Superiority of the Bouyoucos method with sand weighing and the Robinson pipette with a dispersant mixture for quantifying clay in variable-charge acid soils

The Bouyoucos hydrometer method is commonly used to determine the proportional sand, silt, and clay fractions in Peruvian soils. However, this approach often yields variable and imprecise results, particularly in acid soils with variable-charge. This study aimed to evaluate the efficiency of three alternative methods, the Bouyoucos hydrometer method including sand weight (Bhm-sw), the Robinson pipette method (Rpm), and the Robinson pipette method using a mixture of dispersing agents (Rpm-md), compared with the standard Bouyoucos hydrometer method (Bhm). Soil samples were collected from the first three horizons of profiles developed on the Sarayaquillo, Chonta, Chambara, and Oriente Group lithostratigraphic formations. The data was first tested for normality using the Shapiro-Wilk test. Non-parametric data were transformed into parametric variables through quantile normalization, then used analysis of variance and Dunnett’s test to compare the alternative methods with the standard method. Method efficiency was assessed by the highest recovery of the clay fraction. The results showed that the standard Bouyoucos hydrometer method was not efficient for clay fraction determination, as Bhm-sw (p < 0.01) and Rpm-md (p < 0.05) significantly outperformed it, increasing the clay fraction by 15.51% and 11.55%, respectively. These findings suggest both methods may be suitable for laboratory use in other soil classes common in the Peruvian Amazon. The efficiency ranking of the evaluated methods was Bhm-sw > Rpm-md > Bhm = Rpm.