A three-year university research project launched this fall in farm cooperator fields in Southern California and southwestern Arizona will study preplant variable rate applications of phosphorus (P) in commercial winter vegetable production.

The findings could not only help vegetable growers more precisely apply P, reduce P use overall, and increase yields which combined could increase grower profitability. It could also help agriculture create a smaller environmental imprint and improve U.S. food security.

Project leader Charles Sanchez of the University of Arizona (UA) says, “The successful implementation of this project will provide positive economic impacts for growers, reduced environmental impacts on water quality in the region, and enhance food security by using a finite and geopolitical resource (P) more efficiently.”

Phosphorus (P) is a key macro nutrient required in crop production, along with nitrogen and potassium.

The UA project is funded by a $147,000 grant from the Fertilizer Research and Education Program. FREP is financed through fertilizer purchases and administered by the California Department of Food and Agriculture.

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Sanchez, a soil scientist, is based at the Maricopa Agricultural Center (MAC) in Maricopa, along with fellow project researchers Pedro Andrade, a precision agriculture specialist; and research specialist John Heun.

The research team also includes Kurt Nolte, Yuma County Cooperative Extension director based in Yuma.

The field research will be conducted in low desert vegetable production areas, including California’s Coachella Valley in a broccoli field; and in California’s Imperial Valley and Arizona’s Yuma County in iceberg lettuce fields.

About 90 percent of the nation’s winter supply of vegetables is grown in this mild winter weather region.

Each year, most winter vegetable fields require up to 550 pounds of monoammonium phosphate (MAP) which includes about 20 percent P. The fertilizer is knifed into the soil at preplant. The same amount of P is usually applied across the field whether specific areas need this amount or not.

“This project is about joining science and precision agriculture technology teaming up to determine which field areas need more or less phosphorus, and then applying only the needed amounts,” Sanchez says.

The project will utilize GPS-enabled soil electrical conductivity sensors to determine actual needs, and then create prescription maps to precisely apply the mineral.

Three steps

There are three steps to the process. First, soil apparent electrical conductivity (ECa) surveys will determine zones with textural differences. Two systems will be tested.

The Veris 3100 system from Veris Technologies, which includes electronic circuitry connected to soil-engaging coulters will be pulled through the field behind a tractor. The sensor controller measures voltage drop to create detailed maps of ECa which is closely related to soil texture variability in the crop rooting zone.

The other survey method is the EM38 unit from Geonics. The unit will be pulled on top of the ground by a tractor. As it moves, the non-contact sensor generates soil ECa data based on the principle of electromagnetic induction.

Weighing these two systems is important to determine which one could be a better fit for growers and crop advisers. While the EM38 system is a research-grade instrument, it may provide a more accurate snapshot of P needs. Sanchez and Andrade have shown the Veris system provides an adequate level of accuracy and is a field-ready sensor that is simpler to use.

Preliminary work conducted by the research team suggests that Veris and EM surveys could cost about $17 an acre. Surveys would not be required every year so an amortization over 10 years would total about $1.70 an acre.

The second step will compare sampling techniques. Soil samples will be taken using grid- and zone-based sampling techniques and evaluate the economic outcomes of each.

“Grid sampling is a simple procedure but it is very labor intensive and costly,” Andrade said. “Zone sampling requires a minimal number of samples per acre. It is more efficient since it is guided by the natural variation of soil types in the field.”

After the soil sample laboratory analyses, a mathematical algorithm-based prescription map of the actual P amounts in the soil would be created.

The third step will include loading the prescription map into precision ag equipment mounted inside the tractor and interfaced with a variable speed hydraulic drive on the implement side. A Trimble GPS FMX system, including auto-steer and variable rate application software, will trigger other equipment pulled on an implement to inject P into the soil according to the prescription map.

In the Coachella Valley trial, liquid P will be applied. Granular P will be applied at the other sites. P is usually applied granularly in vegetable production in the low desert.

Following the P application, the cooperators will plant and manage the crop as usual through the growing season.

RFID and yield

During harvest, Kurt Nolte will utilize a radio frequency identification system (RFID) he helped develop for food safety and traceability in vegetables. RFID will allow Nolte to determine precise yields in the field and then tied the information back to the actual P applications.

The UA team will assess the season-long data to determine the impact of the site specific P applications.

The bottom line, Sanchez says - “A practice must be economically viable before a grower will use a new practice,” Sanchez said. “My hypothetical calculations suggest that variable phosphorus applications can economically benefit the grower.”

Next year, the program will be tweaked and then expanded in the fall on additional farms. In 2015, the program would be fine tuned further with Extension outreach to share the results with the vegetable industry.

Once the project is completed, the findings would be turned over to the private sector to further tweak the program and possibly make it commercially available to the vegetable industry.

Site-specific fertilization is currently used in the Midwest in agronomic crops, including corn and soybeans. While growers there have not seen much of a yield benefit, Sanchez says consistent fertilizer savings alone have increased economic returns to growers.

“In lettuce, which is sensitive to phosphorus, I believe we’ll see an increase in yield in addition to the cost savings from reduced phosphorus use,” Sanchez said.

Andrade added, “I think the variable rate system could be economically viable to growers since it is an un-lockable extension of auto-steer technology which many growers already use.”

Not only are cost savings to the grower important. Fewer P applications could have a broader societal and environmental impact. Unused P by crops or tied up in the soil can be transported downstream into bodies of water.

Sanchez summarized, “The need to improve P use efficiency in agricultural production systems is urgent, especially in high value crops which receive large amounts of P fertilizer for optimal yield and quality.”

This makes site specific P application in vegetables even more important.

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