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When a Soil Test Lies

March 28, 2009
By: Sara Schafer, Farm Journal Media Business and Crops Editor

Crop consultant Del Glanzer is known for pestering his clients about proper soil testing. He is convinced that at least 25% of soil fertility issues are related to poor soil sampling, which leads to misguided test results—and flawed execution.

"Many farmers are lax in their soil sampling, so they end up with the wrong analysis and bad decisions,” explains Glanzer, who is based in Alexandria, Minn., and works with clients across the Midwest. "With the cost of fertilizer today, you can't afford to be fooled.”

The best rule of thumb in soil sampling is that the plant and the soil sample have to match, Glanzer says. He gives the example of a client's 40-acre field with flat, sandy soils. The client took a composite soil sample in the fall right after harvest that showed adequate fertility levels, including magnesium. Yet, when the corn reached 8" tall, at least three-fourths of the field showed magnesium deficiency in the form of striping on leaves.

"No matter what the soil test shows, the plant is the true test. And the plants said three-quarters of the field did not have adequate magnesium,” Glanzer explains.

In that case, Glanzer and the farmer began sampling both in the fall and late spring. They further divided the field to sample the good and poor spots separately and eventually eliminated the magnesium deficiency problems.

As a crop consultant for more than 30 years, Glanzer has witnessed (and made) all sorts of soil sampling missteps. He and his associate Jared Anez of Anez Consulting in Willmar, Minn., have developed a list of key factors that can derail soil test results. Here are their top five:

1. Sampling in compacted soil.

When farmers sample in compacted soil, they don't realize that compaction restricts the roots' water and air movement, so yields are undermined. "The result is the soil test shows good fertility levels, but with the hard layers the plant can't get the fertility it needs,” Glanzer explains. To help identify compaction problems, he often takes soil samples by hand so he can feel if the soil is tight and compacted. If a problem is present, he encourages the client to deal with it before planting.

2. Failure to adjust soil sampling to growing field size.

Be careful of blending parcels together as fields enlarge and machines get bigger. Old fence lines need to be respected. For instance, a farmer who purchases two 40-acre fields that were once farmed separately needs to ask questions about the land's history, including fertilizer use, manure spreading problems and other factors that could impact the fields' fertility. "We suggest you sample newly purchased fields separately the first year and don't combine them until you are positive the soil test is close,” Glanzer says.

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FEATURED IN: Farm Journal - Early Spring 2009

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