By Brent Crain
Soil testing is a best practice recommended by Michigan State University Extension to optimize agronomic, economic and environmental outcomes from fertilizer practices. MSU Extension recommends that soil be tested every two to four years. Nutrient levels vary by season. To accurately assess changes in soil nutrient levels, it is important to test soil at the same time of year. There are several reasons why testing in fall might be preferable to testing in spring.
There is more time available in the fall to collect soil samples and make fertilizer decisions compared to spring. Based on the soil analysis, fall season allows additional time to think about upcoming management decisions. Farmers usually apply phosphorus (P) and potassium (K) fertilizer in the fall when there is more time and equipment available and soil compaction is less of a concern. This simplifies spring operations and streamlines planting.
On soils with optimum fertility levels, field research has shown that fall applications of P and K would be equally effective compared to a spring application prior to corn and soybean planting. For winter wheat, all the P and K requirements are best applied at fall planting.
Source : msu.edu