Crop consultants and ag retailer agronomy departments can take soil tests almost any time other than winter. Testing can be done in the spring, post-harvest and even during crop growth in the summer months. Each season has pros and cons for why sampling should or should not be performed. The important factor is that the sample results be interpreted appropriately, according to the time of year the samples are pulled, notes SGS laboratories.
SGS explains the difference in soil test results based on the season when the samples are pulled. In the Midwest, most fields are sampled and tested in the fall. Good weather and increased field activity spurs the fall sampling season.
Fall-applied fertilizer rates can be calculated from the latest possible soil test results, and the following winter months gives additional time for fall-applied lime to react in the soil before the following crop year. Some phosphorous/potassium (P/K) variability may occur, especially for K, due to uncontrolled events in the fall season such as weather, leaching and the rate of stalk decomposition.
Lowest K results are often seen when soil is sampled immediately after harvest in a dry season. Spring soil tests tend to be more consistent simply due to less volatile moisture extremes. Spring soil test values are usually within the variations seen in the fall, but some increases with spring sampling may be noted; pH may vary by 0.2 pH units, and K may increase by 40-100 pounds per acre.
P results do not fluctuate as much between seasons, but increased P results may be related to increased soil pH and organic matter in the spring season. Heavier soils (with higher CEC) are more apt for variation than the lighter, sandier soils (lower CEC). Spring sampling also gives a more accurate picture of what nutrients will be available to the plant for that crop year. It also relieves pressure in the more hectic fall season and allows time for planning fall fertilizer applications based off of spring soil test results.
Sampling during crop growth in the summer months may result in the lowest overall soil test results. As the crop grows it takes in nutrients from the soil, depleting the soil nutrient supply. The nutrients bound into the plant tissue are later returned to the field in the fall, winter and spring, while the nutrients bound into the crop yield is removed from the field. Lowest soil test results are usually noted in July through August, when the crop is fully grown and beginning to mature.
If soil tests are not current, testing any time of year is acceptable, although maintaining sampling season regularity does eliminate seasonal bias of results.