Cover crop soil nitrate-N conservation after seed corn
Every year we collected soil samples to a depth of three feet after cover crop emergence in the fall and continuing into spring until after soybean planting. Figures 5-7 illustrate trends in soil nitrate-N as influenced by cover crops. Overall residual nitrate-N levels were much lower in fall 2010 than in 2011 or 2012. Still, there was a significant effect of cover crop presence on soil nitrate-N. Overall nitrate-N levels reflect both management of the preceding seed corn crop and weather effects (temperature, moisture) on N cycling in soil. For example, the steep increase in residual nitrate-N levels for all treatments in the spring of 2012 reflects ideal conditions for N mineralization. In all years, we see an increase in soil residual nitrate-N between November and late April or May the following year where cover crops were planted. This reflects primarily the accumulation of N deposited by cattle or decomposition of cover crop biomass. There also may have been some accumulation of nitrate-N resulting from mineralization of soil organic matter, though in 2011-2012 and 2012-2013 the no-cover treatment did not increase over the winter, and in 2010-2011 the no-cover treatment increased only slightly, suggesting that mineralization and loss of N was about equal over winter.
click image to zoomFigure 6. Cover crop effects on soil residual nitrate-N fall 2011–spring 2012. T=turnip, R=radish; RS=rapeseed; SB=spring barley. Comparing results from soil samples over years, we can definitely see the value of having a cover crop present. In all cover crop treatments, soil nitrate-N declined significantly in the fall compared to the no-cover crop treatment. In fall 2012 grazing varieties of turnip and radish were compared to ordinary varieties. Grazing varieties tend to produce more vegetative matter and smaller bulbs. However, there did not appear to be any difference in how these varieties influenced soil residual nitrate-N.
The use of a winter cover crop following seed corn production appears to be a good practice to minimize the risk of nitrate leaching over winter. In addition, cover crops provide an attractive grazing resource for the fall. From our studies there is no evident advantage to cover crop blends over a single species. The use of turnips alone, or turnips combined with radish, significantly reduced soil nitrate-N levels in the fall, and at much lower cost than more expensive blends including rapeseed or spring barley.