Effect of Spring vs. Fall Sampling on Soil Test K

Potassium (K) soil test levels have the greatest potential to vary between fall and spring. Soil test K levels are usually lowest in the dry conditions of late summer prior to harvest and peak in the early spring. Potassium is released from crop residues as the crop matures and crop residues degrade, and this release continues through the winter months. In addition, potassium is released from the soil through prolonged saturation and freeze/thaw cycles in the winter months.  With these processes combined, the question becomes how much higher soil test potassium levels will be in spring- versus fall- collected samples.

A simple approach is to select a location in a field such as a grid or zone sample location and physically mark it with a flag.  Collect 6-8 cores at a depth of 6-8”. It is important to pull the same number of cores at the same depth each time samples are collected. Collect the cores as close as possible to each other and the flag. Do this in the fall and in the spring.

In most cases, spring and fall potassium levels are usually within 5-10%. Spring soil sampling does not always result in higher potassium soil test level; this depends on crop residue release and weather patterns. Below is an example of some actual field data collected in northeast Indiana in a clay loam soil with a corn/beans/wheat crop rotation with fertilizer applied after the spring sample collection. The key to proper soil sampling is consistency. Sample the at the same depth, following the same crop at the same time of year.