Spring soil nitrogen following the drought of 2012
Last fall, with funding provided through the Illinois Council for Best Management Practices (C-BMP), Growmark, C-BMP, and the University of Illinois, the N-Watch soil sampling program was initiated to see how much inorganic N remained in the soil following the drought of 2012.
Fall sampling revealed fairly high amounts of soil N, with 151 samples statewide averaging 19.5 ppm of nitrate-N in the top foot of soil. We multiply this time 4 to get lb of N per acre, so soils represented by these samples had an average of 78 lb of nitrate-N per acre. Samples from northern Illinois had higher levels (26 ppm) than those from central and southern Illinois (both 18 ppm), even though 2012 corn yields in northern Illinois were higher than in central or southern Illinois. The second foot of soil depth had more than 15 ppm of nitrate-N, which meant another 62 lb of nitrate-N per acre, for a total of 140 lb N per acre.
click image to zoomFigure 1. Precipitation and departure from normal (in inches) in Illinois, October 1, 2012 through March 31, 2013. Source: Midwest Regional Climate Center. One reason for fall sampling is to inventory soil N in order to know the potential for loss of leftover soil N. If the weather stays dry through the fall and winter, we expect minimal loss of soil N. But precipitation returned to normal over most of Illinois during the past 6 months, most areas showing only small departures from normal over the October 2012 to March 2013 period (Figure 1). Normal precipitation from October 1 through March ranges from about 12 inches in the northern edge of Illinois to about 20 inches at the southern tip of the state.
While nitrate-N moves readily down into the soil profile as water moves down through the soil, the lack of rainfall during the 2012 growing season meant that nitrate from fertilizer N and from soil organic matter simply accumulated. Water only moves as far down as it takes to wet the soil, and in a very dry soil, it can take 6 to 10 inches or more of water to wet the soil. Even more water is needed to move through to deeper layers and into tile lines. Indications are that most tile lines in Illinois have been running for the past few weeks in the areas that were dry longer, and for the past couple of months in areas that received more rain and rain starting earlier.
Not surprisingly, a few reports in recent weeks from sampling tile line outflow show elevated levels of nitrate-N. This is normal for spring outflow, but with little or no tile line outflow as N accumulated last summer and fall, and with the large amounts of nitrate we found in fall sampling, the flush of nitrate-N may be larger than normal this spring.