Soil Testing Behind the Combine

As harvest gets underway, savvy soil samplers are following right behind the combine and starting to collect soil samples. These soil samplers understand the many reasons why taking soil samples right behind the combine gives them the best quality soil samples and data.

In the past, the reasoning to wait until later in the fall to start soil sampling was that there may be additional nitrogen that would be converted to nitrate through the fall as small grain straw and crop residue start to decompose. However, we now know that small grain straw has a high carbon content, and it takes a long time for wheat straw nitrogen to convert to nitrate-N in soil for future crops. Research has shown that soil nitrate-N levels after small grain harvest are quite stable with small changes (up or down) through the fall. Soil sampling right after harvest provides actionable soil nitrate-N data for making fertilizer decisions for next year.

Soil testing behind the combine has several other advantages. If you sample right behind the combine, you beat chisel plows and disk rippers to the field. Taking soil samples before fall tillage allows you to obtain clean and consistent soil cores with your soil probe; this is important for high-quality soil samples. If you sample after tillage, you will be dealing with soil clods that do not feed smoothly into the soil probe. Soil sampling after tillage can also lead to inconsistent sample depths, which will affect soil test levels for P, K, Zn, etc.

Here are some comments by Dr. Dave Franzen, NDSU Extension Soils Specialist (retired) about soil testing right after harvest:

“It is more the rule than the exception that soil sampling begins in mid-September, rather than starting immediately following small grain harvest. However, many producers miss an excellent window for soil testing by waiting too long. The reason for waiting is the hope that additional nitrogen will be made available through mineralization (decomposition of crop residue and organic matter). A review of research has shown that soil nitrate levels change very little, up or down, following small grain harvest.”

Soil sampling right after harvest is recommended and has numerous advantages

1. Producers are more likely to use the actual soil test results for deciding fall nitrogen fertilizer rates if the soil test results are in their hands before fall fieldwork begins.
2. Soil sampling before fall tillage provides more consistent 0-6 inch soil cores, which gives the best soil sample quality for phosphorus, potassium, zinc, organic matter, and other non-mobile soil nutrients tested on topsoil.
3. Soil sampling right after harvest guarantees that fields will be soil sampled on time and not missed due to weather problems that could happen later in the fall.