Who samples your soils — an agronomic consultant or your retail fertilizer dealer? If you don’t pull samples yourself, do you know where they are pulled? If you aren’t sure how sampling is done, perhaps this example will convince you to pay more attention to the details behind soil testing results.

The Soybean Watch ’24 field, comprising 50 acres, was split into four sampling areas after harvest, designated northwest, southwest, east and hill. Each represents a different soil type. This was confirmed when Dena Anderson, an NRCS soil scientist, drilled soil profile cores.

Soil samples were pulled from the four sampling areas and sent to A&L Great Lakes Lab in Fort Wayne, Ind., for analysis. At the same time, soil from all four areas was mixed to form a composite sample. This simulated taking one sample for the entire field. It’s not a recommended practice, but some samples are still pulled this way today.

SOIL SAMPLE LAYOUT: This layout of the Soybean Watch ’24 field shows the four sampling areas: northwest, southwest, east and hill. (Google Earth/Getty Images/Lisa Lynd)

Single sample failure

Dan Quinn, Purdue Extension corn specialist, explains why pulling one sample for a large acreage isn’t a smart move. “You will miss key facts you could pick up by sampling different soil types separately,” he says.

“You could not utilize variable-rate application to only put inputs where they have the best chance of returning an economic return quickly.”

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Quinn studied the soil sample results from each sampling area and the composite sample. Here are key takeaways:

1. Failure to pick up lower pH. Two areas are 6.2 or lower, but if you only had results from whole-field sampling, you would not know that.

“Half the field is at the point to consider liming,” Quinn says. “Yet at a pH of 6.6 for the composite sample, it looks like you are in good shape.”

2. Failure to find high pH. Soil pH is 7.2 in the sample pulled from the hill area, which was badly eroded.

“You can start having problems with availability of some nutrients if pH is too high,” Quinn says. “Our goal is 6.5 for corn and soybeans. We get concerned at pH levels above 7.0.

“Acidic fertilizers with nitrogen and/or sulfur may work the pH down over time. But we’re talking a long time for that to happen.”

3. Failure to detect super-low phosphorus value. At 5 parts per million on the Bray P1 scale, or 10 pounds per acre, phosphorus is clearly deficient in the hill section.

“The critical level where applying phosphorus should pay off quickly is 15 parts per million by the Tri-State Recommendations, often used in the eastern Corn Belt,” Quinn says. “You should see immediate yield results for applying phosphate in this area.”

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However, you only know it is so low if you sampled by soil type. Also, note that if your lab uses Mehlich extractions for P, the Tri-State critical level will be 20 ppm.

4. Misleading implications for phosphorus. Based on the southwest sample, the P level is right on the line of where fertilizer would pay immediately. In the northwest and east sampling areas, it is slightly above, at 17 ppm. But in the composite sample, at 13 ppm, it is below Bray P1 critical level.

“So, if you only have composite sample results, you will apply phosphate on the entire field,” Quinn says. “It certainly won’t hurt agronomically, but if you’re trying to just put on maintenance amounts, basically crop removal, to hold down costs, you would miss an opportunity. You would not know where levels were higher or lower.”

5. Missed potassium opportunities. The situation is similar for potassium, Quinn observes. For K, 100 ppm is the Tri-State critical level, or 200 pounds per acre. The east sample level at 112 ppm could likely go another year if you’re shaving budgets, but in all other areas, you should see a yield response for applying potash now.

“Potassium is really deficient on the hill, at 76 ppm,” Quinn says. “With 90 in the composite sample, at least apply maintenance levels of potash. If crop prices were more favorable, you might consider buildup applications of P and K.”

6. Missed opportunity to save money. “The biggest issue with basing your decision on just one sample is missing the opportunity to invest limited dollars where they could help most now,” Quinn explains. “Ideally, based on four samples, you would variable-rate lime, applying in the northwest and southwest areas, but certainly not on the hill nor east section.

“You could also variable-rate P and K, putting maintenance amounts where needed most, and perhaps even applying a buildup rate of phosphate on the hill.”