Modification of the Pipette Method of Mechanical Analysis

In the International grade scale. only four size classes are recognized. These are No. I, 2.0 to 0.2 mm; No. II, 0.2 to 0.02 mm; No. Ill, 0.02 to 0.002 mm; and No. IV, less than 0.002 mm. While seven size classes are recognized in the U. S. Dept, of Agriculture classification, only one of them — clay _ less than 0.002 mm is used in the International classification. For this reason means of obtaining the quantity of soil in classes I, II, and III of the International classification had to be introduced into the analytical procedure used by the U. S. Dept, of Agriculture. To obtain International class I, a new sieve was introduced into the set of sieves ordinarily employed (6). This sieve, with square openings 0.177 mm on a side (U. S. Standard No. 80), was selected on the basis of a calibration curve of sieve openings in terms of settling velocity of quartz particles. Its size agrees very closely with the sieve adopted, for the same purpose, by the Agricultural Education Association of Great Britain (i). To obtain International Class III a pipetting is made for particles less than 0.020 mm. A Lowy 25-0; automatic pipette is used, the pipette tip being placed at a depth of 10 cm below the surface of the liquid, as is done ordinarily for clay particles (6), and the time of settling calculated from Stokes law corresponding to the suspension temperature. Details of this type of calculation have been given before (6). International Class IV is obtained by pipetting for particles less than 0.002 mm diameter. This is the pipetting ordinarily made for "clay" in the regular U. S. Department of Agriculture analysis. Finally, fractional! is obtained by difference. The sum of the weight of fractions I, III, and IV is subtracted from the weight of the organic matter-free sample. Fig. i shows the relation between the size classes of the U. S. Department of Agriculture classification and those of the International Classification. This figure is a copy of the mechanical analysis report card used by the U. S. Dept, of Agriculture. Ordinary routine analyses are reported for both classifications, although no particular use is made of the International classification data at the present time other than for comparison with analyses reported in this scheme by others. It should be noted that data reported in the manner shown in Fig. i may be expressed very easily in terms of the grades used before January i, 1938. For example, the quantity of soil less than 0.005 mm diameter, formerly called clay, is given directly at the lower left hand corner of the card. By subtracting this figure from the sum of the present silt and clay classes, the quantity of material between 0.050 and 0.005 mm is obtained. This fraction was formerly called silt. The coarser fractions remain the same as before and no calculations are necessary. Certain other changes in the mechanical analysis procedure have been made from time to time since the method was originally described (6). These changes may.be seen conveniently from Fig. 2, which is a flow sheet of the revised method. First, the moisture content of the sample is determined directly on the sample for mechanical analysis