Jarrod Miller, Extension Agronomist

University of Delaware recommends potassium (K) applications for low (0-70 ppm) and medium (71-140 ppm) soil test categories. However, soils in Delaware may have K that is not accounted for in a normal soil test, with minerals slowly release K over the growing season. Some of this was established by UD soil chemists in the 1980s, who observed high total K (not all plant available) in our soils, particularly in the sand fraction (https://www1.udel.edu/soilchem/Parker89SSSAJa.pdf). This means that some soils may need less K than is recommended by a regular soil test.

Some recent studies at UD, including K rates and split applied K have observed limited responses to K applications. For six corn K-rate studies at UD (2021-2023), a response to K was only observed in one field, even though fields were in the low to medium soil test range (52-142 ppm K.)  For split applied K studies (pre-plant and in season), a yield response was also not observed for both corn and soybeans in 2019 and 2020, although K uptake could increase.

In 2023, the Delaware Soybean Board sponsored a project observing K applications across soils with varying cation exchange capacity (CEC), which controls nutrient holding in the soil. One of our research farms (Figure 1a) has a zone in the field with very low CEC’s (< 4 meq/100g), that will hold less K and promote leaching. This field had very low K concentrations (20 – 50 ppm) where K was applied at two rates (0 and 60 lbs K2O) in high and low CEC zones. While CEC had a large effect on yields (Figure 1b), applying K did not have any statistically significant effect.

Figure 1: a) A map of soil cation exchange capacity (CEC) at our research farm in Harbeson, DE and b) yields based on CEC (> or < 4) and K applications (0 or 60 lbs K2O). Yields were only significantly greater based on CEC.

This implies that our research farm has additional K not being picked up by a soil test, and we should be more concerned with soil variability and low CEC. Managing CEC is difficult since it is an inherent property of the soil that can only realistically be improved with increased organic matter.

The overall point is that your sandy, Delaware soils may provide more K than you are aware of. The only way to be sure is through soil and tissue testing combined with varying potash applications (can be small areas) to predict field K supplies.

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