Soil testing is an important diagnostic tool for determining the nutrient needs of plants and for environmental assessments. Some soils are inherently deficient in plant nutrients. Other soils had sufficient levels of nutrients in the past, but removal with crop harvest has depleted the reserves. Thus, soil testing is widely accepted and used in most advanced crop-production areas of the world to determine fertilization needs for crops. Soil testing can also be used to identify application rates of waste materials containing nutrients or other elements that could harm the environment. Waste materials such as animal manures and industry by-products may provide various plant nutrients. However, high application rates to soils designed to dispose of the material at a low cost may result in nutrient loads that are harmful to plant, animal, or human health. Nutrient management regulations are being developed to address land application of waste materials. Soil testing is required in many regulations and management guidelines to assess environmentally harmful levels of certain elements and to determine limits to application rates.

Soils are tested routinely for the primary nutrients phosphorus (P), potassium (K), and nitrogen (N). In some regions, soils are also routinely tested for other primary nutrients such as calcium (Ca), magnesium (Mg), and sulfur (S), and for other nutrients required in very small amounts by crops such as boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Zn). Soils receiving waste materials are also tested for elements such as arsenic (As), cadmium (Cd), nickel (Ni), lead (Pb), mercury (Hg), and selenium (Se) among others.

Two of the primary plant nutrients, N and P, may have harmful effects on the environment when applied to soils in excessive amounts. Excessive N and P applications to agricultural fields and ineffective nutrient, soil, and water conservation practices are increasing nutrient pollution in many regions of the world. However, the basic concepts of soil testing also apply to other elements. Phosphorus is included with the group of elements with relatively low mobility in soils (Ca, Cd, Cu, Fe, Mg, Mn, Mo, Ni, Pb, and Zn among others). N, especially in the nitrate form, is included with the group of elements with greater mobility (which include B, Cl, S, and others). Important concepts include the meaning of a soil-test value, soil-testing quality, use of soil testing to determine economically optimum nutrient application rates, and use of soil testing for environmental assessments.