Soil Testing Behind the Combine – “Best Quality Sample and Best Data”

Dave Franzen

Dave Franzen, Extension Soils Specialist, North Dakota State University

It is more the rule than the exception that soil sampling begins in mid-September instead of immediately following small grain harvest. However, many growers miss an excellent window for soil testing by waiting too long. The reason for waiting is the fear that additional nitrogen will be made available through mineralization (decomposition of crop residue and organic matter). NDSU research has shown that following small grain harvest, soil nitrate level changes are usually small and no sampling date adjustment should be made. Soil sampling right after small grain harvest is recommended for its numerous advantages:

  1. Soil sampling prior to fall tillage produces a more consistent 0-6” sample core, which provides the best sample for testing phosphorus, potassium, %OM, zinc, and other nutrients tested on the topsoil.
  2. Growth of volunteer small grains will not hide available nitrogen. Early sampling will indicate the amount of nitrate that will be available for next year’s crop.
  3. Sampling immediately after harvest guarantees that fields will be sampled and not missed due to weather problems that could happen later in the fall.
  4. Growers are more likely to use soil test results to direct fall N applications if soil test results are in their hands soon enough to consider before fall fieldwork begins.