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Fertilizer strategies

It's no secret that fertilizer prices are increasing astronomically. At first, nitrogen got most of the attention. But over the past few months, potash and phosphate have really gotten into the act.

According to USDA figures for the north-central region, anhydrous ammonia (82%) that cost $368 per ton in 2003 had risen to $536 a ton by 2007. Last spring, it was $769 per ton, and some estimates predict $1,000 a ton in 2009.

Diammonium phosphate (DAP), commonly referred to as 18-46-0, cost $249 per ton in 2003. Last spring it cost $751 per ton and is also forecast to cost $1,000 per ton in 2009.

Potash (0-0-60) that cost $162 a ton in 2003 is expected to cost $900 per ton in 2009.

In mid-July, University of Illinois ag economist Gary Schnitkey estimated that fertilizer costs for corn in 2009 will be $215 per acre. "That's an increase of $97 per acre over the 2008 projected level of $118 per acre," he says.

He figures fertilizer for the 2009 soybean crop will cost $98 an acre, up $53 from the 2008 level of $45 an acre.

"Fertilizer is the input with the largest cost increase," he says.

Granted, grain prices are high, too, but fertilizer prices are changing so dramatically that many growers and researchers are taking a hard look at every facet of fertilizer management.

According to University of Nebraska agronomists, DAP was selling for $1,230 a ton in May. If you credit the 360 pounds of nitrogen in a ton of DAP at 60 cents per pound ($216), that computes to $1,014 for the phosphorus.

How do those prices affect fertilizer recommendations?

"With MAP (monoammonium phosphate) and DAP prices nearing or breaking $1,200 per ton," says agronomist Charles Shapiro, "the question can be asked, 'At what price are University of Nebraska phosphorus fertilizer recommendations not profitable?'"

For an answer, Shapiro and several colleagues looked at data collected statewide from irrigated corn sites in 2002-2004 from the Nebraska Soil Fertility Project. There were 34 site-years in the project with Bray P1 soil test values for phosphorus ranging from below 10 ppm to over 80 ppm. Treatments that received 40 pounds of P2O5 yielded, on average, 226 bushels per acre. The check plots (which received no phosphorus), averaged 223 bushels per acre.

There were 10 site-years where soil test levels were between 20 and 25 ppm. Yields where P2O5 was applied averaged 229 bushels compared to yields of 228 bushels where no P2O5 was applied.

"Clearly, there was no benefit to phosphorus fertilization at these soil test levels," says Shapiro. "A 40-pound phosphorus application will cost about $44 per acre. A nine-bushel-per-acre yield increase is needed to pay for the phosphorus application if corn price is $5 per bushel. On 19 sites that were under 20 ppm Bray P1 test level there was an average yield increase of six bushels per acre. Corn would have to be $7 per bushel to pay for this increase.

"However, at the 225-bushel-an-acre yield level, corn is removing about 80 pounds of P2O5 per acre per year," says Shapiro. "If unfertilized, soil test levels will gradually decrease."

Nebraska recommendations suggest applying four pounds of P2O5 for each Bray P1 point below 25 parts per million (ppm). Nebraska agronomists say this will avoid depleting soil phosphorus and maintain soil levels at or above 15 ppm.

One place that warrants a fresh look is the way application rates for P and K are determined. While some growers have routinely used soil tests to guide fertilizer rates, others base application rates on the amount of potassium and phosphorus removed by a crop.

John Lamb, a University of Minnesota Extension soil scientist, and George Rehm, a recently retired Minnesota soil scientist, strongly favor using soil tests rather than crop removal to determine P and K application rates.

They say the crop removal concept "is based on assumptions that are not valid."

"Emphasis on use of crop removal as a basis for fertilizer recommendations suggests that a user of crop removal has no confidence in soil testing as a predictive tool for fertilizer recommendations," says Lamb.

"Use of the crop removal concept assumes that the soil does not supply any of the nutrients needed for crop production," adds Rehm. "It also assumes that all of the nutrients applied in a fertilizer program are used by the growing crop. This, however, is not what happens when nutrients are applied to soil. Depending on the nutrient and the chemistry of the soil, utilization or uptake from various sources has ranged from 20% to 65% of that applied."

Multiyear trials of corn yields from fertilizer applications based on crop removal were compared to yields from applications based on soil tests at the Southern Research and Outreach Center at Waseca. (See Table 1.) The Bray P1 soil test at the beginning of the study was 13 ppm (a medium value), and the expected yield was 170 bushels per acre.

"In all years, corn yields were not significantly affected by treatment," says Lamb.

"With a Bray soil P1 test of 13 ppm at the beginning of the study, a response to broadcast phosphate would not be expected. None was measured."

At the Southwest Research and Outreach Center at Lamberton, the initial Bray P1 test was 8.8, which is low. In that situation, using crop removal to determine rates sometimes led to a rate that was too low for optimum yields.

It's no secret that fertilizer prices are increasing astronomically. At first, nitrogen got most of the attention. But over the past few months, potash and phosphate have really gotten into the act.

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