A field method for determing percentage of coated sand grains

Several USDA-NRCS hydric soil field indicators require estimation of the pe rcentage of organic-coated sand grains (black grains). For example, to meet the Dark Surface field indicator the soil layer must contain at least 70% coated (black) grains. Field experience has Shown that the estimation of th e percentage of coated sand grains is often subjective and highly variable from one soil scientist to another. In order to overcome this variability a set of standards was created using a mixture of black and light gray (repr esenting uncoated grains) sand grains. Weighing out each component for the desired ratio and mixing them in a 47-mm-diam. petri dish we made a set of three standards consisting of 50, 70, and 90% black grains, To test the eff ectiveness of these standards, soil scientists estimated the percentage of coated grains from similarly prepared samples first without the use of the standards and then with the use of the standards for comparison, Individual s improved the accuracy of their estimates by 10 to 60% and their hydric so il identification by 16%, The standards are easily prepared, easy to use, a nd portable.