Nitrogen availability For 2006

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

After the rains of this past week many are concerned about the availability of applied nitrogen. Answering this concern is a bit challenging. The good news was that the soils were actually quite dry prior to the rains, so they were able to absorb much of the initial water. However, many areas of Michigan have received more than 3 inches of rain in a few days. Sandy soils are only able to hold 1.0 to 1.75 inches of water per foot of soil whereas loam soils hold 2.5 to 3.0 inches per foot and clay loam soils hold nearly 3.75 inches per foot of soil. Therefore, the potential for downward movement of nitrate in soils will be greater in sandy soils than in loam and clay loam soils. Once the soil is saturated, water will move downward and carry nitrate with it. Where rainfall was less than 2 inches, movement of nitrates may have only been into the second foot where it may eventually be available to the crop. But, where rainfall was over 3 inches, some nitrate may have been leached beyond the potential root zone and into the subsurface drainage water, especially in sandy soils. Soil temperatures have decreased to near 50°F so that the rate of denitrification in saturated soil will be reduced.

The time of application and the form of nitrogen applied will also have an impact on potential loss. Recently applied nitrogen in the ammonium or urea form is less subject to loss than when applied in the nitrate form.

To get a better handle on the available soil N status, collect soil samples to 12 inches and have them analyzed for the nitrate and ammonium N content. This can provide a general guide to the amount of available N currently present in the top foot of soil. There may also be significant amounts available in the second foot that will be available to the roots as they grow or as water moves upward as the soil dries. Where nitrogen has been knifed in as anhydrous ammonium or liquid urea ammonium nitrate (UAN), soil sampling for determining nitrogen availability will not be reliable. The presidedress soil nitrogen test for corn is most reliable where less than 40 pounds of nitrogen is applied preplant. It also is effective for determining nitrogen release from animal manures or incorporated cover crops. With the cooler soil temperatures, it would be good, if possible, to wait until near June 1 to take soil samples. This will give a better indication of nitrogen release.

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