Agronomy Library > Soil Conservation

Reduce Losses of Fall Applied Fertilizers
Author: Dr. Ross McKenzie, Alberta Agriculture and Food, Lethbridge, AB
Date Created: October 15, 2002
Last Reviewed: February 01, 2007

Farmers frequently ask about the relative effectiveness of fall versus spring nitrogen fertilizer applications. In a nutshell, fall fertilization can range from very effective to disastrous depending on soil moisture, the form of nitrogen used and how it is applied. To understand why we must understand the fate of fertilizer N in soil.

Fertilizer N is applied to soil in the form of urea (CO(NH2)2), anhydrous ammonia (NH3), ammonium (NH4+), or nitrate (NO3-) depending on the product used. Urea and anhydrous ammonia quickly convert to ammonium. It is the ammonium and nitrate forms that are taken up by plants. 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.

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 prior to freeze-up, large losses can 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 which converts nitrate to nitrogen and oxygen gases.

Fall-Applied N as a Percent of Spring Broadcast and Incorporated
 
Soil Climate Categories
Application Method
Dry
Medium
Wet
Irrigated
Spring Broadcast and Incorp
100
100
100
100
Spring Banded
120
110
105
110
Fall Broadcast and Incorp
90
75
65
95
Fall Banded
120
110
85
110
Dry-Well drained soils which are seldom saturated during spring thaw.
Medium-Well to moderately drained soils that are occasionally saturated during spring thaw for short periods.
Wet-Poorly to moderately drained soils that are saturated for extended periods during spring thaw.
Irrigated-Well drained soils in southern Alberta that are seldom saturated during spring thaw.
(Source: Agdex 542-7)

Research has also shown that denitrification will occur in virtually all of our agricultural soils. 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.

What this means in terms of fertilizer management is that no soil type or region of the province is 100% safe when it comes to losses of fall-applied N. The risk of over 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 southeastern Alberta, but even in these regions N losses can be high during a wet spring, such as this year. In genera, however, N losses through denitrification in the drier regions are normally small and fall banded N is equal to spring banded N (see Table). In cases where spring banding causes a significant loss of seedbed moisture, fall banding can be superior to spring banding.

Also keep in mind that denitrifying bacteria are less than 2 millonths of a meter in size. They could care less about the regional climate or moisture level during a given spring. They only respond to what is happening in their tiny corner of your field. What does this mean? It means micro-climate is also important. Even 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? Not likely. There are always spots that are wetter than the rest. Where runoff accumulates after a rain or spring snow melt. Over 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.
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 local conditions.

Before we get into the nitty gritty of management recommendations, let’s use our discussion thus far to develop some general rules about application methods and timing:
-Generally spring banded is the most effective method of application and fall broadcast the least effective.
-Fall banded N will 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.

With this information in mind here are a few tips to consider before you fertilize this 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 for you, then you should get results from fall banding.
-Soil test to determine the optimum rates of fertilizer required. We encourage producers to sample 0-6, 6-12, and 12-24 inches to determine the cumulative N to two feet.
-Apply a conservative rate, say 75% of soil test recommendation, or 75% of what you would expect to apply, if you haven’t got your results back at time of application. This conservative fall rate is a hedge against such things as high soil test N levels, or low spring moisture or low crop prices. If conditions look favorable come spring additional N can be drilled with the seed. Take note however that the amount of additional N that can be drilled with small seeded crops like canola is only 10 pounds with a disk drill and 20 – 35 lbs. with an air seeder.
-Select a fertilizer formulation that is right for your conditions. Generally under low risk conditions such as in southern Alberta, anhydrous ammonia (82-0-0), urea (46-0-0), ammonium nitrate (34-0-0), or liquid nitrogen (28-0-0) 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 the nitrate containing products 34-0-0 and 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.
-Apply N in late fall after the soil temperature has dropped below 7 C and the nitrification process has slowed down.
-Band, don’t broadcast. Banding restricts the contact between soil and fertilizer and as a result overwinter losses are lower.

As you can see there are a number of agronomic factors to consider before you go ahead and fall apply N. You may want to consult with a soil fertility specialist while you’re setting up your fall fertilizer program.

Other management factors should also be considered in deciding to fall fertilize. Here are a few:
-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 favorable 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.

That covers the major points to keep in mind when making your decision to fertilize this fall. It is always a good idea to get several opinions and consider all the factors before you make your final decision.