David Franzen, NDSU Department of Soil Sciences

The 2015 year followed work on the project in 2014. The objective of the study is to determine the critical soil test level of potassium (K) that would define the test where above meant little value to K fertilizer would be realized, and below which would indicate K fertilizer is needed. So far 22 sites have had K rate trials imposed, soil samples collected and corn yield measured. The surprise within the study so far is that using the present K soil testing procedure, which is a specific amount of 1N ammonium acetate with a specific amount of air-dry soil, shaken for a specific time at a specific speed, filtered and the filtrate analyzed for K content, only predicted K response about half the time. We have had very low (80 ppm) sites with no response; we have had high testing (170 ppm) sites where substantial yield increases were obtained with K fertilizer. We also tested the field-moist K test, as used by Iowa State, and have found it 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.

After investigation of possible reasons for the relative lack of prediction of the current test, we sent soil from each site in 2014 and 2015 to a laboratory in Ontario, Canada for K mineral and clay mineral species analysis. What we found is that all of our sites contained 5-10 per cent potassium feldspar- a potassium-bearing mineral of Canadian Shield original, brought down by glaciers to become part of our present soils. By the book, potassium feldspar should only be available over a lifetime, but in careful laboratory experiments by a researcher out of Delaware with a similar K response experience as ours, 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. Soils with dominant illite clay (so far the Sheyenne Delta soils-Wyndmere, Walcott, Milnor) were far less likely to respond to K even if soil extractions were very low. In contrast, soils with high smectite content (Arthur) were more likely to increase in yield with added K than expected. Smectites tend to ‘suck 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.

We are beginning (mid-December 2015) to run a resin-based extraction. The resin-based extraction runs for a much longer time than the dry K test recommended today. The hope is that this test would measure K release over time and be more like what is happening in our soils. The results of this will be available early 2016.

We plan to have another 6 locations of K tests in 2016 and further investigate alternative soil testing procedures so that the tests are more predictive of K needs than the present state of the science.