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N source, application as important as rate

Application method and source can influence grain yield much more than the applied nitrogen rate in corn production, according to Erick Larson, Extension corn specialist for Mississippi.

Larson points out that no-till research studies in Missouri and Tennessee show UAN-solution (N-sol) and urea broadcast on the soil surface reduced corn yield potential 9 percent to 23 percent compared to ammonium nitrate broadcast, N-sol injected, or anhydrous ammonia injected.

The surface application would reduce economic returns from $50 to $184 per acre, compared to ammonium nitrate or N-sol injected, at current corn prices.

A big reason for that is urea-containing nitrogen sources, including N-sol (32 percent, or 28-0-0-5) and urea (46-0-0 or 41-0-0-5), are much more subject to volatilization loss when applied to the soil surface, either broadcast or dribbled in a band.

Larson notes that surface-applied urea sources readily volatilize when substantial crop residue or vegetation is present on the soil surface, temperatures exceed 55 degrees, and rates exceed 100 pounds of nitrogen per acre. The potential for volatilization exists until rainfall incorporates the nitrogen.

One problem this spring was the lack of rainfall in the Mid-South during March. Volatility losses from broadcast urea nitrogen sources “are likely considerable.”

Surface applications do have a benefit, Larson says. For example, with surface applications applied preplant, “you're not likely to have problems with early-season nitrogen uptake, and you might have if you sidedressed it all postemergence.

“But you don't want to surface-apply a lot of your nitrogen because you subject it to a fair amount of risk. Minimizing the broadcast amount applied early in the season would help reduce potentially significant volatility losses in dry years.”

While producers can increase nitrogen sidedress applications after corn emergence to compensate for volatility losses, there are some things they can do to minimize volatilization in the first place.

According to Larson, urease inhibitors, such as Agrotain, may be applied to granular urea or N-sol to reduce volatility potential. The product temporarily slows the activity of the urease enzyme. However, timely rainfall or overhead irrigation is still necessary to incorporate urea-based nitrogen into the soil for plant utilization and to stop volatility.

Producers should avoid making a broadcast application of nitrogen after corn emergence, if possible. Larson stresses that broadcast applications of N-sol can kill any exposed tissue of emerged corn plants. Although the growing point of young corn plants remains underground and protected until about the V5-V6 growth stage (12 inches tall), intentionally burning exposed plants “is not a good idea.”

Severe fertilizer burn temporarily eliminates the ability of the plants to produce energy through photosynthesis, Larson says. “Thus, plant growth falls back upon energy reserves in the seed. If warm, dry or otherwise favorable growing conditions prevail, corn plants will likely generate new leaf tissue relatively quickly and experience little stand reduction.

“If wet, cold, unfavorable conditions persist, recovery will be very slow and the likelihood for stand failure increases greatly, particularly if disease or insect pests infect or infest the plants during this vulnerable period.”

For sidedress nitrogen applications, Larson stresses that nitrogen placement close to the row is not necessary or even preferred for corn production, since nitrogen is relatively mobile in the soil-water solution, compared to other nutrients.

Corn also has a fibrous root system, which develops substantially more lateral growth than tap-rooted crops, such as cotton. In fact, corn roots will likely extend to the row middles before plants are knee-high. More than 90 percent of nitrogen uptake will occur after corn is more than knee-high.

Larson generally advises corn growers to place sidedress knives in the row middles to avoid substantial root pruning. This is most important with sidedressing corn taller than 12-inches, when you are likely to get more benefit from a split applications. This suggestion applies to all normal (single) row widths and twin-row patterns (based upon wide rows). “The outside knife on each side of the applicator should be modified to apply one-half of the intended nitrogen rate, since it will run between the same rows twice.”

Denitrification occurs when nitrate nitrogen is converted into nitrogen gas by microorganisms and escapes into the air. Warm soil temperatures accelerate this process.

Denitrification loss related to extended soil saturation in the bottom of the furrow (in bedded fields) will primarily apply to the first substantial rainfall event, which will disperse nitrogen through the profile, Larson says. Thus, seasonal effects should be insignificant.

Research indicates denitrification rates range from 2 percent to 3 percent per day at soil temperatures of 55 to 65 degrees or more if soil temperatures are warmer. “I would anticipate 10 percent to 30 percent of preplant or pre-emergence applied nitrogen may potentially be wasted in seasons with normal rainfall.”

Mississippi's warm, wet spring climate generally dictates better performance from timely split applications of nitrogen fertilizer relative to corn crop demand (spoon-feeding), or substantial nitrogen may be lost prior to crop uptake, Larson says. Corn extracts less than 10 percent of its seasonal nitrogen uptake before rapid vegetative growth begins, which is normally after May 1.

Producers can improve their nitrogen use efficiency considerably by applying only a minimal portion of nitrogen shortly after emergence, followed by the bulk of their nitrogen fertilizer just prior to this period of maximum crop demand. Early fertilization can waste considerable nitrogen, particularly if wet weather prevails, before rapid corn growth begins.

The state's standard nitrogen recommendation is to apply no more than one-third of total nitrogen near planting/crop emergence and apply the remaining nitrogen about 30 days later (corn exceeding 12 inches tall or V6 growth stage).

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