Potassium Recommendations for Corn in North Dakota


22 May 2017

2016 Annual Interpretive Summary

Existing potassium (K) fertilizer recommendations for corn production in North Dakota are based on field calibration response trials conducted decades previously. The objective of the study was to determine the critical soil test K concentration that would define the level above which little crop response to additional K fertilizer would be realized, and below which K fertilizer is needed. During the three years of this study, K rate trials have been conducted at 28 sites where soil samples have been collected and corn yields measured. The surprise within the study so far is that using the present K soil testing procedure, K response was accurately predicted only half the time.

After the investigation of possible reasons for the relative lack of prediction of the current test, we sent soil from each site in 2014, 2015 and 2016 to a laboratory in Ontario, Canada for K mineral and clay mineral species analysis. What we found is that all of our sites contained 5 to 10 % K feldspar, a K-bearing mineral. We found that the clay mineral in our experiments also contributed to the K response. Soils with dominant illite clay were far less likely to respond to K even if soil extractions were very low. In contrast, soils with high smectite content were more likely to increase in yield with added K than expected. Smectites tend to ‘draw in’ K from the soil solution when dry, whereas illites tend to keep releasing K, wet or dry. The test to determine clay and mineral content is expensive, so asking a producer to run this test would not be practical.

Planned K recommendation changes, that will be introduced summer, 2017, will be a split recommendation system based on soil clay chemistry. If the clay species is illitic, use of the present critical level of 150 ppm will be recommended, and those soils that are sandy loam and less than 2.5% organic matter would be annually fertilized, with no recommendation for buildup, because they cannot be built to 150 ppm. Those soils that are smectite-dominant would have a higher critical level of 200 ppm.

Additional work is needed to map smectite/illite clay content within the state. The resulting maps will help growers and their crop consultants determine their K fertilization strategies.