Accuracy varies for commercially available soil test kits analyzing nitrate-nitrogen, phosphorus, potassium, and pH

Soil testing is an important component of a plant nutrient management program for farmers, home gardeners, and agricultural service personnel. Results from five commercially available colorimetric soil test kits were compared with standard laboratory analyses for pH, nitrate-nitrogen (NO3), phosphorus (P2O5), and potassium (K2O) content for Salinas day loam soil with three cropping histories. The kits ranked in accuracy (frequency of match with analytical laboratory results) in the following sequence: La Motte Soil Test Kit, Rapitest, Quick Soiltest, Nitty-Gritty, and Soil Mt at 94%, 92%, 64%, 36%, and 33%. NO3 was most accurately determined by Rapitest and Quick Soiltest, P2O5 by Rapitest, and pH by La Motte Soil Test Kit. K2O was determined with equal accuracy by all but Soil Mt. The composition of the extractants may be an important factor affecting the accuracy of the test kit. For example, all kit extractants for E2O were composed of the same chemical and matched analytical laboratory results 82% of the time. By contrast, kits using an acid-based extractant for NO3 analysis more frequently matched the analytical laboratory results than kits using a zinc-based extractant (P <= 0.0001). La Motte Soil Test Kit had the largest range of pH measures, whereas Rapitest was relatively easy to use and interpret and is a practical choice for home gardeners or landscapers; both were more than 90% accurate for this soil type. Although an important limitation of commercial test kits is the approximate or categorical value of nutrient content (i.e., low, medium, high), accurate test kits can yield results quickly and economically for improved nutrient management.