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Is My Fertilizer Still There?

Posted Aug. 1, 2006

One of the most common inquiries I get around this time of year in very dry years goes like this: "I put out fertilizer on my pasture this spring, and it hasn't rained much. I've had very little grass growth. Is the fertilizer still there? If so, how long will it stay there?"

The quick answer is it's probably still there. The more informative answer is a bit more involved, but I'll try to explain it as simply as possible.

If you used phosphorus (P) or potassium (K), they're still there. Phosphorus is lost only through crop removal or erosion. If you didn't have enough rainfall for crop growth, it stands to reason that you didn't have enough for erosion to occur. Potassium losses are much like phosphorus losses, except that K can also be lost to leaching in very sandy soils. However, since rainfall has been very limited, it's unlikely you've had leaching losses of potassium. If you applied P and/or K this spring under the conditions listed above, you almost certainly still have them in the soil, although the P may be less available to plants if your soil pH is either highly acidic or basic.

The amount of nitrogen left is a more difficult question. If you used urea on the soil surface and it did not rain for five days or so, you probably lost some of it to the atmosphere. How much you lost depends on the soil moisture, temperature, wind speed and soil pH when you applied the urea, as well as how long after application it rained. You lost more of the urea if you put it on wet ground, it was very hot and windy, your soil pH is over 7.9, and it was more than a week after application before it rained. If it rained within three to five days after urea application, volatilization losses were minimal. If you had the worst-case conditions listed before and it did not rain for two weeks, you could have lost 30 to 40 percent of the urea applied. If you used a different nitrogen source, like ammonium nitrate or ammonium sulfate, losses to the atmosphere were minimal.

How long will the nitrogen fertilizer stay around? The short answer is until it rains enough for crop removal or for losses to occur from very heavy rainfall. After it's incorporated into the soil through tillage, irrigation or rainfall, nitrogen can be lost through crop removal, leaching or denitrification. Crop removal takes rainfall to make the plants grow. Leaching and denitrification require heavy rainfall for nitrogen to be lost. Until these factors occur, the nitrogen will still be there.

If you want to take a soil sample to determine how much residual nitrogen is present, take 0- to 6-inch and 6- to 12-inch deep samples. You may need dynamite and a pick to get the 6- to 12-inch sample if you are as dry as we are.

I am writing this article on June 29. I hope by the time you read it, rainfall will be abundant and the points in this article will be moot. If it's still this dry... best wishes and good luck.