Basics

How do I interpret my soil test results?

Soil sampling for the soybean cyst nematode (SCN) has increased dramatically in the last couple of years. Several private soil fertility laboratories have begun to offer SCN soil analysis as a service. As growers continue to increase their sampling for SCN and begin to compare results from various fields and various laboratories, there are six questions that may be asked to help them interpret their soil sample results.

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How can the variability in my soil sample results be explained?

(from Dr. Terry Niblack , University of Illinois at Urbana-Champaign & Dr. Pat Donald, University of Missouri)

Soybean cyst nematodes (SCN) are not evenly distributed in a field. For example, in a single field with an average number of eggs of 50,000, the actual number of eggs in any one section of that field might be anywhere from 0 to 100,000. Due to this natural variability in the distribution of SCN, it is very important to take a sample. Some of the questions we've received about this variability in SCN egg counts are given below, along with the answers.

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How many bushels per acre soybean yield loss can I expect for a given SCN egg population density (egg count)?

How many bushels per acre soybean yield loss can I expect for a given SCN egg population density (egg count)? In other words, what is the relationship between soybean yield loss in relation to the number of eggs per 100 cc of soil?

It is impossible to provide such precise information about the potential for yield loss because the amount of damage caused by SCN is very dependent on numerous interrelated factors including:

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How high should the SCN population density (egg count) be before growing a SCN-resistant soybean variety?

Here at ISU, we have two management recommendation for SCN-infested fields. The recommendation given is based on the SCN egg population density (egg count) of the sample submitted.

Recommendation #1 - for fields with egg densities up to 5,000 eggs per 100 cc soil

We recommend that growers follow a six-year crop rotation scheme as illustrated below:

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Sampling to evaluate the success of an SCN management program

Recommended technique: Collect soil samples and have the SCN population densities determined.

When to sample
You can take a soil sample to check on the progress of a SCN management plan at almost any time of the year. But if you want to monitor the effectiveness of your SCN management program over several years, you should sample at the same point in the management program each time (say after a year of resistant soybeans or nonhost crop).

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