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 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 leeches 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.
Because certain characteristics of soil change with time it is essential that soil is analyzed as soon as practical. If it can not be tested within 24 hours of sampling soil should be frozen to reduce changes due to biological and chemical activity. Longer periods between sampling and testing may require the soil to be air dried. Properly dried soil may be stable for periods of 6 months or more.
Soil testing is often performed by commercial labs that offer an extensive array of specific tests. Choosing the test lab site is just as important as the test results. There are many soil testing labs in the United States, but finding the right one for you will take some research. It is most beneficial for the producer to find the local most lab, as the workers will have a greater knowledge and more experience working with the local soils.
Tests include, but aren't limited to, major nutrients - nitrogen (N), phosphorus (P), and potassium (K), secondary nutrients - sulfur, calcium, magnesium, minor nutrients - iron, manganese, copper, zinc, boron, molybdenum, aluminum, physical properties - soil acidity, electrical conductivity, soil organic matter, moisture content, and soil contaminants (e.g., fuel components such as benzene, toluene, xylene, petroleum hydrocarbons).
Soil testing can be an easy, cost effective way to manage agronomic as well as horticultural soils. It tells key nutrient levels, as well as pH levels, so the producer can make the best choice when purchasing fertilizers and other nutrients.
Less comprehensive do-it-yourself kits are also available, usually with tests for three important plant nutrients - nitrogen (N), phosphorus (P), and potassium (K) - and for soil acidity (pH). Do-it-yourself kits can usually be purchased at your local cooperative or through the university or private lab you choose. Prices of the tests will vary on the lab/university you purchase it from and also on what kind of test you want to do. Lab tests are more accurate, though both types are useful. In addition, lab tests frequently include professional interpretation of results and recommendations. Always refer to all proviso statements included in a lab report - these may outline any anomalies, exceptions and shortcomings in the sampling and/or analytical process/results.
"Background concentrations of lead that occur naturally in surface agricultural soils in the United States average 10 parts per million (ppm) with a range of 7 to 20 ppm. Soils with lead levels above this range are primarily the result of lead contamination. There are two major sources of lead contamination: 1) lead-based paint where contamination may occur when paint chips from old buildings mix with the soil; and, 2) lead from auto emissions. Studies conducted in urban areas, have shown that soil lead levels are highest around building foundations and within a few feet of busy streets.
"The most serious source of exposure to soil lead is through direct ingestion (eating) of contaminated soil or dust. In general, plants do not absorb or accumulate lead. However, in soils testing high in lead, it is possible for some lead to be taken up. Studies have shown that lead does not readily accumulate in the fruiting parts of vegetable and fruit crops (e.g., corn, beans, squash, tomatoes, strawberries, apples). Higher concentrations are more likely to be found in leafy vegetables (e.g., lettuce) and on the surface of root crops (e.g., carrots)." UMinn
Lead Level / Extracted Lead (ppm) / Estimated Total Lead (ppm)
Good Gardening Practices to Reduce the Lead Risk