With fertilizer prices on the rise, it’s more important than ever to understand your soil test levels and crop response to applied fertilizers. Fall is a great time to soil sample before freeze up. The non-mobile nutrients, phosphorus (P), potassium (K), pH, soluble salt content (EC) and most micro and secondary nutrient soil tests are minimally affected by sampling time. However, try to avoid soil sampling after an extreme dry period, as soil K levels will be lower due to the effect of shrinking clay mineralogy trapping K and, therefore, making it unavailable to plants. Soil biological activity affects nitrogen (NO3-N) and sulfur (SO4-S) soil tests. It is recommended to wait until soil temperatures reach less than 50 degrees Fahrenheit in the fall before sampling. Above this temperature, nitrogen and sulfur are released from organic matter and crop residue. Other factors, such as warm winters with early springs, sampling small grain stubble with excessive regrowth and amount of snow cover can lead to changes in NO3-N soil test levels before planting the following spring. Soil sampling for nitrate and sulfate is probably best done in the spring immediately before fertilizer applications.
Generally, soil tests for P, K, pH, EC, calcium (Ca), magnesium (Mg) and micronutrients will not change immensely from year to year; these can be analyzed every two to three years. Conversely, levels of NO3-N, SO4-S and Chloride (Cl-) can change from year-to-year and should be analyzed every year a non-legume crop is planted.
Soil samples should represent a uniform area of your field; take time to assess fields and split them up by differences, such as texture, color, slope, amount of erosion, drainage and pest management. Other options for producers using precision technologies may be small grids (two to five acres) or productivity zones based on soil differences. Individual samples would be submitted to the lab from each grid point/area or management zone. This data can be used to generate variable-rate fertilizer and seeding application maps. Regardless of what sampling method you use, some basic guidelines should be followed.