The goal of a soil sample is to make a fertilizer recommendation for crop production.
- To provide that recommendation, calibration studies are done to measure crop response.
- For Ohio, the Tri-State Fertilizer Recommendations provide the calibration study history for recommendation development.
- While there are other “recommendations” used in Ohio, few have done the comprehensive work to truly provide this information.
- For more information on the Tri-State Fertilizer Recommendations or on developing a soil sampling strategy several references are provided at http://agcrops.osu.edu/specialists/fertility/fertility-fact-sheets-and-bulletins.
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 livestock production (a former pasture, had manure applications or produced hay) or other unique characteristics may require a different sampling strategy. Field areas represented by any single sample should be less than 25 acres. Use of a yield monitor or grid sampling can lead to development of crop management zones, easing the burden of future sampling.
Understand that a single soil sample is not a single core but a composite of numerous cores collected over the field area represented by the sample.
- Where broadcast applications have occurred a composite sample of 10 to 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 to 25. In my opinion every field in Ohio has had band applications at some time in the past, so use 20 to 25 cores per sample.
The samples are bulked, mixed and then a subsample of about one pound of soil (a pint) is submitted to the lab.
Nutrients in the soil are naturally stratified with higher nutrient levels on the surface. This is due to plant residue breakdown and fertilizer placement. Each core taken should be taken to the same depth in the soil profile. Generally a 6- or 8-inch sample should be taken.
The primary goal is to measure the ability of the soil to provide the soluble nutrient needed for crop production for two of our three macro nutrients (phosphorus and potassium) plus measure soil acidity which governs availability of micro nutrients. A secondary goal is to compare soil test results over time. No soil test result should be considered in isolation, look at past results before making major modifications.