Precision Soil Testing

Growers should rely on grids less, collect samples more often.

Despite the difficulty in sampling for phosphorus, there are steps producers can take to be more precise (i.e., yearly sampling), Image by Jim Patrico

Modern variable-rate technology (VRT) gives growers the ability to apply nutrients according to prescriptions determined by soil samples. However, there is an ongoing conversation among agronomists about how well correlated sampling methods are to actual phosphorus (P) levels in the soil.

Two soil scientists involved in that “conversation” are Josh McGrath, associate Extension professor of soil science at the University of Kentucky, and Brian Arnall, precision nutrient-management specialist at Oklahoma State University. Both men point to research demonstrating interpolation of grid soil sampling, where the common resolution of 1 acre to 2.5 acres is too imprecise to measure soil phosphorus for an accurate prescription. Interpolation is an estimation of value between two known points.

“Our ability to deliver phosphorus through VRT far outstrips our ability to cost-effectively sample for the element and accurately predict phosphorus levels between sample points,” McGrath explains. “Given current sampling and interpolation technology, unless we pull samples at a quarter-acre (or less) rate, research shows we’re better off combining the grid soil samples for a field average to develop our recommendations.”

Citing comprehensive 1999 Oklahoma State studies of variations of soil-test values, Arnall says researchers found significant variation of both mobile and immobile plant nutrients within a meter of soil-sample sites.

Outdated Tools. “Essentially, the spatial variance in soil-test values is greater than our ability to treat,” he explains. “That variability is the reason for discrepancies in lab recommendations.”

McGrath and Arnall say new sampling procedures are necessary to match the need for precision application of nutrients and the capabilities of modern precision agriculture systems. Some of those procedures may be suitable only for research, but both agree current tools are outdated.

Despite the lack of precision in current soil-sampling methods and interpretation of lab results for immobile nutrients such as phosphorus, the soil scientists say there are steps growers can take to improve their precision with plant nutrients without the expense of sampling at quarter-acre resolution.

McGrath says because interpolated soil-sample maps of greater-than quarter-acre grids are unreliable at best, growers should use the money they’re spending on grid sampling to sample more frequently.

“Sample every year, and shift the grid,” he explains. “Also, it might help to intensively sample zones determined by layering yield or other field information such as soil texture, slope, aspect, elevation, drainage, etc., over soil analyses maps.” The goal is to find zones in the field that respond similarly to phosphorus.

Positive Return? “Still, the problem is, we should take those samples and put them in the same bucket. Even with a decent soil-test map (grid or zone), our recommendations are very coarse and were intended to be an average,” he explains. “I tell growers we have to live with what we have, so they need to be assessing what they are doing on their own farms to determine if there is a positive return on investment for their phosphorus application regimen.”

McGrath suggests growers start with the “old-school, flat-rate approach” and then run four strips to eliminate phosphorus as a limiting factor. Then, compare variable-rate applications with the flat-rate method, and check out which is more profitable at the end of harvest.

Arnall is building a study with cooperators across the Southern Plains. He hopes it will someday include 500 farms’ grid-sampled soil analyses for further study of the problem. He agrees with McGrath that growers should layer field and yield information over grid-sample maps to create identifiable zones within fields.

“By doing more soil samples within each of these zones, growers can estimate a base line of available P and adopt a sufficiency program in their particular crop,” he says. Sufficiency fertility programs are intended to apply just enough P to maximize yield for the year of application. They do not take into account crop removal.

“Building to that sufficiency level might be a four- to eight-year project,” Arnall explains. “Once that level is attained, however, growers can use yield data to estimate P removal rates and combine that with annual soil-sample analysis to maintain sufficiency.”

Arnall says such a sufficiency program is not intended to provide optimum economic returns in a given year. Instead, it is offered as a way to minimize the probability that phosphorus will limit crop yields while providing for near-maximum yield potential.