Source: United Soybean Board

As harvest comes to a close and input prices continue to fluctuate, soybean farmers have actively begun planning for next year. One tool that will help make these plans is a fall soil test. After harvest and before the ground freezes is an ideal time to dig into your soil to discover what nutrients remain and learn more about the pests that are present, especially soybean cyst nematode (SCN).

"If sampling for SCN, you have more time to plan if you sample in the fall than if you sample in the spring," said Greg Tylka, professor of plant pathology at Iowa State University. "If you have your results now, you have all winter to plan."

Samples for soil phosphorus, potassium and pH tests should also be taken in the fall to allow more time to receive and interpret results for spring applications, says John Sawyer, Iowa State University extension soil fertility specialist and professor of agronomy. Sawyer recommends that soybean farmers look at the soil test pH and phosphorus and potassium levels before making decisions about fertilizer and lime applications.

"The main thing a soil test can provide is a balanced fertilizer program that will increase yield per acre by maintaining adequate fertility," said Ken Dalenberg, soybean farmer from Mansfield, Ill., and United Soybean Board director. "Providing nutrients to plants can help mitigate some of the devastation of cyst nematode."

CN soil tests are important even if you know what fields are infected with this top yield robber in order to check the nematode population and make sure it's not increasing. These numbers will also tell you if your management is working or if you need to switch varieties or rotate in a non-host crop.

"You can have SCN, and your plants may not look sick," Tylka said. "We have research funded by the checkoff that shows in the Midwest up to 40 percent yield loss from SCN may not leave plants looking sick."

Taking a representative soil sample requires a little time. Tylka recommends using a soil probe to pull soil cores about 8 inches deep.

  • For an SCN test, more than 20 soil cores should be taken from every 20 acres.
  • With larger fields, divide them into 20-acre sections.
  • Collect a 20-core sample from each section.
  • All of the cores from one section should then be broken up and mixed together in a bucket.

  • A scoop of the mixed soil can then be placed in a bag to be sent to the soil lab.

"You're only collecting 20 little cores," said Tylka of SCN. "If you get results that say you don't have it, it's a good sign, but don't assume that the field is not infested with SCN. There's a quote from the astronomer Dr. Carl Sagan I use a lot: 'Absence of evidence is not evidence of absence.'"

When testing for soil nutrients, each sample should represent approximately a minimum 10 acres of uniform soil that have been treated similarly. Ten to 12 soil cores should be collected and mixed for each sample. For more intensive grid sampling, soil samples should be more intense, with 2.5 acre sample grids. With multiple samples per field, farmers can better understand tests levels for making whole field fertilizer and lime applications, or use the results for variable application to different field areas.

"Over the years it has typically been suggested to sample on a four-year cycle, especially if you are rotating corn and soybeans," Sawyer said. "Sampling more frequently can help establish a trend line, so doing it every two years is helpful for that."

More soil testing is taking place this year as farmers are trying to plan fertilizer applications even more carefully due to high fertilizer prices.

"A lot of sampling is going on, and, with the high prices of fertilizer, making the right decision is especially important, which can best be done having recent soil tests," he said.

Soil tests at a university lab usually cost $20 to $30. To find a lab that tests for SCN and nutrients, contact your local county extension office or a university's extension plant pathologist or nematologist.