Potassium Recommendations for Corn in North Dakota

Existing K fertilizer recommendations for corn production in North Dakota are based on field calibration response trials conducted decades previously. This project is being conducted to determine if the present recommendations are accurate, considering changes in potential corn yields due to higher yielding cropping systems used by farmers. The objective of the study is to determine the critical soil test concentration of K that would define the level above which little value to K fertilizer would be realized, and below which K fertilizer is needed. So far K rate trials have been conducted at 22 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. There have been very low-testing sites (80 ppm K) with no yield response to added K; and conversely, high testing (170 ppm) sites where substantial yield increases were obtained by applying K fertilizer. As part of the study, the field-moist K test, as developed by Iowa State University, was used to assess soil K availability, but unfortunately it has been similarly unpredictive. We also tested the proposed CEC base saturation-based K test and found it the worst predictor of any testing procedure we have investigated. Investigation of possible reasons for the relative lack of prediction of the current test, included sending project soil samples for K mineral and clay mineral species analysis, are underway. All of the field sites contained 5 to 10% K feldspar, a K-bearing mineral. By the book, K feldspar should only be available over a lifetime, but in careful laboratory experiments by Dr. Don Sparks out of Delaware, K responses similar to ours were found. It was found that equilibrium between soil solution and K feldspar was measured in hours and days, not years. We also found that the clay mineral in our experiments also contributed to the K response. A series of cation exchange resin-based extractions run on soils from the 22 field project research sites is underway. The resin-based extraction runs for a much longer time than the dried soil sample K test presently used. The hope is that this resin test will measure K release over time and perhaps better predict K response for corn. There will be another six locations tested in 2016.