A soil test is the analysis of a soil sample to determine nutrient content, composition and other characteristics. Tests are usually performed with a soil test kit to measure fertility and indicate deficiencies that need to be remedied. Soil testing is often performed by commercial labs that offer an extensive array of specific tests. 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). Lab tests are more accurate, though both types are useful. The quality of the original soil sample plays a key role in determining the practical value of test results. Soil characteristics can vary significantly from one spot to another, even in a small garden or field. Sample depth is also an important factor. And the presence of various nutrients and other soil components varies during the year, so sample timing may also be important. Mixing soil from several locations to create an "average" sample is a common procedure. All of these considerations affect the interpretation of test results.