The same 9-acre field that Nathan Bush used to demonstrate how to lay out soil sampling zones illustrated several key points once soil tests results came back. There was enough difference between the two zones to show what would have happened if only one sample had been pulled and sent to the lab for the entire field. Part of the field might have received more fertilizer than needed, and/or part of the field might have not received enough fertilizer, Bush says.
Bush is a soil consultant and agronomist for Greene Crop Consulting Inc., Franklin, Ind. He laid out a field for sampling (see Soil sampling requires judgment calls on this website), making judgment calls based on total number of samples desired and ability to manage the field. He pulled samples and sent results to Brookside Labs in Ohio.
Instead of sending just the two samples he pulled, Bush mixed the two together and sent a third sample. It represented what he might have pulled if he had sampled the entire 9-acre field as one unit and not split it into two management zones. See the chart below for lab results. Sample A is the higher organic matter soil with more drainage issues on the left side of the field map. Sample B is the somewhat lower organic matter soil on the right side of the map. “Mixed” is the combination of the two samples, representing one sample pulled from the field.
Soil sample results revealed that no parts of the field appear to need lime. All samples tested higher than 7.0, which means they’re not acidic soils.
“By testing the area with somewhat lighter soils separately, we discovered the pH is higher there, at 7.7,” Bush says, adding that pH is not a linear measurement. A difference from 7.1 to 7.7 is larger than it might seem.
“It’s getting high enough that it could affect availability of some nutrients, especially micronutrients in the soil,” he says. “Applying fertilizer would likely help bring it down over time. We wouldn’t have found it if we had just pulled one sample from the field.”
Looking at phosphorus levels, the 52 pounds per acre reading for Sample A is at the lower end of the acceptable range. If only one sample had been pulled and the result was 40 pounds per acre, you might have selected a higher fertilizer rate for the area represented by Sample A than you would have based on 52 pounds per acre. By applying a lower rate in the area represented by Sample A, you could save money on fertilizer, Bush says.
With only one sample, you also would have missed the fact that Sample B at 28 pounds per acre of P is considerably below the recommended level for phosphorus, Bush says. That area would be considered low for P. You would definitely expect a yield response for applying fertilizer there.
The story is similar for potassium. Comparing Sample B and the mixed sample, it’s clear that if only one sample was pulled, Bush might not have detected that soil represented by Sample B would be expected to respond to potash application.
It pays to sample areas that appear different separately, Bush concludes.