SOIL PARTICLE SIZE FRACTIONS DETERMINED BY GAMMA-RAY ATTENUATION


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Abstract

Soil texture-or the particle size fractions of sand, silt and clay-is an important 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 size fractions are conventionally determined either by the pipette or the densimeter method. The gamma-ray attenuation technique has also been used to determine the particle size distribution (PSD) curve and the particle size fractions of soils. Because it takes about 20 min to perform a complete analysis 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 quickly than the original procedure of gamma-ray attenuation. Instead of measuring a complete PSD curve, the cumulated percentage of particles in only two positions of the sedimentation container are measured for specific sedimentation times related to the limits of sand/silt (50 μm) and silt/clay (2 μm). Because this new procedure allows the particle size fractions of 10 soil 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 were each analyzed with the new gamma procedure and the pipette method. Although the average standard deviation of all particle size fractions for the gamma 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 texture. The larger deviations are due mainly to the statistical behavior of gamma ray emission. The new gamma method yielded an acceptable linear correlation with the pipette method (r2 = 0.976) for all particle size fractions.We propose that the new procedure should be used routinely for particle size analysis. Its advantages are associated with automation of the analysis, decreasing time and work for handling samples (sieving, collecting, weighing, drying, measuring temperature) and minimum influence of the operator expertise.

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