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Soil test values vary most by soil type and management because the underlying geology of soils and what man does to the soil greatly affects mineral composition, air-water balance, and biological activity. This is why Ag Spectrum uses both soil types (ST) and management zones (MZ). Ignoring ST/MZ boundaries leads to mixed composite samples that do not accurately reflect the distribution of soil properties across a field.
Soils constantly change, so it is hard to get a fair measure of a soil’s annual potential to provide nutrients to plants from a single time point. One has the best chance of getting a fair and representative value when soils are least biologically active. This is typically late fall, pre or post-harvest, or early spring before soils warm and cover crops start actively growing if they are present. Because soil moisture around the time of sampling also plays a significant role in how much of each nutrient will be measured in the lab, it is best to sample in the same seasonal window, either before significant snowfall or after winter snow melt.
The protocols used by Ag Spectrum are standardized and implemented with due diligence at Midwest Labs that are specifically calibrated to our recommendations. Other labs will do the same tests as Midwest Labs, but they may choose to use other test methods. For this reason, Ag Spectrum requires all customers to use Midwest Labs for their soil and input testing to get the most accurate and optimal input recommendations.
-submitted by Dr. Brian Gardener, Technical Director
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