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4 steps to soil testing

09/27/2013 @ 5:12pm

After you’re done with harvest, you may want to consider heading back out to the field and collecting soil samples. Dr. Robert Mullen, director of agronomy for PotashCorp, says soil testing provides an accurate way for farmers to assess their fertilizer needs.

Mullen recommends soil testing every two to four years. Considering the price of fertilizer, it’s important to make sure you have accurate data when it’s time to make fertilizer purchases. The key to soil testing is being consistent says Mullen, because if you aren’t, your results may be misleading and not representative of your farm. Below are four simple guidelines to follow when collecting your samples.

1) Use clean sample tools.
Not using clean tools can contaminate your soil samples. Mullen encourages using a plastic bucket instead of a metal one. “If we have a metal bucket that has some rust in it, or has some flaking, or we have a zinc-coated metal bucket out there, that’s going to contaminate that sample and give us bad information about that soil’s ability to supply nutrients,” says Mullen.

2) Sample at a consistent depth.

Nutrients vary at different depths, and Mullen’s recommendation is to collect samples between the 6-inch and 8-inch. depth. “Make sure you are consistent about the depth of that sample. You don’t want to be collecting 3-inch samples when your target is 8 inches,” says Mullen. Not staying at the desired depth will influence the fertilizer recommendation.

“If you’re collecting a lot of shallow samples, the information you’ll get back is going to say it has a high nutrient-supplying capacity,” says Mullen. “Conversely, if we sample too deeply, we’re going to get information that tells us our soil does not have much in the way of nutrient supplying capacity, because we’re diluting away from that main concentration,” he adds.

3) Collect enough cores.
You should collect 15 to 20 cores per sample area. “The total amount of soil that’s being collected may be 1.5 pounds. The weight of soil, over an acre to a depth of 6 inches, is about 2 million pounds,” says Mullen. Once you multiply 2 million pounds by 20 acres you end up with 40 million pounds of soil. “We’re representing 40 million pounds of soil with a pound and half that we’re putting in a plastic bucket and mixing. So it’s absolutely critical that we collect an adequate number of samples,” says Mullen. Not collecting enough cores will cause your results to be skewed.

4) Thoroughly mix cores.
“If that composite is not a good representative mixture of all the soils that we’ve collected, we can dramatically skew the results when it comes time for the analysis,” says Mullen.

Mullen recommends soil sampling in the fall, because, in general, there’s more time to collect the samples, run the analysis, and purchase fertilizer based on the test results in the fall than in the spring. However, if you choose to collect samples at a different time of year, he recommends being consistent in collecting at that particular time of year each time you sample.

If you decide to hire consultants to collect the samples, be sure to discuss the particular qualities of your fields with them. “No one knows a farmer's field better than the farmer,” says Mullen. “A farmer knows in his field where the bad areas are, where the crop is not performing as well, where there’s a knoll that doesn’t produce regardless of management practice. Those are probably not the correct areas that you want to include in that composite when you are representing a fairly large acreage.” While he doesn’t recommend including poor producing areas in the composite for the larger area, he does recommend taking samples from the poor areas separately to test for deficiencies.

Overall, Mullen says that consistency is the key to obtaining accurate samples. “Be consistent when you sample, and be consistent about when you collect,” he says.

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