The revised potassium (K) fertilizer recommendation for corn in North Dakota shows that yields will unlikely be impacted by K when soil test levels are greater than 150 ppm and the soil contains high amounts of illite clays, or the ratio of smectite clay to illite clay is < 3.5.

Field trials were conducted to determine if K soil test and clay are low for three Carrington research fields to for K recommendation purposes. Trials were established on three fields (Field 4, Field 6, and Q5) at the Carrington REC and fertilized with pre-plant K at 0, 50, 100, and 150 lbs/ac, and top-dress K at 40 lbs at V4, V8, and V12, respectively. Soil K and clay types were analyzed.

Corn did not show K deficiency at any of the three sites, suggesting K was not deficient. As a result, yields did not improve from K fertilization even though leaf K concentration was significantly greater for the K fertilized treatments - an evidence of luxury consumption of K from fertilization (Table 1). Leaf K was also increased by top-dressing corn with K at V4 and V8. The lack of K impact was likely due to adequate soil K present in readily available form or supplied by the clay minerals.

It should be noted that soil tests can sometimes show low available K but the crop does not respond to K. This often happens because K bound to illite clay is usually not accounted for during routine K analysis; yet, it can be released from the clay and become available for root uptake.

Table 1. Soil analysis of K, smectite and illite clay ratios, and mean yields and leaf K concentration of corn in response to potassium fertilizer application on three fields at the Carrington REC.

Why is it that some years, K deficiency sometimes appears on corn at these sites even though soil K may be adequate (150 ppm), or has high illite clays, or low smectite:illite ratio? Because of drought. Soil moisture is needed for K to dissolve, to stay in available form, and to be moved by rain. Unlike very mobile nutrients like N and S in soil, K has very low mobility. Thus crop roots need to grow beyond their immediate vicinity (where nutrients may have been depleted) in order to have access to the less mobile nutrients like K and P. Soil moisture helps to move dissolved K closer to roots. When the soil is dry for prolonged periods, soil test of available K probably needs to be greater than 150 ppm.  

Conclusion: Potassium fertilization is not recommended for the Carrington REC field plots, unless for corrective measures, when the crops express deficiencies during drought conditions. It is important to soil sample and test for K to help determine future course of action. Further reading in the Crop and Pest report (https://www.ag.ndsu.edu/cpr/soils/potassium-deficiency-in-season-diagnosis-and-correction-in-corn-07-11-19).

This research was conducted with funding from the North Dakota Corn Council.

Jasper Teboh, Ph. D.
Jasper.Teboh@ndsu.edu
Soil Scientist