Providing adequate nitrogen (N) is a major factor in building a successful corn production program. Several N fertilizer materials are available, giving farmers flexibility in planning fertility programs. Research has shown little if any difference in corn yield as a result of using different N sources in conventional tillage production systems if soils are well drained. The reason seems due to achieving quick contact between soil and fertilizer. On wet soils there is less chance of loss from ammonium than nitrate forms of nitrogen.
In no-tillage production systems, however, achieving quick N-soil contact may not be possible when the fertilizer material is applied by broadcast methods. Unincorporated crop residues may intercept much material and hold it above the soil surface until rainfall washes it into the profile. Prolonged contact with crop residues may cause loss of applied N and reduced yield. The extent of N loss will be affected by the type of N fertilizer, the application method used, soil surface pH, soil drainage, the weather, and the nature of the crop residue. Weather will be the major factor, but because a farmer can rarely predict it in advance, those involved in no-till corn production should consider carefully sources of nitrogen and methods of application, designing a program which minimizes risk, regardless of weather.
ANHYDROUS AMMONIA is the preferred source of N in no-till systems. Because this material is injected into the soil, it does not interact with surface residue and no problems associated with tillage or residue are normally anticipated. Subsurface N application has little effect on surface pH, making it easier to maintain a favorable pH for weed control. It may be necessary to mount coulters in front of knives to cut through heavy residue. As in conventional tillage systems, 25-50 lb N/A should be applied on the surface or with the planter to promote early root growth when anhydrous ammonia is the primary N source. Anhydrous ammonia may cause significant acidification at the point of injection. Farmers should use tissue analysis to monitor possible buildup of nutrients and toxicities related to soil acidity.
UREA can be broken down to ammonium carbonate by an enzyme present in crop residue and then escape into the atmosphere as ammonia gas. This can be a severe problem on no-till fields covered with heavy residues such as cornstalks or with a high surface pH, when a warm, dry period follows the urea application. Because weather is critical in determining N-loss from urea, farmers who choose to broadcast urea are urged to do so early in the season (early April) because cooler temperatures will slow N loss, or prior to anticipated rains which will wash the urea into the soil. Banding urea between corn rows and below residues may offer another way to avoid losses. Farmers should never broadcast urea over freshly limed soils as high pH can greatly increase N loss. Autumn is the best time to lime no-till fields, if one is to minimize N losses.
NITROGEN SOLUTIONS (28-32% N) contain significant amounts of urea, though N losses are normally less of a problem than when prilled or granular urea is used. Some losses can occur, however, in dry springs. Banding, split applications, and injection have improved yields from solutions in research studies. As with urea, the best time to apply solutions is shortly before rain. This N material should be applied with caution to standing residue, such as a rye cover crop. Nitrogen losses from this situation can be severe.
AMMONIUM NITRATE is available in certain areas of the state, though material handling problems have limited its widespread adoption. This material contains no urea and is a safe material for surface application. In comparative studies, ammonium nitrate has produced corn yields higher than broadcast urea in years when significant urea loss occurred.
In summary, a preference list of N fertilizers would read anhydrous ammonia, ammonium nitrate, N solutions, and finally urea. Broadcasting urea and N solutions may cause farmers serious problems in certain years. Much of the problem can probably be overcome by proper application.
Surface applied N fertilizers will greatly depress surface soil pH in no-till fields. Proper soil sampling and liming procedures should be followed to ensure effective weed control when using triazine herbicides.
All of the above mentioned nitrogen fertilizers are subject to N loss by denitrification. The use of a nitrification inhibitor is strongly recommended in fields where denitrification may be a problem.
Prepared by:
Donald J. Eckert
Professor, Natural Resources
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