Declining Soil Test Potassium Levels

In 2015, the International Plant Nutrition Institute (IPNI) released their report on soil test levels. Potassium was one of the soil nutrients that were exhibiting a steady decline in soil test levels. A&L Great Lakes Laboratories regularly contributes to the IPNI data set, and we also analyze our data for the Eastern Corn Belt region.

In the graph above the green bars indicate the average potassium soil test levels in ppm. The dashed line is the trend line of the soil test values, indicating an average 1.3 ppm per year decline. In addition, the blue line on the graph indicates the percentage of samples that are likely deficient. This trendline is of particular concern since it exhibits a steady increase in the percentage of soils which are likely deficient in soil test K levels.

While it is difficult to attribute these declines to only one factor, yield and fertilizer application trends show an overall net negative balance. On average, potassium is being removed from the soil faster than it is being supplied. It takes, on average, the addition of 8 pounds of K₂O raise a soil test by 1 ppm. Inversely, the removal of 8 pounds of K₂O lower a soil test by 1 ppm. On an annual average, crop removal of K₂O exceeds application by 10 pounds per acre per year. Looking at USDA data, the crop removal of potassium at USDA average yields for corn and soybeans began outpacing average potash applications in the late 1990’s, just before the steady increase of deficient soils begin in the early 2000’s. 

There are many factors that may potentially contribute to these trends. Better crop management practices and improved genetics are leading to rapid increases in yields. If those higher yields are not accounted for when generating fertilizer recommendations, particularly if actual yields exceeded yield goals, nutrient recommendations may be inadequate to supplant what the crop actually removed. Predicting future yields in these high yield environments can be difficult, so it may be more beneficial to base crop removal on yields obtained in previous years, and to adjust future removals to accommodate high yielding crops that occurred since the previous fertilizer application.

Another factor may be the financial, logistical, and equipment limitations brought about by the high amounts of fertilizer material which are required to meet these higher crop removal needs. As an example, if applying nutrients in the form of MAP (11-52-0) and potash (0-0-60) on a two-year application cycle to replace the nutrients removed from a 240 bushel corn crop and a 70 bushel soybean crop, a total of 500 pounds of fertilizer per acre would be required. If an application is capped below this level due to fertilizer budgets, equipment limitations, or concerns of overloading the soil’s ability to retain nutrients, applications adequate to meet crop removal may not be made. Often these maximums are in the range of 400 to 500 pounds, not covering crop removal in some yield environments.  

A final management practice that may be reducing the amount of potassium held by the soil is the application of high calcium products at the same time as potassium applications. The soil cation exchange capacity (CEC) has a limited ability to hold cation nutrients, and if a large quantity of calcium is added to the system, it can lead to losses of soil potassium. This is amplified when multiyear applications of potassium are made, or when large applications of calcium products are made in a similar time frame as a potassium application.