource on the nutrient supplying capacity or fertility of soils in North America (IPNI, 2015). The percentage of samples that tested below the critical level, the soil test level above which the soil can supply adequate quantities of K to support optimum yield, ranged from 8 to 79 percent (Figure 1). The highest percentages of K soil tests below the critical level were found in the Southeast, with Georgia having the most K deficient soil test results (IPNI, 2015; Figure 1). In a second project, soil samples were collected between the fall of 2015 and the spring of 2016 in the Corn Belt, soils testing below the critical level ranged from 2 percent in Nebraska to 56 percent in Iowa (Schulte and Heggenstaller, 2017). Potassium deficiency was most frequently observed in Michigan where 55 percent of samples tested below the critical level for K (Schulte and Heggenstaller, 2017). The use of commercial K fertilizer has declined in the recent past, while crop removal has increased, resulting in mining of soil K reserves (Singh et al., 2017).

By Sally Flis, Ph.D. and CCA Director of Agronomy, The Fertilizer Institute, Washington, DC

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