With heavy rains, how likely is N leaching?

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Q. In the last three weeks Nebraska, and other Midwestern states have received more rain than in the previous three months. Should I worry about N leaching?

A. Leaching of both residual soil nitrate and applied N is always a concern in the spring. The exact amount of leaching is difficult to quantify. The answer depends on site-specific soil conditions and the effectiveness of the rainfall. In many years this question is complicated by unknown soil moisture when the rains occur. This year most of the state was fairly dry until recently. Some leaching occurs when water moves into the soil, but not all the nutrients (present as anions and cations dissolved in the soil solution) are leached.

For example, assume you have a silt loam soil that holds 2 inches of plant available water per foot. The top 4 feet can hold 8 inches of infiltrated water before it moves deeper. If nitrogen was applied as a surface broadcast application immediately before rainfall, it would move down with the rainfall. Its distribution in the soil would not be uniform, but 3 inches of infiltrated rain would distribute the nitrogen in the top 18 inches of soil. There would still be some nitrogen in the top 6 inches, but the “bulge” of the nitrate concentration would be at about 12 inches. Corn roots will grow into that nitrogen and there should not be an N deficiency as early plant needs would be met.

If there were 6 inches of infiltrated rain, the nitrogen would be distributed over 3 feet. There would be very little N in the top foot, with the N bulge now about 18 inches deep. There would be limited N for a young corn plant and a delay until the corn  roots reached the nitrogen available between 18 inches and 3 feet. If the same scenario happened on a sandy loam with 1 inch/foot water holding capacity, all these depths would be doubled, and there might be problems with the 3-inch rain.

This assumes that leaching is the only loss mechanism. If the soil is saturated for several days, N loss could occur from denitrification.

One of the best ways to determine where your N is located is to take a soil sample up to the 4-leaf stage. This follows guidelines for the PSNT (pre-sidedress N test) which is not routinely used in Nebraska. (See 2012 CropWatch article, Using the PSNT for Spring Testing of Nitrogen Availability.)

For those who did not have time to put on their nitrogen preplant this year, applying early sidedress nitrogen will be very effective since now there is soil water for corn to grow. There is potential for ammonia volatilization if urea or UAN is surface-applied to wet soils, and temperatures get warmer without more rain. The best case situation would be to apply these nitrogen sources and then have about a ½ inch rain following the application. If rain is not forecasted for your area, consider using a urease inhibitor to help reduce loss.

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