TILLAGE DEPTH, SOIL TESTING AND LIME APPLICATION
Tillage depth can affect the stratification of soil acidity. The term “stratification” is just a fancy word for separating something into layers. Which is what can happen when nitrogen fertilizer is applied to the soil surface and either not incorporated (as in the case of notill) or incorporated at less than a normal moldboard plow depth of 6-8” (as in the case of an offset disk or a chisel plow).
We’ve long known of the potential problem in long-term notill where nitrogen fertilizer is topdressed or sidedressed during corn production. All nitrogen fertilizers will acidify the soil, and eventually the soil pH in the top 1” or so of soil may become so acid that it can reduce the effectiveness of some preemergence herbicides. This is called an “acid roof”, but it’s much less likely to occur if manure is the major source of nitrogen because the pH of dairy manure is close to neutral. When soil sampling notill fields, two composited samples should be taken on each field, one at the normal 6-8” depth and the other just the top 3” of soil. I’d also suggest taking two samples if routinely using a multi-function tillage tool designed for one-pass field preparation, since these implements work the soil no more than a few inches deep. Taking two samples is probably more important if nitrogen fertilizer has been the major source of N, but it’s a good idea to do this anyway since it may reveal some stratification of phosphorus and potassium as well.
If the soil analysis — either from the top 3” or the entire plow layer — indicates a need for lime, is it necessary to moldboard plow the field following lime application? Nope, research has found that as long as the top few inches are increased to the recommended pH and the soil below that is at least 5.0, crops won’t be affected. And I’m not aware of any agricultural soils in the Northeastern U.S. that are as low as 5.0.
— Ev Thomas
