One of several factors to define in your soil test collection method is sample depth
Soil testing is a very profitable practice to manage fertility input cost and promote environmental stewardship. A standard soil test measures two of our three macro nutrients (phosphorus and potassium) plus soil acidity which governs availability of nutrients and other soil functions. The primary goal is to measure the soil’s ability to provide the soluble nutrient needed for crop production. A secondary goal is to compare soil test results over time relative to crop response and fertilizer additions, or what is known as “adaptive management”. The key to accomplishing both goals is to take a quality soil sample that represents the field area being sampled.
Planning ahead and establishing standard sampling criteria are good first steps to taking a quality soil test that can track trends over time. Preplanning involves determining the field area that will be collected for the sample. There are a number of different methods on dividing up a landscape that can be effective. The overall goal is to have sample areas in the field that have similar crop yields, crop rotation histories, fertilizer application methods and sources of applied nutrient. Fields or field areas with a history of manure, banded fertilizer application or other unique characteristics require a different sampling strategy. Field areas represented by a single sample should be less than 25 acres.
The second focus area for a quality soil test is the sample collection process. First, a soil sample is not a single core but a composite of numerous cores collected over the field area. Where broadcast applications have occurred a composite sample of 10-15 cores is suggested. Where a history of banded application exist in a field or manure application, then increase the number of cores to 20-25. The samples are bulked, mixed thoroughly and then a subsample submitted to the lab.
The second critical collection item to define is the sample depth. Nutrients in the soil are naturally stratified with higher nutrient generally found on the surface due to residue breakdown and fertilizer placement then decreasing deeper in the soil profile. Each core taken should be taken to the same depth in the soil profile. Generally an 8 inch sample should be taken.
Finally, some other factors in sample collection should be included. Scrape the soil surface before taking a core so the sample core does not contain residue or live plant material. If manure has been applied wait at least 6 months before sampling, or if fertilizer is applied wait 2 months. To compare trends over years, the soil sampling should be done at approximately the same time of the year. Early spring or fall when crops are not actively growing are ideal times to sample.
The whole goal of soil sampling is to make a fertilizer recommendation for crop production. To provide the recommendation calibration studies are done with the soil test to measure crop response. For Ohio, the Tri-state fertilizer recommendations provide the calibration study history for recommendation development.
(Source – http://www.farms.com/news/preplan-for-a-quality-soil-test-result-80402.aspx)