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The accuracy of soil and plant analytical results are occasionally called into question by laboratory clientele. Although laboratories generally conduct internal quality assurance procedures, there are few external performance testing programs for the industry. In the spring of 1994, a proficiency testing program was initiated for soil and plant samples for agricultural laboratories in the Western United States to provide an external quality control for the laboratory industry. The program involves the quarterly exchange of soil and plant samples on which soil salinity, soil fertility, and plant nutrition analyses are conducted. For the 1994 program, 103 laboratories from 23 states and Canadian provinces participated. Overall, soil salinity and pH analyses were highly reproducible across soils and laboratories through the year. Variability in soil nitrate results were lower than noted in past interlaboratory exchanges independent of soil sample. Results for extractable soil bicarbonate phosphorus were variable independent of soil and indicate a significantly positive skewed distribution. Plant nitrogen (N) and phosphorus (P) results were consistent across samples, laboratories, and methods. Variability in plant nitrate increased with decreasing tissue concentrations. Overall accuracy and precision of reported results, based on the use of a NIST certified reference botancial sample, were excellent for N, P, potassium (K), calcium (Ca), and copper (Cu). Generally for any given analysis the results of approximately 10% 451

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Recently, one of the authors received a question about determining the gypsum requirement (GR) for reclamation of sodic and sodium‐impacted soils. The purpose of this article is to provide information to aid in understanding the laboratory procedure for determining GR and in developing and executing an effective reclamation program. Gypsum is frequently, although not always, used as part of the reclamation program. Earn 0.5 CEUs in Soil &Water Management by reading this article and taking the quiz at http://www.certifiedcropadviser.org/education/classroom/classes/802.