Quality Control for Soil Testing

The large number of routine soil samples extracted and analyzed daily in the Soil Testing Laboratory (100-400) requires that a precise system of quality control be used. The system insures that samples remain in their proper order so no mix-up is made in the sample sequence and the quality of the analyses will remain constant from day to day and throughout the year. "Check" samples are purchased or established by collecting, drying, homogenizing, and storing several kilograms of a soil that has varying levels of pH, P, K, Mg, and Ca. A "standard" value for pH and extractable nutrients and a standard deviation is established by analyzing this soil ten to twenty times on different days and establishing a mean value. This soil is designated as a Check sample, given a letter, and used daily in maintaining a quality control for soil samples.

The laboratory participates in the North America Proficiency Testing soil sample exchange programs. This allows the Soil Testing Laboratory to check its "Check" sample values with those from other laboratories, as well as compare analytical techniques.

In a routine, daily soil-test run, two Check samples and one Blank are placed in every group of 100 soil samples as the samples are numbered. An information sheet is included every time a Check sample or Blank is placed in the samples, and the Check or Blank is numbered and analyzed as a routine soil sample. The supervising technician is responsible for comparing the laboratory results of the Check samples and Blanks to make sure they are reading within an accepted range. By careful attention to the values of the Checks and Blanks, errors can be avoided before they leave the laboratory. Samples are held for 1 month after analysis to allow for recheck if necessary. Growers or Extension personnel may request that all or part of their analyses be rechecked for accuracy. This allows for a monitoring of quality control as well as maintaining confidence in the lab.