Intensive soil sampling and variable-rate application of phosphorus and potassium improve nutrient management. But it usually isn't profitable in the short term, according to multi-year, on-farm testing by Iowa State University (ISU) soil scientist Antonio Mallarino.

Nevertheless, soil sampling in grids of about 2.5 acres, followed by variable-rate fertilizer application, does have a place under certain circumstances, he says.

Mallarino and associates compared variable-rate strips to fixed-rate strips. For variable-rate strips, they soil sampled in research grids of 1/5-¾ acre. They then applied P or K in variable-rate strips as called for by the sampling.

In the fixed-rate strips they applied a uniform rate of P or K.

The research began in 1996 and has continued, with corn and soybeans rotated each year since then in each test field.

“In most fields there has been no yield difference between the fixed and variable-rate applications of either P or K,” Mallarino reports. “The exceptions were two fields for P and one for K where the variable rate produced a slightly higher yield than the fixed-rate method.

“Except for those fields, the average amount of P or K fertilizer applied was lower with the variable-rate method.”

Yet even when less fertilizer was applied or yields were higher with the variable-rate method, costs exceeded benefits. Lime applied in variable rates was profitable, however, because it isn't applied as often as P and K and savings in amounts applied can be significant.

“It's fairly obvious from what we've seen that the current costs of variable-rate P and K application would offset any yield advantage and savings in fertilizer,” Mallarino notes. “When you add in the extra expense of intensive grid sampling, it may become unprofitable.”

However, the ISU research shows that variable-rate fertilization tends, over time, to reduce soil-test variability. It also reduces fertilizer applications to high-testing areas, which allows for more efficient maintenance fertilization.

“In addition,” he notes, “variable-rate application of P or manure can reduce potential P runoff to surface water.”

Nationally, variable-rate P and K has generally not been profitable with bulk commodities such as corn, soybeans and wheat, reports Jess Lowenberg-DeBoer, Purdue University ag economist. It tends to pay on high-value crops — seed fields, sugar beets, potatoes, etc.

“Variable-rate P and K are more likely to be profitable as part of an integrated site-specific management system that allows costs to be spread over a range of inputs,” Lowenberg-DeBoer says. “For example, applying variable-rate lime often more than covers soil sampling costs. Then variable-rate P and K would only need to cover variable-rate application costs.”

He says variable-rate P and K also are more likely to pay if soil tests are taken primarily from areas where nutrient levels are known to be low and a yield response can be expected. Where levels are high, even though variable, a yield response is not likely.

Yet Nyle Wollenhaupt, a soil scientist, says variable-rate nutrient applications and improved nutrient management are just part of the story. “Soil and fertility data are critical to making improved pH, tillage, planting, seed selection, drainage, crop chemical and other field management decisions,” says Wollenhaupt, affiliated with SOILTEQ, an AGCO division manufacturing variable-rate application gear.

Soil and fertility baseline information shows compliance with environmental regulations, he notes.

“Our efforts, as we move forward, should focus on developing new strategies and technologies to gather soil and fertility data more efficiently and cost-effectively, and to better identify how this data can benefit the entire farming operation to achieve increased profitability.”

From a different approach, independent crop consultant Phil Cochran, Paris, IL, has been applying variable rates of potash and lime since 1982. But his costs don't outweigh the benefits.

Cochran generally samples by soil type (management zones). He maps fields and the areas that get more or less of a nutrient. The applicator then uses his map to apply the fertilizer or lime.

Cochran's clients save more than enough in fertilizer costs to pay his fee. They find that their fertilizer costs are below average for their farm size. “Yields are average or slightly above average for their farm size,” he says.

Mallarino says reducing intensive sampling costs is key to making variable-rate fertilization cost-effective. He suggests two ways:

  • Space sampling over four to six years. It's less precise but workable. In non-test years, use yield maps to apply P and K based on removal.

  • Take soil samples every two to four years by management zone.

“A combination of these procedures — an initial intensive grid soil sampling followed by management zone sampling — is another possibility,” Mallarino points out.