Five Tips For Reviewing SCN Soil Test Results

As the results from fall soil tests begin to arrive, it’s time to evaluate what to do next.

“The most commonly asked questions we get from farmers when they receive their SCN soil test results are about what the results mean and what to do with that information,” said Dylan Mangel, plant pathologist with the University of Nebraska–Lincoln.

To help growers create an effective management plan for controlling soybean cyst nematode (SCN), The SCN Coalition shares five key steps to take.

1. Egg Vs. Cyst Counts
While most SCN test results will report egg counts, some provide cyst and juvenile counts instead – which may not be the most accurate.

For example, Iowa State University Extension and Outreach reports SCN juveniles are short lived and their populations do not accurately represent SCN levels overall. They consider cyst and egg counts to be far more informative.

At the same time, The SCN Coalition says cyst counts should be multiplied by 100 to get a better idea of the egg count.

“Egg counts provide a more accurate assessment of SCN pressure in a field, despite the variability in soil test results,” said Horacio Lopez-Nicora, plant pathologist and nematologist with The Ohio State University. “Cysts tend to cluster and contain varying numbers of eggs, leading to significant fluctuations in soil sample outcomes. Thus, these results should be considered an approximate estimation of the actual SCN population in the field.”

2. Population Levels Aren’t One Size Fits All
Population levels in SCN soil tests are typically reported as low, medium and high. But those levels don’t always mean the same thing between fields.

“It’s important to keep in mind that what may be considered a high SCN egg count in some states is low or moderate in others, because different environmental factors like sand content and soil pH can impact SCN reproduction,” explains Lopez-Nicora.

Be sure to pay attention to the soil volume measured in the test as well. Comparing a sample reported in 100 cubic centimeters of soil to one reported in 250 cubic centimeters of soil will give misleading results. 

3. Calculate Profit Loss
SCN soil test results can also be evaluated with the SCN Coalition’s new online tool - the SCN Profit Checker calculator - which allows growers to put a number to how much the soybean cyst nematode is costing them on a field-by-field basis.

Users input field-specific information, like the SCN egg count and SCN female index on PI 88788, and see an estimate based on expected yield and soybean price.

4. Build a Management Plan
Due to the soybean cyst nematode’s ability to rapidly grow in populations, the SCN Coalition recommends taking action whether results come back with low or high numbers.

The group suggests three key components to any management plan:
• Rotation of SCN-resistant soybean varieties
• Rotation to nonhost crops (corn, alfalfa, oats, etc.)
• Consideration of nematode-protectant seed treatments

Iowa State University Extension Publication IPM 63 provides specific management recommendations based on results of samples processed by the Iowa State University Plant Disease Clinic.

5. Compare Results Over Time
Iowa State University Extension and Outreach shares the methods for collecting SCN samples can sometimes result in low population numbers not showing up on a report. 

This means continuing soil testing for multiple years is not only a good way to evaluate management practices, but also identify potentially inaccurate test results.