Nitrogen Fertilizer Application -- Fall versus Spring
Farmers frequently ask about the relative effectiveness of fall versus spring nitrogen (N) fertilizer applications.
“Fall fertilization can range from very effective to disastrous depending on soil moisture conditions, the form of nitrogen fertilizer used, and how it is applied,” says Ross McKenzie, Agronomy Research Scientist, Alberta Agriculture and Rural Development (ARD), Lethbridge.
Various soil processes and environmental conditions will affect the fate of fertilizer N in soil. Fertilizer N is applied to soil in the form of urea 46-0-0, anhydrous ammonia 82-0-0, or liquid fertilizer 28-0-0 which contains urea, ammonium, and nitrate. Urea and anhydrous ammonia quickly convert to ammonium, which won’t leach and normally won’t volatilize. It is the ammonium and nitrate forms that are taken up by plants, but generally most N is taken up in the nitrate form. If the soil is warm, moist and well aerated, ammonium is rapidly oxidized to nitrate through the nitrification process, which is a soil biological process performed by highly specialized soil bacteria.
Banding N fertilizer 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 over winter 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 prior to freeze-up, and large N losses can occur when soils are water saturated during the spring. The losses are caused by anaerobic bacteria through a process called denitrification. Denitrification converts nitrate to gaseous N forms such as nitrous oxide. Once this process occurs the nitrogen fertilizer is lost from the soil.
“Research has also shown that denitrification will occur in virtually all of our agricultural soils,” says McKenzie. This is not surprising since denitrification is not a particularly specialized function. Many different types of soil bacteria use denitrification as an alternative form of respiration when oxygen is in short supply.
“In terms of fertilizer management, this means that no soil type or region of the province is completely safe when it comes to losses of fall-applied N,” says McKenzie. “In most years, N losses through denitrification in the southern Alberta are relatively small and fall banded N is usually 90 to 100 per cent equally effective to spring banded N.”
In cases where spring banding causes a significant loss of seedbed moisture, fall banding can be superior to spring banding. Typically, denitrification can be significant on irrigated soils after extended saturating rain conditions in spring and summer.
“Denitrifying bacteria are less than two millionths of a meter in size and respond to environmental conditions in the micro-pores of soil,” notes McKenzie. “This means that the microclimate in soil is important.” In fields with localized wet, depressional areas, denitrification can occur. Over winter N losses can vary greatly over a short distance in fields with rolling topography. Fall-applied N can be very effective on upland areas of a field and totally ineffective in depressional areas just a short distance away.
It is important to remember fall-application always puts your fertilizer N at risk. The level of risk is generally assessed at the regional level, but whether or not losses occur is a function of very localized moisture and temperature conditions. General rules about N application methods and timing include:
- Generally spring banded is the most effective method of application and fall broadcast the least effective.
- Fall banded N can be as effective as spring banded if there is no extended period of saturation in the spring.
- Fall banded N may be more effective than spring banded when lack of seedbed moisture is a concern and the spring banding operation dries the soil and reduces the quality of the seedbed for spring seeding.
With all this information in mind here are a few tips to consider before fertilizing in the fall:
- If your soils tend to be saturated with water for extended periods in the spring, then fall-application is probably not a good option. However, if saturated soil conditions are normally not a problem, then you should get good results from fall banding.
- Soil test to determine the optimum rates of fertilizer required. Producers should sample zero to six, six to 12, and 12 to 24 inches to determine the cumulative N to two feet. Based on soil analysis, then you can decide the ideal rates of N fertilizer to apply.
- Select a fertilizer formulation that is right for your conditions. Generally, under low risk conditions such as those in southern Alberta, anhydrous ammonia (82-0-0) or urea (46-0-0) perform equally well when fall banded. However, soils in southern Alberta tend to be alkaline and losses through ammonia volatilization can occur if the bands are too shallow or the soil is dry and cloddy.
- Avoid the use of nitrate containing products such as 28-0-0 on soils that tend to be saturated in the spring. Nitrates are subject to both denitrification and leaching losses under wet spring conditions, so it is best to apply N as late in the fall as possible to reduce the potential of the nitrogen fertilizer converting to nitrate in the fall.
- Apply N in late fall after the soil temperature has dropped below seven degrees Celsius and the nitrification process has slowed down.
- Band, don’t broadcast. Banding restricts the contact between soil and fertilizer and as a result over-winter losses are lower.
Farmers should consider consulting with a soil fertility specialist when setting up a fall fertilizer program. Other management factors farmers should weigh when considering whether to fall fertilize include:
- Fall fertilization can improve your time management. By applying fall-fertilizer, a field operation can be eliminated in the spring and allow earlier planting.
- Fertilizer prices and payment schedules tend to be more favourable in the fall, making it economical to fall apply.
- Availability of product and application equipment is often better in the fall than during the peak demand periods in spring.
- Soils tend to be drier in the fall, so N application equipment is less likely to cause soil compaction.
“These are the major points farmers should keep in mind when making a decision on whether to fertilize in the fall,” says McKenzie. “It is always a good idea to get several opinions and to weigh all the factors before you make your final fertilizer timing decision.”
Source: Alberta Agriculture And Rural Development