Particle size distribution by Bouyoucos in slightly alkaline soils from department of Cordoba, Colombia

The aim of this study was to evaluate the Bouyoucos method, at different stirring times and using two chemical dispersants, for particle size determination and distribution in slightly alkaline soils of Cordoba, Colombia.  Eight soils were mechanically dispersed applying a slow stirring; 60 rpm with five stirring times as follows: 2, 4, 6, 10 and 16 hours and a control experiment at 0 h. Chemical dispersion of samples was performed using two dispersants: Calgon (0.058 molL-1 (NaPO3)6 + 0.075 molL-1 Na2CO3) and a mixture of Calgon and 0.1 molL-1 Na4P2O7 at pH 9.1, which was proposed by authors. In addition, particle size distribution was determined by Pipette method, which was used as reference method. Tests of means and regression models were performed for sand, silt and clay contents using SAS statistical software. According to the results, significant differences were not found between 10 and 16 h of stirring, therefore maximum state of dispersion had achieved at stirring time of 10 h. Content of sand fraction showed an exponential decreasing which can be described following model given by y=a+be^(((-x)⁄c)), with determination coefficients higher than 0.90. Clay fraction showed exponentially increasing behavior of first order given by y=a-be^(((-x)⁄c)) with determination coefficients greater than 90%. Soil textural classification was performed by Bouyoucos method using the proposed dispersant at 10 h of stirring was 75% similar to that determined by Pipette method whereas for Calgon, was 62.5%.