When it comes to soil sampling, if you don’t do the sampling yourself, do you know where the samples are pulled from? According to Farm Progress, those who are unsure about how the process works, consider the example of the Soybean Watch ’24 field, which was divided into four distinct sampling areas: northwest, southwest, east, and hill. Each area represents a different soil type, confirmed through drilling soil profile cores by Dena Anderson, an NRCS soil scientist.

The samples from each area were analyzed at A&L Great Lakes Lab in Fort Wayne, Ind., while a composite sample from all four areas was also created, simulating a single sample for the entire field. Though some still use this method, it’s not recommended, as Dan Quinn, Purdue Extension corn specialist, explains that combining samples from different soil types can lead to missed insights and limit the effectiveness of variable-rate applications.

Quinn reviewed the soil samples from each area and the composite sample and found several key issues:

1. Low pH missed: Two areas had a pH of 6.2 or lower, but a composite sample showed a pH of 6.6, which appeared fine. This could lead to missing the need for lime in parts of the field.
2. High pH missed: The hill area had a pH of 7.2, which can affect nutrient availability. A pH above 7.0 can cause problems, and it would take a long time for fertilizers to lower it.
3. Low phosphorus: The hill area had very low phosphorus at 5 ppm, which is deficient. Applying phosphorus here would show immediate yield results.
4. Misleading phosphorus levels: In the composite sample, phosphorus appeared below the critical level at 13 ppm, while some areas had higher levels. This could lead to unnecessary fertilizer application.
5. Potassium opportunities missed: The hill area had very low potassium (76 ppm). Applying potash in these areas would help yields, but the composite sample missed this need.
6. Missed savings: Using only one sample could waste money. With multiple samples, you could apply fertilizer more efficiently, targeting areas where it’s needed most and saving on costs.

A slideshow of the 2024 Soybean Watch project is available on FarmProgress’s website.