Fall fertilizer applications can make things run more smoothly in the spring, especially in years where spring seeding is delayed and time is precious.
Urea and anhydrous ammonia are perfect forms of nitrogen fertilizer for fall banding, but timing is critical to minimize nitrogen loss, said Trish Meyers, crop enhancement specialist with Agricore United.
Urea and ammonia cling tightly to the soil in a form called ammonium. In warm, moist soil, the ammonium converts to nitrate, a form of nitrogen that plants can use. The risk with nitrate is that it can be gassed off in the spring from saturated soil or leached from the rooting zone, especially with high rainfall and light soil and when vegetation hasn’t started to grow.
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When urea and anhydrous ammonia are applied to cool soil, conversion to nitrate is a slow process. The ideal soil temperature for fall banding is generally considered less than 10 C.
Soil temperatures can be taken with a soil thermometer placed eight to 10 centimetres deep into the soil at a consistent time each day, usually mid-morning. Taking temperatures from various locations will provide a good representation of the field. Once soil temperature is below 10 C, it doesn’t fluctuate much.
If the soil is warmer than 10 C, soil microorganisms start converting urea and ammonia to nitrate. The rate of conversion depends on soil moisture and temperature.
In a loam soil at 10 C, it takes six weeks for half of the urea or anhydrous to convert to nitrate. At 4.5 C, it takes longer than four months for half to convert. But at 24 C, it only takes one to two weeks to convert all of the urea or anhydrous ammonia to the vulnerable nitrate form.
Conversion of urea and anhydrous ammonia to nitrate in the spring is necessary for crops to take up nitrogen and produce healthy vegetation. Banding urea and anhydrous ammonia in soil at less than 10 C will minimize problems with early conversion and deliver the best results for your crop.
