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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.


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.


What is the best time to sample for SCN?

If a grower simply wants to sample a field to check for the presence of SCN, I'd recommend sampling the field after soybean harvest. The chances of finding SCN after a soybean crop are greater than sampling after a corn crop because soybeans are a host for SCN. Of course, if the grower discovers SCN in a soil sample taken from a field in which soybeans were just harvested and corn will be grown the next year, he or she also should collect another sample after the corn harvest in those fields if soybeans will be grown again following the corn crop.


Field evaluation of SCN-resistant soybean varieties

Investigators: Gregory L. Tylka, Gregory D. Gebhart, Christopher C. Marett
Funding: Iowa Soybean Association, seed company fees

An effective and affordable way to manage SCN is to grow resistant soybean varieties. SCN-resistant soybean varieties suppress SCN reproduction, reducing the yield loss caused by damage from nematode feeding. SCN resistance preserves the yield of soybean varieties growing in SCN-infested fields.

Field evaluation of SCN management products

Investigators: Gregory L. Tylka, Gregory D. Gebhart, Christopher C. Marett, David Soh
Funding: Iowa Soybean Association, company fees

Each year, one or more foliar-, seed-, or soil-applied products are advertised for use in managing SCN. Growers and agribusinesses in Iowa turn to Iowa State University for an unbiased evaluation of the effect of these products on SCN populations and soybean yield.

Studies of the interactions of SCN with brown stem rot of soybean

Investigators: Gregory L. Tylka, Girma Tabor, Charlotte Bronson, David Soh
Funding: North Central Soybean Research Program, Iowa Soybean Association

Iowa fields are commonly infested with both SCN and brown stem rot (BSR). In the early 1990s, researchers observed that BSR-resistant soybean varieties had much greater than expected levels of BSR disease in fields infested with SCN than in those without SCN.