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Success of fall-applied nitrogen often a very local issue

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“Fall fertilization with nitrogen (N) can range from very effective to disastrous depending on three factors.”

An Alberta Agriculture soil scientist says the success of fall fertilization often depends on very local conditions.

“Fall fertilization with nitrogen (N) can range from very effective to disastrous depending on three factors,” says Dr. Ross McKenzie, agronomy research scientist with Alberta Agriculture and Rural Deve lopment , Le thbr idge . “These three factors are soil moisture, the form of nitrogen used, and how it is applied. To understand the effect of these factors, we need to understand the fate of fertilizer nitrogen (N) in soil.”

Fertilizer N is applied to soil in the form of urea (CO(NH2)2) or anhydrous ammonia (NH3). When applied in 28-0-0 liquid fertilizer, the nitrogen forms are ammonium (NH4+) and nitrate (NO3-). Urea and anhydrous ammonia quickly convert to ammonium in warm moist soil conditions. If the soil is warm, moist and well-aerated, ammonium is rapidly oxidized to nitrate through the nitrification process. This is a biological process performed by highly specialized soil bacteria. Nitrate is the form of N most crops take up from soil.

Banding slows the nitrification process by creating an environment near the band that inhibits the activity of the bacteria converting ammonium to nitrate. Therefore, if urea or anhydrous ammonia is banded in late fall, most of the N is retained in the ammonium form until the soil warms up in the spring.

If the fertilizer is broadcast or banded in early fall, likely most of the ammonium will be converted to nitrate before freeze-up. Large losses can then occur when soils are water saturated during and just after snow melt in early spring. The losses are caused by an anaerobic process called denitrification that converts nitrate to gaseous forms of nitrogen such as nitrous oxide, which is lost from the soil.

Nitrate can also be lost from wet soils as a result of leaching, as nitrate is very soluble and mobile. Leaching is a serious concern in sandy soils and denitrification is a greater concern in loam and clay soils.

“Research has also shown that denitrification will occur in virtually all of Alberta’s agricultural soils,” says McKenzie “This isn’t surprising since denitrification isn’t a particularly specialized function. Many different types of soil bacteria use denitrification as an alternative form of respiration when oxygen is in short supply.”

VARIES BY REGION

This means no soil type or region of the province is 100 per cent safe when it comes to losses of fall-applied N. The risk of winter N loss is highest in regions with moister climates, such as west-central Alberta. There is less risk in regions that tend to be drier, such as south eastern Alberta, but even in these regions, N losses can be high during a wet spring.

In general, N losses through denitrification in the drier regions are normally small, and fall-banded N is equal to spring-banded N. In cases where spring banding causes a significant loss of seedbed moisture, fall banding can be superior to spring banding.

“Denitrifying bacteria are minute in size,” says McKenzie. “They only respond to what is happening in their tiny corner of the field.

“What this means is the specific soil environmental conditions are very important. During dry springs, there are localized wet areas such as depressions where denitrification can occur. Think about this in terms of your own fields. Are they uniformly flat and well drained or are the fields gently to moderately rolling with number of lower depressions in the field? There are always spots that are wetter than the rest of the field where runoff accumulates after a rain or spring snow melt. Winter N losses can vary greatly over a short distance. Fall-applied N can be very effective on upland and totally ineffective in a depression just a short distance away.”

McKenzie adds that it is important to remember that fall application always puts fertilizer N at risk. The level of risk is generally assessed at the regional level, but whether losses occur depends on very local environmental soil conditions.

For more information on fall fertilization, call the Alberta Ag-Info Centre toll-free at 310-FARM (3276).

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