A micro‐pipette method for soil mechanical analysis

Abstract Determination of soil texture, particularly the clay (<2 μm) fraction, is an important measurement in most soil investigations. In this study a modified method for mechanical analysis was evaluated that eliminated the need for bulky laboratory equipment and long settling times associated with standard pipette and hydrometer methods. Termed a “micro‐pipette”; method, the modified procedure uses 2 to 4 g soil with 40 mL of dilute dispersant, shaken overnight in 50 mL centrifuge tubes. Clay is determined by sampling 2.5 mL using an adjustable volume pipettor from a depth of 2.5 cm after approximately 2 h of settling, as calculated from Stokes’ law. The dried suspension weight is used to compute clay content after correction for salt content of the dispersant. Sand can be determined by sieving at 50 μm after clay analysis, with silt calculated by difference. Using 12 soils with a range of particle sizes, the proposed method was found to give textural values nearly identical to those found with the standard pipette method. Variability between replicates was higher for clay content with the use of 2 g in the micro‐pipette method, but was not different from the standard pipette when 4 g were used. Variability was also higher in the micro‐pipette sand determination with both soil weights, although was still within acceptable limits (coefficient of variation < 4%). The major advantage of the method is that one person can run analyses on 50 soils per day (including duplicates) in limited laboratory space with standard laboratory equipment. The method is also useful for determining water‐dispersible clay in soils, and for measuring sediments in runoff and natural waters.