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Residual Nitrogen Could Lower Fertilizer Bills

Posted Sep. 11, 2006

Soil testing pays, and I am certain you've heard this statement from one of Noble's soil and crops specialists before. Well, if you are going to establish winter pasture this fall, the statement may be especially true. We already have begun seeing soil test results with high residual nitrogen. These levels have been high enough at times to cause us to not recommend nitrogen fertilization for fall/winter pasture forage production.

Consider the savings this fall if you invested the time and energy in soil sampling. Let's assume you did not soil-test and applied 80 pounds of nitrogen per acre around planting time using ammonium nitrate (34-0-0) at $315 per ton (46 cents per unit nitrogen) to support fall forage production. The cost of the nitrogen fertilizer would be $36.80 per acre. If residual nitrogen was present, how would you know without a soil test? You might find 30 or more pounds of residual nitrogen per acre by soil testing. Now, let's assume the soil test indicates there are 40 pounds of residual nitrogen per acre. You have cut your fall fertilizer bill in half, saving $18.40 per acre simply by soil testing.

You might be wondering where this carry-over nitrogen came from. Well, it is a result of the drought, which started last fall in most areas. As a result, there was little or no winter pasture forage production last fall or winter, and the unusually dry spring and summer has limited weed growth through the fallow period. Since nothing has been growing to consume the nitrogen and there has not been adequate rainfall to leach nutrients from the soil profile, some of the applied nitrogen fertilizer is still plant-available.

It is important to note that even if your soil test indicates a good amount of residual nitrogen, there may still be a need to fertilize. If, after emergence, a stand shows nitrogen deficiency symptoms (yellowing of the lower leaves), apply a minimum of 50 pounds of nitrogen (i.e., 150 pounds 34-0-0) per acre to correct the problem.

Thus far, I have only mentioned carry-over nitrogen associated with winter pasture production. But what about carry-over nitrogen in bermudagrass pasture? Well, it depends on the amount of nitrogen applied and the amount of bermudagrass forage production after fertilization. Nitrogen is directly related to yield, and most people experienced 30 to 50 percent of normal yields so far. If you were one of the lucky ones who actually received some timely rains, it is likely most of the applied nitrogen was used and carry over will be minimal. However, most have not had sufficient rainfall, and, as a result, much of the applied nitrogen will carry over for uptake this fall.

If you did not fertilize your bermudagrass this season or feel you had adequate forage production which will limit carry-over nitrogen, take advantage of the early fall rains by fertilizing your bermudagrass pastures. In September, apply 50 pounds of actual nitrogen per acre using ammonium nitrate. Do not expect great yields this fall, but some forage may be accumulated for grazing to ease the feed bill in late fall to early winter. Most importantly, fall fertilization with decent rainfall will assist in drought recovery by regaining some plant vigor, leading to healthier stands with better productivity next spring.

One last thought regarding bermudagrass pasture and the drought: The loss of vigor in bermudagrass going into the late fall and winter will allow cool-season annual grasses such as ryegrass and Texas wintergrass to take over. These cool-season annuals may be a blessing to some early next spring by providing grazeable forage. However, come May, too much can suppress bermudagrass production by competing for water, nutrients and sunlight. To promote bermudagrass production, remove cool-season annual grasses by either grazing or haying by mid-April.

Should you have any questions regarding fall fertilizer management, contact a Noble soil and crops specialist at (580) 224-6500.