Best management practices for obtaining representative samples for analysis:
Soil sample every 2 to 4 years for a given field; sampling every year rarely adds additional information.
Sample when crops are not growing in the field (applies to standard soil sampling).
Avoid fields where fertilizer, manure, or liming materials were recently applied.
Sample fields at the same time every year so that analyses are more comparable over time. Sampling 3 to 6 months prior to the next crop will allow enough time for any pH or nutrient adjustments. For many crops, this time is post-harvest in late autumn.
Thoroughly clean the soil probe or instrument to remove all residual soil or debris. It does not take much of a foreign substance to significantly contaminate a sample.
Be sure the sampling container is similarly clean to prevent any contamination. Plastic or stainless steel containers are preferable to other materials.
Move residue, debris, and any vegetation from the soil surface at the sample site. Failing to do so will cause an incorrect (too high) organic matter measurement.
Pull soil cores from a depth of 6 to 8 inches, or depth to plow layer where soil mixing occurs. Long-term no-till fields, perennial forages, ridge tillage, and similar systems may be sampled also at a shallower depth to note any shift in soil pH in the top 2 to 4 inches.
Obtain 15 to 20 soil cores for an area of 20 acres or less. For larger areas, submit multiple samples for more accurate soil information, even if the field appears uniform.
Mix cores together well for the sample. Usually, samples can be sent moist after mixing. For standard soil tests, air drying is permissible, but do not heat cores to speed drying.
Follow the soil laboratory's instructions for submission, using the containers recommended or supplied by the lab.