One of the most common questions growers ask Decisive Farming Agronomists this time of year is whether or not to apply Nitrogen fertilizer in Fall or to wait until Spring. The response to them is it depends. The efficacy of Nitrogen applications depends on the following factors:
- Soil temperature and moisture content
- The type of Nitrogen are being used
- The method for fertilizer application
Soil temperature and moisture content
To understand how these factors influence the successful application of Fall Nitrogen, let’s look at the processes Nitrogen undergoes when in the soil.
Fertilizer Nitrogen is applied to the soil, either as urea, anhydrous ammonia, ammonium or nitrate, depending on the product used. Urea and anhydrous ammonia in soil quickly convert to ammonium. In warm, moist, and well-aerated soils, ammonium is rapidly oxidized to nitrate by highly specialized soil bacteria through a process known as nitrification.
Substantial Nitrogen losses can occur in soils as we enter winter with a large pool of available nitrate and then become water-saturated during and just after the snow melt in early spring. Soil N is lost when micro-organisms in anaerobic conditions convert nitrate N to nitrous oxides and/or nitrogen gas in a process called denitrification.
Generally speaking, once soil temperatures drop to below 5 degrees Celsius, the conversion rate from ammonium to nitrate slows down, to the extent that it almost stops.
It needs to be noted that any Fall applications creates risk for potential Nitrogen losses if the soil is saturated. Expects applications to be very effective on well drained upper slopes, but completely ineffective in nearby poorly drained depressions.
Type of Nitrogen being used
Applying Anhydrous Ammonia after the soil has reached low temperature will stay as Ammonia and only convert to Nitrate in the Spring when the soil warms up. Note that applied Nitrogen will also not be detected in any soil tests for Nitrate residues in the soil.
Urea will have a similar effect as Anhydrous Ammonia, if band applied, although not as stable as Anhydrous Ammonia. However, Liquid N, in the form of Urea Ammonium Nitrate contains 25% converted Nitrate and its stability is low and will have the potential for greater losses.
Principles of Nitrogen fertilizer application methods and timing
- Spring-banded N 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 springtime seedbed moisture is limited.
- Fall banded N also has an advantage in areas with high snowfall in that the conversion warms the soils up slightly and helps with drying off. In some Northern areas, this may help farmers to be able to seed as much as a week earlier than usual.
Tips to consider for Fall applications
In summary, the following tips will help guide you to a successful Nitrogen applications in Fall and/or Spring:
- Test your Soil to determine the optimum rates of fertilizer needed. Best practices for soil testing are samples taken at 0 to 6” and 6 to 24” increments to determine the total nitrate N.
- Avoid Fall Nitrogen applications in areas where soils are poorly drained and tend to be saturated with water for extended periods in the spring
- Fall banding can be an effective N application method where saturated soil conditions are not an issue
- Apply a conservative rate, typically about 75 per cent of the crop requirement in the Fall. This will hedge against potential over-winter losses, low spring moisture or low crop prices. If conditions look favourable in the Spring, additional N can be applied at seeding.
- Avoid the use of the nitrate products (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 5°C and the nitrification process has slowed down.
- Band, do not broadcast. Banding restricts the contact between soil and fertilizer, concentrating the fertilizer and delaying nitrification. Consequently, over-winter N losses are lower with banding than with broadcasting.
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