The Mwendwa Protocol: A modification of the Bouyoucos method of soil texture analysis

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Abstract Background This study aimed to modify the Bouyoucos method of soil texture analysis by proposing the best sample treatment and analysis protocol that would give the most accurate results. The Bouyoucos method lacks sample pre-treatment whereby samples are dispersed for only 2 minutes after being soaked in 5% sodium hexametaphosphate (calgon) for 15 to 20 hours. In this study, the Pipette method was used as the standard method due to its increased precision and reproducibility. Different treatments modified from the Bouyoucos procedure were compared with the Pipette method aiming to identify the treatment whose soil texture proportions were closest to those obtained through the Pipette method. The treatments involved variation in the concentration of the digesting material during sample pre-treatment, concentration of the dispersing material, time of hydrometer readings, method of agitation before taking the first hydrometer reading and method of dispersing. Obtained data was analyzed statistically using Genstat and SPSS. Results Percent sand obtained through the Pipette method significantly correlated with that obtained through Shaking at the 0.01 level (r = 0.862; P-value = 0.001) and that obtained through Shaking + Stirring at the 0.05 level (r = 0.737; P-value = 0.015). The sand measurements made using the hydrometer variations overestimated the sand fraction in the soil samples. None of the Bouyoucos treatments estimated the percentage silt with sufficient accuracy. There was a positive correlation in the clay proportion between the Pipette method and Shaking + Stirring treatment (r = 0.644, P-value = 0.044) at the 0.01 level. There was a negative correlation between the clay proportion obtained using the Pipette method and no-digestion treatment (r = -0.234). The 4%Calgon treatment was negatively correlated with the Pipette method (r = -0.712; P-value = 0.021) at the 0.05 level. Treatments involving shaking and increased concentrations of hydrogen peroxide and calgon estimated the clay proportion with sufficient accuracy. Conclusions These observations are indicative of the need for soil pretreatment with hydrogen peroxide and also the use 10% calgon as the dispersing agent during soil texture analysis. A modification to the Bouyoucos method was proposed and dubbed The Mwendwa Protocol.

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