Considerations for flooded corn and soybean
The transformation of ammonium to nitrate, or nitrification, is done by soil bacteria that need oxygen. In fields where ammonium-base fertilizers were applied within a few days before soil conditions became excessively wet, the potential for N loss from the fertilizer is minimal as there was not enough time for the fertilizer to nitrify. Since urea is soluble in water, the only concern would be if substantial precipitation occurred soon after urea was applied in well drained fields. In sandy soils or heavily tile-drained soils it is possible to move urea or nitrate as much as a foot for each inch of rain. On the other hand, movement is only approximately five to six inches for each inch of rain in a clay loam or silt loam soil. That said, between rain events nitrate will start to move back up as evaporation from the soil surface create an upward suction force that moves water and nitrate closer to the surface. Similarly, evapotranspiration from actively growing crops will result in a similar suction force in addition to some nitrate uptake by the crop.
Nitrogen loss will occur in fields where N from fertilizer or organic N from the soil was present in nitrate form before the soils became excessively wet. In fine-textured soils, water saturated conditions cause N lost through denitrification. Denitrification rates increase after about a day under oxygen-depleted conditions that result when soil pore space is filled with water. Under these conditions, soil microbes utilize nitrate for respiration, and N is released as a bi-product in gaseous forms that are lost to the atmosphere. For each day the soil remains saturated with water under warm soil temperatures, it is possible to lose as much as 5% of the nitrate-N in the soil. In coarse-textured soils or soils intensively tiled, N loss occurs mostly by leaching below the root zone or into tile lines.
Nitrogen deficiency is characterized by yellowing along the leaf midribs, starting at the leaf tip and moving toward the stalk (Figure 1). Nitrogen deficiency will first be observed on the lowest leaves of the plant. The chlorotic areas will turn brown as the season progresses (Figure 2). Severe N deficiency will result in a poorly-developed crop canopy that will be unable to intercept all of the sunlight during grain fill. Nitrogen-deficient leaves also have a lower capacity for photosynthesis, further limiting the potential for grain fill.
Nitrogen uptake by corn from emergence through the V6 (six leaf collar) stage only represents about 5% of the total plant uptake. However, starting at about the V8 (eight leaf collar) stage, there is rapid accumulation of N by the plant, with about 60% of the total N uptake occurring between V8 and silking. Thus, it is important that N-deficient areas are detected early, and that supplemental N is sidedressed on these areas as soon as possible.
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