Published: November 20, 2007 02:53 pm            

Drought in 2007 creates fertilizer questions for ’08

Originally published in the November 16, 2007, print edition.

Since many areas of Minnesota experienced drought conditions this season, the leading question for most corn farmers going into next year is what to do about nitrogen management. The cost of fertilizer, particularly the price of nitrogen, keeps crowding into the decision-making process more and more.

A farmer is led to ask, “if we expected a 180-bushel crop but harvested only a 120-bushel crop, what happened to the nitrogen that didn’t get used?”

“Maximum nitrogen uptake generally happens before corn tassels,” said John Lamb, a University of Minnesota Extension soils scientist. “For many drought areas, N uptake stopped even earlier. August rains came along generating a flush of nitrogen mineralized out of the organic matter. So there likely are some nitrogen reserves in your soil. But if you’re going back into corn, you need to know nitrate nitrogen levels in your soil. That simply means soil test.”

Thanks to those August rains, the mineralization process also produced nitrate-N in soybean fields which continued to feed off that source right up to maturity. So if drought didn’t kill your soybeans, there may be little residual nitrate-N in your soybean fields this fall, Lamb said.

For a corn-on-corn program, Lamb gets specific. “Start with a nitrate test this fall after soil temperatures drop below 50 degrees Fahrenheit and use that as a guide to your corn-on-corn fertility program for 2008.”

He emphasized the importance of waiting until soil temps in the top 6 inches are 50 F or lower to get the most accurate amount of residual soil nitrate-N. This also means delaying nitrogen application until soil temps are less than 50 F.

The cooler the soil temperature, the less likely the fall-applied ammonia will convert to nitrate, and be lost into your tile system, or simply denitrified and unavailable to your growing crop next spring.

“It boils down to seeing exactly what that residual factor is and using that as a credit with additional N needed for your total program,” said Lamb.

Better genetics lessening N need?

Lamb said newer genetics showing up in new corn hybrids are suggesting the corn plant is becoming a more efficient user of nitrogen. “We used to talk of 1.2 pounds N per bushel of anticipated yield as your guide. Now we’re seeing as little as 0.6 pounds N per bushel is generating maximum yields. So our new N recommendation isn’t based on yield potential nearly as much as it used to be.

“Now we talk in terms of soil productivity. Factors affecting soil productivity include: degree of erosion, soil texture, is the rooting depth impeded and climate of the area where the crop is going, like cold season and short season versus warmer, longer-season areas.”

Lamb also suggested that new genetics such as Roundup Ready resistance and the new corn rootworm-resistant strains have lessened crop stress in the field and thus increased N use efficiency. “In essence you don’t have weeds using up N, and with less overall stress on the corn plant, it simply can function more efficiently on all of its intakes.”

Corn yield trials at the Southern Outreach and Research Center in Waseca show declining corn yields in a continuous corn program. Some producers, however, say that after the second-year yield reduction, yield loss levels off. “Some of that may be because sharp growers learn how to better manage their residue,” Lamb said.

Though not professing to be a tillage expert, he indicated an even distribution of trash residue right out of the combine is key. Come spring, some light tillage to further break up residue, followed by trash whippers on your planter, is his advice for a corn-on-corn program.

Recharge of soil moisture is key

Vital to rebuilding 2008 yields in the 2007 droughty areas of Minnesota is a generous recharge of soil moistures, including recharges of subsoils down to 8-foot to 10-foot depths.

“If subsoil moisture levels are still running low next spring, you may want to reevaluate both your fertility program and your planting schedules,” Lamb said.

He tends to agree with Dale Hicks, a long-time Extension agronomist, who recently predicted that Minnesota state corn yield averages could be 200 bushels within eight to 10 years.

“Genetics continue to advance productivity,” said Lamb. “Top growers are learning more about the key issues of soil management, especially as it relates to residue and supplemental fertilizer. And yield trend lines of recent years certainly point to that 200-bushel eventuality on a state-wide basis.”

“Six or seven years ago I would have laughed at that notion, but this year we’ll be disappointed with less than 170 bushel yields. Yet just two years ago Minnesota averaged 174 bushels per acre, one bushel more than Iowa.”

Lamb said determining an appropriate N rate is not easy because of the transient nature of N in soils. In western Minnesota, these guidelines of soil productivity, previous crop, ratio of N price to corn price and overall risk assessment also need to be adjusted to residual nitrate-nitrogen measured by fall soil tests.

With the current volatility in energy costs, fertilizer N cost has risen dramatically. That’s why the ratio of the price of N per pound to the value of a bushel of corn should now be part of your N rate decision. If N fertilizer costs $0.30 per pound N or $492 per ton of anhydrous ammonia, and corn is valued at $3, the ratio would be $0.30/$3 = 0.10.

This new price-value ratio gives each producer the flexibility of choosing N rates based on maximum yield versus acceptable yield ranges. The terminology MRTN refers to maximum return to N value and is shown in the accompanying table.

Micronutrient needs

Research trials throughout Minnesota indicate zinc is the only micronutrient that may be needed for corn. The soil test of Zn is reliable and will accurately predict the needs for this essential nutrient; Zn is not needed on all fields, however, and recommended rates will vary from 2 to 10 pounds per acre depending upon soil test and broadcast versus band applications.

The use of iron, copper, manganese and boron is not suggested for corn fertilizer programs in Minnesota. Highly recommended for your fertilizer planning is an Extension publication FO-3790-C, “Fertilizing Corn in Minnesota.” Check your local Extension office, log on to www.soils.umn.edu/extension/extension_publications or call (800) 876-8636.

        

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