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SDSU:?Soil sampling after the drought is essential

By Staff | Nov 11, 2012

BROOKINGS, S.D. (SDSU) – Because of the drought this growing season, soil sampling after harvest becomes important said Ron Gelderman, a South Dakota State University soils Extension specialist.

“Soil sampling should be part of any nutrient management program,” Gelderman said, “but is even more important after a dry year with limited yields.

“For those fields that were severely moisture stressed, available nitrogen carryover may be higher than normal.”

Gelderman said all growers should take 2-foot soil samples throughout their fields and have them analyzed for nitrates; especially for those fields going into a non-legume crop.

“If the rotation hasn’t yet been set,” he said, “sample and analyze as if it will be a non-legume crop.

“As the old adage goes ‘It is better to have it and not need it, than to need it and not have it.’

The additional sampling cost is inconsequential compared to potential fertilizer savings or yield gain,” he said.

Gelderman said that past drought years have shown higher than average carryover levels.

For example, the average carryover nitrate level following corn is about 70 pounds per acre. In a dry year, he said it would not be unusual to measure 100 to 120 pounds per acre of carryover N after poor yields on some of these moisture stressed fields.

“That is a difference of 30 to 50 pounds or about $18 to $30 an acre in savings with today’s N prices,” he said. “Some laboratories have been reporting average carryover N values of 20 pounds an acre higher than average for this fall.”

However, Gelderman said this is not always the case. Which is why testing is a must this fall.

“One of our moisture-stressed nitrogen rate trials on corn near Beresford had a near average carryover levels. In that case, if the grower guessed at carryover levels of 30 pounds an acre more than average, yield might very well be 5 to 10 bushel an acre lower than would be with the proper test and recommendation.

“The point is we cannot predict what the carryover levels will be. Therefore, every field should be tested.”

Since within many fields there could be high carryover N variability, Gelderman said, that may reflect the high yield variability due to differential soil moisture within that field, Gelderman recommends zone sampling.

“A zone sampling program based on yield zones may show some large nitrogen fertilizer savings for next year,” he said, ” and will put nitrogen where it is needed and not oversupply other areas of the field where it is not.”

Given the poor yields in some fields, there would be less phosphorus and potassium removed with the grain as well. However, Gelderman said measuring the availability of carryover P and K is more difficult than for nitrogen.

“The soil P and K test may have increased slightly (due to less removed with the lower yields), but yield, tillage, residue removed, soils, precipitation and temperatures all can influence how much of these nutrients become available for next year’s crop.

“It is best to follow soil test guidelines for those nutrients and not give a credit for any unused nutrients.”

In a dry fall, he said it is not unusual to find K testing even lower than in a normal year.

That’s because, Gelderman said, the low rainfall after harvest has not moved the K from the plant residue into the soil.

Potassium will move quite readily with water while plant N and P are tied up with organic compounds and will depend on microbial decomposition become they become available.

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