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.