When a Soil Test Lies

March 28, 2009 07:15 AM

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.

3. Forgetting a field's production history.
The life of the field has a huge impact on soil test results, Glanzer says. Knowing the field's disease, insect and weed history can impact decisions for the growing season. Yield maps and yield histories can point out the differences, but watch for other signals, as well. For example, certain weeds can point to other problems. Yellow nutsedge, for instance, thrives in soils with poor drainage.

4. Sampling where fertilizer was spread unevenly.
Manure spread on only a portion of the field can greatly skew a soil test, Glanzer says. If you happen to sample where manure was accidentally dumped at one time, that will throw a soil test off for years.

5. Going with a cheap test.
Sometimes you need more than just a soil test for phosphorus, potassium and pH, Anez says. This is especially true for soils where pH is an issue. "One of the downsides of grid sampling is that the cost gets so high farmers start to eliminate things to test for,” Anez says. "Even if you don't order a full sample every time, you should try to test for sulfur and trace elements when you can.”

One of Glanzer's favorite sayings is: If you don't know where you're going, then it's hard to get there. That is certainly true with crop yields and fertility management. All farmers, he says, whether they are crop or livestock producers, need to be developing a short-term and long-term nutrient management plan.

"This gets back to the point of having an end goal in place,” Glanzer says. "The producers who keep their eye on their goal will end up saving money on fertilizer this year.”

Spring Nitrate Test Tips

The large amount of rain and potential leaching last year spurred many producers to use a late-spring soil nitrate test to estimate additional nitrogen (N) fertilizer to be sidedressed. The test is often called a presidedress nitrogen test (PSNT).

The test helps estimate the likelihood of yield response to any additional fertilizer when soil nitrate concentrations found in the surface are below the critical nitrate range, says Peter Kyveryga, senior research associate with the Iowa Soybean Association's On-Farm Network.

Optimal range. A range of 20 to 25 parts per million of nitrate-N is considered optimal. Lower the optimal range, however, when rainfall in May is more than 4" and when the test is used for soils receiving manure and anhydrous ammonia. The test should be done when the corn is 6" to 12" tall.

It is a good idea to use the late-spring soil nitrate test if all or the majority of N fertilizer was applied before planting and there is concern about losses, Kyveryga says. However, just as with a traditional soil test, bad sampling for a PSNT can lead to wrong results. The largest errors often occur because the nitrate concentration of the soil sample does not represent the nitrate concentration of the field, he says.

"The most difficult fields to sample for this test are the ones where nitrogen was applied in a band as anhydrous ammonia or injected swine manure,” Kyveryga says. Also, when fields are saturated or flooded, the test can show inconsistent results, he adds.

The best way to sample fields with banded N is to collect three sets of eight cores, positioned at various distances between two corn rows. With this method, the person doing the sampling moves in a random pattern within the test area. Each time a core is collected, its exact position is selected relative to the two nearest corn rows: The first core is collected in a row; the second is collected one-eighth of the distance between any two rows after moving to another part of the test area; and the third is one-quarter of the distance between two rows after moving to another part of the test area.

The process continues until the eighth core is collected 7⁄8 of the distance between any two corn rows. Soil from all cores should be crushed and thoroughly mixed before a subsample is sent for analysis.

The late-spring soil nitrate test is a diagnostic tool that can reduce some uncertainty in N management, but it cannot predict the magnitude of yield response in individual fields, Kyveryga cautions. "Understanding what this soil test can do and cannot do is crucial when using this test during the growing season,” he adds. For more information, visit www.isafarmnet.com.

You can e-mail Jeanne Bernick at jbernick@farmjournal.com.


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