K & Soil Clay

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.