Soil particle size fractions determined by gamma-ray attenuation

Soil texture-or the particle size fractions of sand, silt and clay-is an im portant characteristic of soils and is used for their characterization and classification. Textural analysis also helps agronomists and farmers in the recommendation of herbicide and fertilizer applications. Soil particle siz e fractions are conventionally determined either by the pipette or the dens imeter method. The gamma-ray attenuation technique has also been used to de termine the particle size distribution (PSD) curve and the particle size fr actions of soils. Because it takes about 20 min to perform a complete analy sis using the gamma-ray technique, only 25 to 30 samples can be analyzed in 1 day. A new procedure is presented to determine the soil particle size more quick ly than the original procedure of gamma-ray attenuation. Instead of measuri ng a complete PSD curve, the cumulated percentage of particles in only two positions of the sedimentation container are measured for specific sediment ation times related to the limits of sand/silt (50 mu m) and silt/clay (2 m u m). Because this new procedure allows the particle size fractions of 10 s oil samples to be determined in approximately 1 hour, as many as 80 samples can be analyzed in 1 day. Triplicate soil samples of different textures we re each analyzed with the new gamma procedure and the pipette method. Altho ugh the average standard deviation of all particle size fractions for the g amma procedure was greater (1.6%) than that for the pipette method (0.7%), the new procedure appears to be an acceptable method for analyzing soil tex ture. The larger deviations are due mainly to the statistical behavior of g amma ray emission. The new gamma method yielded an acceptable linear correl ation with the pipette method (r(2) = 0.976) for all particle size fraction s. We propose that the new procedure should be used routinely for particle siz e analysis. Its advantages are associated with automation of the analysis, decreasing time and work for handling samples (sieving, collecting, weighin g, drying, measuring temperature) and minimum influence of the operator exp ertise.