Drought leaves fertilizer for winter wheat crop

Though benefits of a drought are at best a left-handed compliment, growers in North Carolina and Virginia may benefit from planting wheat behind drought plagued corn and soybeans to take advantage of fertilizer left in the soil.

In Virginia, Virginia Tech Small Grain Specialist Wade Thomason says late season drought has reduced corn yields to about 65 percent of early season expectations, or about 80 bushels per acre.

While growers can't recoup the loss of this valuable grain, they can benefit from about a third of the nitrogen, potassium and phosphorus used on these crops.

Low crop yields during a drought year mean significant amounts of unused nutrients may remain in the soil at the end of the growing season.

A number of corn acres will be planted to wheat this fall and many growers are interested in reducing fertilizer applications to wheat if carryover from the low-yielding corn crop is truly available, Thomason says.

If phosphorous or potassium was applied but not used because of lower than expected yields, it usually remains in the top few inches of soil. It will not be lost over the winter. Therefore, the unused portion can be credited against nutrient needs for next year's crops.

One method for estimating unused phosphorous and potassium is based on the ratio of the actual drought-year yield and the yield goal used to determine nutrient applications that year.

For example:

Drought year application = 75 lb./acre phosphate= 300 lb./acre potash.

Drought year yield goal equals 6 tons per acre, but actual yield equals 2 tons per acre.

Actual yield/yield goal = 2/6 = 1/3, therefore, 2/3 of drought-year application is unused.

Estimated carryover = 2/3 × 75 lb./acre = 50 lb./acre phosphate= 2/3 × 300 lb./acre = 200 lb./acre potash.

Comparison of the actual yield with the expected yield shows that the drought-year yields were one third of the goal. Under the assumption that nutrient removal is proportional to yield, approximately two-thirds of the phosphate and potash applied in the drought year was not used and likely will be available to the next crop.

A routine soil test is the best option for detecting carryover nutrients, especially if large amounts of fertilizer were applied. But since it takes a relatively large amount of fertilizer to change phosphate and potash soil test values, a single year's application may not be detectable. Therefore, if you have recent soil tests for your fields and applied the recommended rate of fertilizer to your corn, your soil test would still be valid for the wheat, Thomason contends.

One exception to this may be where you had a ‘Low’ soil test level, as a single application of fertilizer at the recommended rate may or may not be sufficient to raise the soil test level for the second crop. In this situation a second soil test would be desirable.

In general it is recommended you soil test every 2 to 3 years, with shorter intervals on sandy soils and longer intervals on heavy or clay soils. The soil samples should be taken to a depth of 6 inches or 8 inches for conventionally tilled soils and to about 2 inches to 4 inches for soils under conservation or no-tillage, he adds.

While phosphorus and potassium carryover from drought are relatively easy to calculate, nitrogen is a bit more tricky. Following a drought year, most nitrogen carryover exists as nitrate in the plant root zone. However, the possibility of over-winter loss of residual nitrate makes estimation of carryover more difficult than for phosphorous and potassium.

Nitrogen carryover is most likely when the drought year crop was corn or other non-legume crops. In North Carolina corn acreage jumped nearly 40 percent from 2006 to 2007. Despite the acreage increase, total yield is expected to be one percent to five percent percent below the 2006 crop. Most of this loss is attributed to drought, leaving thousands of acres in North Carolina and in other drought plagued Southeastern states in a position to have large nitrogen carryover.

Other factors contributing to nitrogen carryover are moderate to high nitrogen application and below average yields.

With corn prices at near record prices, most growers were inclined to invest in the crop, including moderate to high nitrogen application. For sure yields were below expected — by over 40 percent in North Carolina and near 35 percent in Virginia. Again, not good news for growers expecting to make a profit on corn, but good news when using corn land for winter wheat.

USDA guidelines for testing for residual nitrogen include using a preplant soil nitrate test to determine how much nitrate has remained in the soil until the next growing season. If laboratory analysis for nitrate is not possible, the Nitrate Quick Test Kit used for the pre-sidedress soil nitrate test for corn can be used. If nitrate in the top 6 inches of soil is greater than 30 parts per million (ppm), then no nitrogen is needed at planting.

If the soil nitrate test level is less than 30 ppm, apply 20 to 30 pounds of nitrogen. It is critically important for high yields to have nitrogen available for young wheat plants to develop fall tillers.

If samples are not delivered in one day, these should be air dried by spreading on clean paper for 24-48 hours. For optimum results, the soil testing lab should know if the grower used manure, or if the previous crop was a legume.

Because so many growers have gone to banding starter fertilizers in corn, to get an accurate picture of available nitrogen, samples should be taken between rows to avoid these starter fertilizer bands.

Many growers have expressed concerns about the drought increasing the need for lime. In fact, Virginia Tech Soil Scientist Mark Alley says drought conditions actually have little affect on soil pH, and if anything the drought may reduce the need for lime.

Alley says maintaining appropriate soil pH is crucial for maximizing the uptake of essential plant nutrients. Optimum plant uptake of most nutrients occurs at a soil pH near 6.2. If needed, based on soil test recommendations, lime should be applied at the appropriate rate prior to planting.

Heavy rainfall tends to leach out calcium, while under drought conditions the soil pH will not change greatly. Therefore, if anything, this year's drought will decrease the need for lime.

While saving money by utilizing available nitrogen left over from drought plagued crops is important, it is critical to know exactly what nitrogen levels are in the fall wheat crop. With so many growers having disappointing corn crops, it is expected that some will be tempted to skip or delay preplant nitrogen application on wheat.

Thomason says this is not a good idea because nitrogen stress early in the season will prevent adequate tillering and root development, which reduces overall yield potential.

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