By Jeffrey S. Graybill, Extension Agronomist
Fall is historically the time of year when we find time to pull soil samples. A soil test will not only give you the fertility level of a field, but most labs also provide crop specific recommendations for nitrogen (N), phosphorus (P), potassium (K), calcium, magnesium and sometimes micronutrients.
Additionally, a soil test measures pH. Annual applications of N fertilizers and manure increases soil acidity (lowers pH). For example, 7 pounds of lime are needed to neutralize the acidity created by 1 pound of ammonium sulfate. In acidic soils, the natural availability of many plant essential nutrients is reduced, while the availability of undesirable elements such as aluminum will be increased. Limestone has carbonates, which remove the acidity from the soil, and also supply the essential elements of calcium and magnesium.
A soil test should be taken every 3 years and whenever you make a significant change to your cropping system. A soil test sample should be taken to rooting depth in no-till fields. Be sure to take about 15-20 core samples in representative areas of each field. Avoid or separately sample problem areas. The desired pH varies somewhat by crop but should be between 6.5 and 7.0.
Two additional reasons for soil testing are to monitor change in soil fertility and to get an indication of your soil quality. By monitoring soil tests over several years, we can make cultural changes to maintain soil nutrients near the “optimum” level. An important measure of soil quality is the organic matter content. An organic matter test (which must be requested) along with the cation exchange capacity (CEC) reported on your soil test can indicate improving soil quality and productivity. Both of these will change very slowly, but have been shown to increase in many long term no-till systems.
No-tillers note: It’s not uncommon for the surface pH of no-till fields to be significantly lower than the rest of the soil profile. This low pH can have a yield-robbing effect on plant growth and can also reduce herbicide effectiveness. If your standard soil test does not call for lime but you are a no-tiller, then a separate shallow sample (no more than 2 inches deep) should be taken to measure the surface pH. Samples can be sent to the lab separately, or for a faster and less costly method, use a field pH kit. If the surface acidity is below 6.2, apply one ton of lime (2,000 pounds calcium carbonate equivalent). Limestone applications, especially no-till (unincorporated) applications, should be done several months before the crop is planted to allow for rainfall incorporation and activation.