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Fluid Journal : Fluid Journal 1996-1998
420 400 380 360 340 320 Yield - cwt/A 32-0-0 28-0-0-5(S) Figure 3. Effect of DCD on irrigated potato yields in Idaho, soil N, most is not available to the crop. The active component of this organic fraction can contribute 30 percent of all mineralized N while comprising only 4 percent of the total N pool. The remaining inactive portion represents stored N that may not be available for a long time. Plants cannot use organic N in significant quantities. Organic N must be converted to inorganic forms. The conversion of organic N forms to inorganic plant-available forms is called mineralization. Nitrogen bound in organic forms is released when soil organisms digest residues. The first product of mineralization is the ammo- nium ion (NH4+). Once NH4+ is released, it can be oxidized to nitrate (NO3-). The oxidation of NH4+ to NO3- is called nitrification. So much for basics. A look now at a perennial problem and an answer. Nitrogen loss Agronomists know that a crop will take up 40 to 70 percent of applied N. Through research, they have identified several principal ways in which it can be lost. Immobilization. When soil organisms use nitrogen, it is no longer available to the plant. Soil organisms, like plants, use fertilizer and soil nitrogen to grow. This process of nitrogen tie-up is called immobilization. Immobilization may be somewhat higher for ammonium-N than for nitrate. Soil bacteria require about one pound of N for every 11 lbs of carbon in the process of decomposing residues in the soil. The more carbon in the residue, the more likely is immobili- zation by soil bacteria. This loss of N is really only a temporary because as the soil organisms die, the N held in their cells is released for use by plants and other soil organisms. Leaching. Nitrate is subject to leaching if precipitation or irrigation is heavy enough to move water down- ward. One inch of water on silt loam or clay loam can move NO3 down four to six inches. Because infiltration and percolation are very rapid on sandy soils, an inch of water can move nitrates up to one foot. Less movement of NH4+ will be observed, but on low CEC soils even the ammonium ion can be washed downward. Volatilization. Nitrogen can be lost by ammonia volatilization. Ammonia volatilization is the loss of nitrogen as ammonia (NH3) gas into the atmo- sphere. Under certain conditions, urea or ammonium nitrogen can be converted to NH3 nitrogen. If this occurs at or near the surface, N will be lost to the atmosphere. Surface applications of amino-based fertilizers such as UAN or urea are subject to losses from volatilization. In most neutral to acid pH solutions, nitrogen exists as NH4+ or NO3- ions. Urea is soluble and exists as a urea molecule [CO(NH2)2]. When a urea- containing solution is sprayed on the soil Winter 1996 220 210 200 190 180 170 160 150 140 130 120 110 100 N N + N-Serve® UAN - lbs/A (PP = Preplant, SD + Sidedress) Figure 4. Response of center pivot irrigated corn to N-Serve; N source UAN, N- Serve used at rate of 1 qt/A. Yield (bu/A) 200 100PP 100PP 100PP 75PP 200 100PP 100PP 100PP 100SD 150SD 200SD 75SD 100SD 150SD 200SD Site 1 Site 2 Table 1. Effect of rainfall on volatilization Rainfall Time Estimated loss in. days % 0.4 less than 2 0 0.4 2to3 <10 0.1 to 0.2 less than 5 10to30 .25 to .35 less than 9 10to30 0 6 >30
Fluid Journal 1993-1995
Fluid Journal 1999-2001