North Dakota was primarily a wheat state until the early 1990’s. A string of wheat crop failures due to disease and insects, and the simultaneous development of early-maturing corn and soybean varieties, as well as the ease of weed control from their glyphosate-tolerant genetic traits spurred a great increase in corn and soybean acreage, with soybean now the dominant state crop and corn number 3 behind spring wheat/durum. The cropping change resulted in a corresponding decrease in soil K test values, since the previous small grain rotation removed very little K, while corn and especially soybean remove up to 4 times more K per season. A K rate study was conducted from 2014-2016 in fields with K soil tests between ~100-200 ppm. Data were collected from 29 sites. The initial results indicated that the dry K soil test was superior to other extractions, but even the dry test only predicted correct corn response a little over half the time. The clay chemistry was investigated, and the predictability of the K test increased when the ratio of smectite clays to illites was considered. The ratio of 3.5:1 smectite:illite was the ratio above which a 200 ppm critical K soil test level was identified, and below, a 150 ppm critical K soil test value was adequate. A map of clay chemistry in North Dakota was produced from multiple surface soil samples collected in every county to help growers determine their probable clay chemistry ratio.
Dave Franzen is a Professor of Soil Science and Extension Soil Specialist with North Dakota State University. He received his BS (1975), MS (1976) and PhD (1993) from the University of Illinois. He worked as agronomist/manager of a chain of fertilizer/ag-chemical retail outlets in central Illinois for about 18 years, and joined NDSU in 1994. His early research focused on soil sampling strategies to direct site-specific nutrient management, but expanded to reconstructing state fertilizer recommendations when the past fertilizer recommendations did not support the new information found from zone/grid soil sampling. His research has spanned corn, soybean, sugarbeet, spring wheat/durum, sunflower and barley, and he has overhauled the fertilizer recommendations for 18 major state crops based on his and other NDSU colleague’s research. The consideration of clay chemistry in formulating potassium recommendation is a first within the USA.