Potential N Loss after Heavy Rains
May 15, 2015
The heavy rains recently received by many areas of Nebraska (Figure 1) have several implications for managing soil fertility for crop production. The most pressing of these is potential loss of nitrogen (N); specifically runoff, denitrification (loss to the atmosphere), and/or leaching.
If fertilizer had been recently applied to the soil surface, without incorporation or gentle rain of half an inch or more to move N into the soil profile, substantial N loss may have occurred in runoff. Rainfall has been very intense in some areas, with significant erosion. This eroded topsoil carries away nutrients and may reduce soil organic matter levels and the soil's ability to mineralize N from organic matter throughout the growing season.
|Table 1. Potential loss of N via denitrification for soil temperature and time in anaerobic conditions.|
|5||55 - 60||10|
|10||55 - 60||25|
|3||75 - 80||60|
|*Denitrification loss will be less with soils having less than 1% organic matter.|
When soil becomes saturated with water, N can and will be lost through a process call denitrification. Denitrification is the conversion of nitrate to one of several gaseous forms of N that will be lost to the atmosphere. This process usually occurs when conditions exist for the absence of oxygen (anaerobic), such as in saturated soils. The amount of loss varies, depending on temperature and time sustained in these anaerobic conditions. Table 1, from the Nebraska Extension publication, Nutrient Management for Agronomic Crops in Nebraska: Nitrogen (EC155), shows the rate of N loss.
Leaching of N in the form of nitrate is particularly a problem where water movement in the soil is not restricted. Leaching occurs when there is a water soluble and mobile form of N, nitrate, is dissolved in the soil water, and moves down through the profile. If there is enough movement of water, the nitrate will leach beyond the rooting depth of the crop. Eventually this nitrate will reach a water source such as an aquifer. Of immediate concern to the grower is the loss of N that was intended to supply the crop. The amount of loss and potential for an N deficiency later in the growing season is governed by several factors: the amount of N applied, the soil water, soil texture, and the intensity and amount of rainfall that occurred. There is no simple estimation for the amount of N that was lost from leaching, but there are actions the grower can take to ensure the crop is adequately fertilized before yield-limiting conditions exist.
ManagementField Assessment. Carefully monitoring the crop for N status is another option, primarily between now and silking. Most corn hybrids will take up the majority of their N requirement in this period. The growing crop will present a better picture of integrated N availability from the soil than will a soil sample.
Visual observation. Visual observation for signs of N deficiency (lower leaves yellowing, inverted V yellowing pattern of leaf tips) is one option, although yield potential may be reduced by the time N deficiency is visible.
Sensors. A chlorophyll meter or other crop canopy sensor may be useful in detecting N stress before it can be seen. To calibrate chlorophyll meter readings, it is best to have one or more reference strips in the field with N applied at a rate high enough that yield would not be limited. A reference strip can be useful in assessing N sufficiency visually and from aerial imagery. For more information, see the UNL Extension NebGuide, Using a Chlorophyll Meter to Improve N Management (G1632).
Long-Term Planning using the UNL Nitrogen Loss Assessment Tool: N-LAT
Reducing N loss needs to be a management objective largely dependent on pre-N application decisions. An important consideration is the potential for N loss to one or more processes for a given field condition as determined by soil, climate averages, and management. The Nebraska-specific tool N-LAT (Nitrogen Loss Assessment Tool) is easily used to estimate average N losses for four processes and to determine likely effects of management alternatives on these losses. See NebGuide G2249, Nitrogen Loss Assessment Tool (N-LAT) for Nebraska: Background and Users Guide, and downloand N-LAT at (N-LAT) from the CropWatch Soil Management page.Brian Krienke
Soils Extension Educator