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In agriculture, a soil test is the analysis of a soil sample to determine nutrient content, composition and other characteristics, including contaminants. Tests are usually performed to measure fertility and indicate deficiencies that need to be remedied.
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The quality of the original soil sample plays a key role in determining the practical value of test results. Most labs will provide documentation outlining the proper procedures for collecting soil samples.
Labs, such as Iowa State and Colorado State University, recommend that you take between 10-20 samples for every 40 acres (160,000 m2) of the field. Sampling implements must be properly cleaned prior to sampling, and must be cleaned between samples to avoid cross-contamination (especially when sampling and testing for soil contaminants). The tool should be free of rust, and washed with distilled water. Doing so will clean the tool, but also not add any minerals or elements from regular tap water or chemicals that could change the composition of the soil.
Soil characteristics can vary significantly from one spot to another, even in a small garden or field. Taking samples everywhere in the field is crucial to get the most accurate measurement of nutrients and other organisms. An example of this is along gravel roads where the soil could have more lime from the dust from the roads settling down in the soil, or an old animal feedlot where phosphorus and nitrogen counts could be higher than the rest of the field.
Sample depth is also an important factor. It is recommended that you take the samples from tillage depth, as this is where the majority of the nutrients and elements are placed mechanically. The presence of various nutrients and other soil components varies during the year, so sample timing may also be important. A good time to take a sample for testing is in the fall after harvesting is finished, but this isn't the only time it should be done.
Sampling and testing in the fall is beneficial because the producer will get the results back in time to formulate the fertilizer plan for the following growing season. Another time sampling and testing can be done is spring. This is a good way to see what nutrients survive over winter when the soil freezes, as well as if any leaches away from melting of snow and thawing of the soil. This way the producer can know if more or less fertilizer needs to be purchased.
Mixing soil from several locations to create an "average" (or "composite") sample is a common procedure but it must be used judiciously as it can artificially dilute quantities/concentrations of soil components and may not meet government agency requirements for sampling. Make a reference map for your filing system so you know where you took them, and how many samples you took in the field. All of these considerations affect the interpretation of test results.