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Fluid Journal : Fluid Journal 1993-1995
Winter 1995 160 150 140 130 120 110 100 90 80 Yield - bu/A 1 150 lbs/A N 2 50 lbs/A early 3 100 lbs/A N early Figure 1. Effects of different UAN management systems on corn yield following corn and soybeans over two years, Eckert, Wooster and Springfeild, Ohio. Soil Applied N Previous Crop Soybeans NoN Broadcast1 Surface-band1 Split2 Split3 160 150 140 130 120 110 100 90 80 Yield - bu/A N rate - lbs/A At Planting Split --- 50 lbs/A N broad- cast at planting; balance banded at 4-leaf stage 0 100 200 Figure 2. Corn yield as affected by N rate and timing of UAN application over three years, Eckert, Wooster and Hoytville, Ohio. used, and these optimum rates are not necessarily higher for conservation tillage. Ensuring adequate applications of N, therefore, is a major step toward achieving optimum N-use efficiency in conservation tillage. Volatilization. Conditions for N loss are ideal when broadcasting fertilizer onto moist, warm surfaces with heavy residue cover. Crop residues contain high concen- trations of an enzyme called urease, which converts urea to ammonia and carbon dioxide. This reaction is essential for efficient use of urea-N by plants. Normally, it causes no problem when it occurs under- ground, because the ammonia dissolves in soil water and quickly attaches to the soil's cation exchange complex. When urea- containing fertilizers are applied to the soil surface, however, the ammonia generated from urea usually evaporates into the atmosphere, resulting in loss of N fertilizer. Yields may be reduced severely as a result. Placement is key N losses can be reduced and yields maintained by adopting practices that minimize contact between urea-containing materials and urease-rich crop residues. Since placement is crucial, we'll review the pros and cons of various placement systems. Broadcasting. Broadcasting granular urea or urea-containing materials is not advisable, unless it will be incorporated very shortly after application. Broadcast applica- tions of UAN should be limited from a fourth to a third of the crop's total N requirement, and should be confined to "weed and feed" or at-plant application programs only. Broadcasting all N as UAN in heavy residue, such as corn stalks, may reduce yields almost as much as broadcast- ing urea does. In lighter residue situations, such as soybean stubble, its effect on yield reductions may be mitigated (Figure 1). However, despite this tendency, losses are possible. Therefore, UAN placement programs other than broadcasting are usually more desirable or consistent. In the Wooster and Springfield studies cited in Figure 1, N rates were 150 lbs/A. Injection. Injecting UAN several inches below the soil surface prevents urea-loss and mobilization problems because it is placed below the residue. The result will be improved yields. Banding UAN on the soil surface reduces contact between the fertilizer and residue, reducing potential for yield reduc- tions as shown in Figure 1. Split applications. Here the bulk of N is applied after crop emergence as a surface- banded or injected sidedress. This method has also proven more effective than broadcast, especially in corn following corn (Figure 1). N in these treatments was broadcast at planting and surface-banded at sidedressing. In experiments at Wooster and Hoytville, similar responses were shown between at-planting and split applications (Figure 2). Split applications also have the advantage of allowing producers to evaluate the condition of the crop before applying the majority of N, permitting increased rates of application under ideal conditions and reduced rates under poor ones. Starters. In any system where N is to be applied in a band away from the seedlings (i.e., in row middles or as a sidedress), some starter N should be applied to ensure that young plants do not become N deficient before roots grow into the main concentra- tion of N. Such at-planting applied N can be placed in the row with the planter or included as UAN in a "weed-and-feed"
Fluid Journal 1996-1998