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Are you thinking of putting some or all of your nitrogen on in an early preplant application? You should reconsider. The importance of timing in N management cannot be overemphasized as the interval between date of application and crop uptake determines the length of exposure to N loss processes. Combining early preplant herbicide programs with a fertilizer application may appear efficient in terms of input costs and field traffic, but a broadcast fertilizer application as much as 30 days before planting can lead to a substantial loss of the applied N.
Multiple pathways exist for N loss after a surface application of UAN, for example. Surface volatilization of N can occur when urea forms of N break down to form ammonia gases, and there is not enough soil water to absorb the gases as they form. Typically, this path to N loss occurs when urea or solutions (28%) are applied to the field but heavy residue prevents direct contact of the material with the soil. Nitrate N can be lost through denitrification and leaching. Denitrification is promoted by wet soils, compaction and warm temperatures, conditions that cause soil bacteria and microorganisms to meet their biological needs for oxygen by scavenging it from nitrate thereby volatilizing the N as N gas or nitrous oxide. Leaching losses occur when excessive rainfall moves N off the field either in surface runoff or through deep percolation.
Recent Indiana field trials on no-till corn illustrate the potential for N loss from early preplant applications. Broadcast application of UAN two to three weeks before planting resulted in 142 bu./a. grain yields, 14 % lower yields than in treatments where an equivalent amount of N was broadcast preemergence (Table 1). The most efficient method of N application was injection at planting (179 bu./a. yields) where N losses were reduced by ensuring good material contact with the soil and by minimizing the amount of time between application and crop uptake.
| Table 1. Effect of time and method of UAN application on yield of no-till corn in Indiana, 1993-1994. | |
| Time and Method of Application | Grain Yield |
|---|---|
| bu./a. | |
| No N Control | 89 |
| Early preplant Broadcast | 142 |
| Preemergence Broadcast | 165 |
| Injection at Planting | 179 |
| Source: Mann, Hawkins, and Mengel, Purdue University | |
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