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Growers ask SGS many questions regarding the timing on testing their soil samples. Some test their soil in spring, some choose post-harvest, and others prefer 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 grower understands how to interpret SGS soil test results based on the time of year the samples are pulled.

In the US 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. For fall-season sampling and testing, SGS offers competitive laboratory turnaround times and the best soil nutrient results possible.

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 lbs/ac. 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 to 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 the July – August months, 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.

Contact SGS for more information on in-season sampling options and regardless of the season in which you choose to sample, SGS is ready to work with you in optimizing your crop yields.

Neil Stoller
Agronomist, CCA/TSP
117 E. Main St.
Toulon, IL 61483
t: (309) 286-2761
f: (309) 286-6251
Website: www.sgs.com/us-soil