Revisiting soil texture analysis: Practices towards a more accurate Bouyoucos method

Abstract

The Bouyoucos method lacks sample pre-treatment whereby samples are dispersed for only 2 min after being soaked in 5% sodium hexametaphosphate (calgon) for 15–20 h. This study aimed to improve the accuracy of the Bouyoucos (Hydrometer) method of particle size analysis by proposing the best sample pre-treatment and analysis practices. The Pipette method was used as standard reference due to its precision and reproducibility. Treatments modified from the hydrometer method were compared with the Pipette method through Pearson's correlation. The treatments involved variation in the concentration of the pre-treating and dispersing material, time of hydrometer readings, method of agitation and that of dispersing. The proposed improvement suits the Food and Agriculture Organization (FAO) and United States Department of Agriculture (USDA) standard 2–50 μm system. Measurements made using the hydrometer variations overestimated the sand fraction in all samples. There was a positive correlation in the clay proportion between the Pipette method and Shaking + Stirring treatment (r = 0.644, p-value = 0.044). Treatments involving shaking instead of stirring, increased concentrations of hydrogen peroxide and calgon estimated the clay proportion with sufficient accuracy. These observations are indicative of the need for soil pre-treatment with hydrogen peroxide to remove organic matter and also the use 10% calgon as a dispersing agent.