Narrow Seed Rows, High Rates of Urea: A Recipe for Disaster?

By Juanita Polegi, P Ag

Conservation Agrologist

Here's the situation: A farmer wants to move into a low disturbance direct seeding system. He's using knives on his air seeder. He wants to put high rates of 46-0-0 down with the seed. He knows that the germinating seedlings can be damaged by "fertilizer burn." If the soil conditions aren't perfect, that burn could seriously affect the establishment of the stand. What to do?

Until recently, the farmer had a few options available. He could either band the nitrogen prior to seeding, dribble band it post-seeding or cut the amount of nitrogen applied at seeding and then run the risk of not having sufficient N available throughout the growing season. Now there is another option, Agrotain, produced by IMC. The cost is 10 cents per pound over the cost of the N (as of spring 2000).

Agrotain is a urease inhibitor. That means that it slows the activity of the enzyme urease, which is what converts urea into ammonia. It's the ammonia that causes seedling burn so as the urease activity slows, the conversion of urea to ammonia slows. That in turn, allows more time for the moisture and leaching to move the urea away from the seed row. As the urea moves away from the seeds and is converted to ammonia, the ammonia will be less concentrated near the seeds. The lower concentration of ammonia in the seed row will ultimately reduce the fertilizer burn on the seedlings.

When the white urea granules are sprayed with Agrotain, they turn a pale green. They are handled in the same manner as the 46-0-0 and flow pretty much as the plain white granules.

Last spring, Ernie Patrick of SA&F and I seeded some canola plots at ECRF on May 11. Using the farm's Seed Hawk, its 2-knife system enabled us to sideband the fertilizer or put the fertilizer with the seed down one knife. We applied 3 rates of urea based (46-0-0) fertilizer: 35, 65 and 100 lbs/ac actual N. Each rate was seed placed, sidebanded or had been coated with Agrotain. The soil moisture was more than adequate and a snowstorm rolled in at the end of the day.

Plant counts were taken June 16. As the table below shows, at the 30 lb rate, there was no real effect on the # of plants per metre² row or their development. All treatments fell within the target of 80 - 180 plants per m². At the 65 lb rate, we start to see some effect of the higher N rates on the seed placed N and seed placed N coated with Agrotain. The two treatments were below the target populations, while the side banded N met the target. (We're not sure why the # of plants dropped between the 35 and 65 lb N side banded treatments. We didn't expect that, especially when the number of plants rebounded at the 100 lb N side band treatment). The high rate of seed placed N seriously affected plant numbers and development. The number of plants per metre² in the 100 lb N seed placed treatment was one third of the sidebanded treatment. The Agrotain helped the numbers somewhat in that the number of plants per metre² in the seed placed Agrotain treatment increased from one third to one half the number of plants in the side banded treatment.

The damage caused by the high rates of seed placed fertilizer carried through to harvest. As the graph below illustrates, significant drops in yield occurred at the 100 lb N rate. The side banded N treatment yielded 31.2 bu/ac while the seed placed fertilizer with Agrotain yielded 28 bu/ac and the seed placed N dropped to 19.8 bu/ac.

At high rates of seed placed N, we expect to see fertilizer damage, especially when the seed and fertilizer are in such a narrow band as created by the knife. What was surprising from this trial was that it took excessive rates of N before that damage was significant. The excellent soil moisture most likely explains this at the time of seeding and the naturally high fertility of the soil in the Canora area. If moisture had been limited, especially during seeding, or the trial had taken place at a location where the organic matter content of the soil hadn't been so high, we would most likely see damage at lower rates of N and ultimately, lower yields. And at sites where the soil fertility was low, the 30 lb N/ac rate didn't yield anywhere near the higher rates, as there just wasn't enough N available to push it through to harvest.

Eric Oliver, Conservation Agrologist with SSCA, observed that if the Agrotain sits in very dry soil for a week or two, it will volatalize leaving the pure urea behind. This condition occurred at both Kindersley and Clavet. For this reason, Agrotain hasn't yet been recommended for use on canola, as the canola isn't nearly as forgiving as wheat.

For single pass seeding operations, double shooting is the ideal. If the seeding equipment you have won't enable you to do that, you must decide if you're going to move to two passes, one for banding the fertilizer, the other for seeding, or if you're going to single shoot. If you're single shooting, the narrower the seed row, the higher the risk of fertilizer damage to germinating seedlings. As seed placed fertilizer rates increase, so does the risk of damage. An option may be to obtain urea fertilizer coated with Agrotain. The Agrotain won't completely prevent seedling damage but it appears that it will help to minimize the damage.

Some of the information for this article was provided by Dr. Cynthia Grant of the AAFC Brandon Research Centre.