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This year's drought will affect next year's fertility management

By Jean Caspers-Simmet

Date Modified: 10/10/2012 3:45 PM

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KANAWHA, Iowa — The drought will affect next year's crop fertility decisions.

Iowa State University Extension field agronomist John Holmes offered up some things to consider at last week's field day.

Corn removes 0.4 pounds of phosphorous per bushel and 0.3 pounds of potassium per bushel and soybeans remove 0.8 pounds of P and 1.5 pounds of K, Holmes said.

Farmers should compare the removal rates with 2012 crop yields to calculate what is left.

"Phosphorus and potassium stay in the soil so you can use less next year," Holmes said. "The soil test is the real way to do that, but the real kicker is that the dry weather conditions do affect the soil test."

Soil test phosphorus won't likely be affected by dry soil conditions, but soil test potassium results may be lower than they would be with normal conditions due to less recycling to the soil and less replenishment of soluble soil potassium.

"This time of year, the potassium is in the stalk and it needs rain to leach out so the soil test may not be reliable if you sample after harvest and it doesn't rain," Holmes said.

Soil pH test results may be a little more acidic than in normal conditions, Holmes said.

Corn stover and soybean straw baled right after harvest will remove phosphorus and potassium from the soil .Nitrogen is applied as anhydrous or 28 percent or 32 percent, Holmes said. It starts out in the ammonium form and goes to nitrate. When it's in the nitrate form, it can leach. But to leach, rain is needed.

"In this case we don't have water moving through the profile and we also have organic N that's contributed throughout the season due to break down of organic mater so we probably have a lot of nitrate nitrogen in the soil," Holmes said. "If it rained a lot and saturated the soil it would denitrify or the nitrate could also leach out of the soil. Both those cases are there, but at this point there's a lot of nitrate in the soil. If you were so inclined, you could soil sample to two feet and get a pretty good reading on what soil nitrate is in the soil."

Next spring if it isn't raining a great deal, farmers could do a spring soil nitrate sample and get a good read on what's available, Holmes said.

"Here's a pearl of wisdom from the late Fred Blacker (who created the spring soil nitrate test)," Holmes said. "He said if your soil nitrate reading comes back high (30 to 35 parts per million) he would guarantee you there was enough nitrogen in the soil to grow a corn crop."

Holmes said if farmers are going to apply fall nitrogen, they should apply at the low end of the recommended rate and maybe even less.

Farmers also need to consider the potential for herbicide carryover, Holmes said. Talk to ag suppliers about potential for residual on products used this year.