Unlike nitrogen, phosphorus is highly immobile in soils, and subject to various transformations, depending on the chemical environment.

Use of RTK guidance systems not only helps farmers plant straighter rows and eliminate guess rows — it’s also allowing University of Arizona researchers gather precision information to help cotton producers reduce phosphorus (P) rates, do a better job of taking advantage of residual P and increase lint yields.

GPS WITH sub-meter accuracy not only allows for precise injection of phosphorus fertilizer under the seed line by a modified Telone injector, but it also allows researchers studying residual P levels to soil sample the area under the band year after year.

According to research, about 11 pounds of P is needed for every bale of cotton produced. The soil does have some supplying power for P, but many Arizona growers have to apply supplemental P due to high pH soils.

Unlike nitrogen, phosphorus is highly immobile in soils, and subject to various transformations, depending on the chemical environment.

Randy Norton, soil scientist at the Safford Agricultural Center, Safford, Ariz., noted, “On the high pH soils of Arizona, calcium and magnesium can take P out of solution and make it unavailable to the plant. Phosphorus is most available to the plant at a pH range of between 5.0 and 6.0. Most soil pH in Arizona can be as high as 8.5.”

To increase soil availability of P for cotton producers, soil scientists suggest placing it as close to the root zone as possible, concentrating the P in a band.

“Basically, we want to minimize soil contact with the P,” Norton says. “We do not recommend broadcast applications because you’re maximizing the soil contact.”

While timing for P applications is not nearly as critical as it is for nitrogen application due to P’s immobility, Norton says the best time to apply it is early “so that it’s there for proper root development.”

A 15-year Arizona study conducted by Norton, Jeff Silvertooth, agronomist at the University of Arizona, and others on cotton yield response to P showed that at soil test levels below 5 parts per million there is a good chance for a positive response to a P application. The likelihood of a positive response to a P application decreases above 5 parts per million.

A current study conducted by researchers on the high pH soils of the Safford Valley in the upper Gila River Valley, is looking to take research on banded P a step further, with the help of GPS technology.

Growers in the Safford Valley began to invest in GPS guidance systems in 2003, primarily for help in constructing straighter and more uniforms rows.

Recently, rising fertilizer prices have prompted researchers and growers to explore more efficient methods of P placement using GPS. The price of diammonium phosphate started to increase significantly in January 2007, reaching $1,200 per ton in the summer of 2008, before starting to fall.

GPS systems with real-time kinematic (RTK) differential, or sub-inch accuracy, allows growers “to put up rows, inject fertilizer directly under the row and then plant the seed right on top of the row, to within sub-inch accuracy year after year.”

The concept excited researchers because they could conduct tests on banded P, then return to the same band year after year to conduct studies on residual P.

“The idea was that we could go to lower rates or possibly apply P every other year,” Norton said.

Researchers at Safford Agricultural Center began their trial in 2007 on Pima clay loam soil.

Initial soil tests at the research center in the spring of 2007 indicated a pH of 8.5 with a P level of 4.5 parts per million, just below the level at which a response to P might be expected. Sodium levels were also high. Soil samples were pulled right under the seed line at a depth of 18 inches.

The studies compared side-dress and injected applications of P at three rates. Phosphorus was injected with a modified disk-hipper-Telone injector.

In 2007, researchers saw a significant response in lint yield, 135 pounds, for 60 pounds of injected P2O5, versus the control, where no P was applied. No differences in lint quality were observed, but researchers did see trends of higher strength for injected P.

There was also a significant response of 109 pounds to the side-dress application at 100 pounds of P2O5 per acre. Researchers also saw a trend for higher strength with side dress P.

In 2008, there was no response whatsoever to applied P across all treatments, which Norton said was likely due to residual P levels.

“When you’re right around 5 parts per million, there’s a good chance you won’t see a response, which is what happened in 2008.”

Residual P in the area under the seed line, sampled in the fall of 2007, confirmed this. Under 100 pounds of P2O5 injected, they found P in concentrations as high as 18 parts per million, compared to 7.4 for 60 pounds injected, 6.1 for 40 pounds injected, and 5.5 for the control.

In side-dressed P, residual P was also above 5 parts per million, but there were little differences in residual P among the application rates, ranging from 6.1 to 6.4 for P applied at 40, 60 and 100 pounds.

In the fall of 2008, samples were taken at two depths, 12 inches and 24 inches, however, composite samples were taken across all treatments rather than taking samples from each experimental unit.

“We saw some stranger results,” Norton said, “but we did see an increase in our injected rates and no significant increase in residual P in our side-dress application.”

Scientists did not repeat the experiment in 2009, but expect to resume studies in 2010.

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