Abstract
The dispersion of the samples in soil particle-size analysis is a fundamental step, which is commonly achieved with a combination of chemical agents and mechanical agitation. The purpose of this study was to evaluate the efficiency of a low-speed reciprocal shaker for the mechanical dispersion of soil samples of different textural classes. The particle size of 61 soil samples was analyzed in four replications, using the pipette method to determine the clay fraction and sieving to determine coarse, fine and total sand fractions. The silt content was obtained by difference. To evaluate the performance, the results of the reciprocal shaker (RSh) were compared with data of the same soil samples available in reports of the Proficiency testing for Soil Analysis Laboratories of the Agronomic Institute of Campinas (Prolab/IAC). The accuracy was analyzed based on the maximum and minimum values defining the confidence intervals for the particle-size fractions of each soil sample. Graphical indicators were also used for data comparison, based on dispersion and linear adjustment. The descriptive statistics indicated predominantly low variability in more than 90 % of the results for sand, medium-textured and clay samples, and for 68 % of the results for heavy clay samples, indicating satisfactory repeatability of measurements with the RSh. Medium variability was frequently associated with silt, followed by the fine sand fraction. The sensitivity analyses indicated an accuracy of 100 % for the three main separates (total sand, silt and clay), in all 52 samples of the textural classes heavy clay, clay and medium. For the nine sand soil samples, the average accuracy was 85.2 %; highest deviations were observed for the silt fraction. In relation to the linear adjustments, the correlation coefficients of 0.93 (silt) or > 0.93 (total sand and clay), as well as the differences between the angular coefficients and the unit < 0.16, indicated a high correlation between the reference data (Prolab/IAC) and results obtained with the RSh. In conclusion, the mechanical dispersion by the reciprocal shaker of soil samples of different textural classes was satisfactory. The results allowed recommending the use of the equipment at low agitation for particle size- analysis. The advantages of this Brazilian apparatus are its low cost, the possibility to simultaneously analyze a great number of samples using ordinary, easily replaceable glass or plastic bottles.
Highlights
Soil texture is based on different combinations of sand, silt, and clay separates that define the particlesize distribution of a soil sample (Gee & Or, 2002)
The acceptable range depends on the coefficient of variation (CV) of the results for each particle-size fraction for each soil sample, according to the following criteria: a) for coefficients of variation (CV)¬!¬40 %, the acceptable range is the average ± 1.0 standard deviation (s) calculated from the results obtained by all laboratories; b) for CV between 20 and 40 %, the acceptable range is the average ± 1.5 s, and c) for CV < 20 %, the acceptable range of results is the average ± 2.0 s
These data sets of particle-size analyses determined independently and as a part of a soil analysis quality program, were considered appropriate to draw conclusions about the accuracy and on the precision of the results obtained by mechanical dispersion with a reciprocal shaker (RSh)
Summary
Soil texture is based on different combinations of sand, silt, and clay separates that define the particlesize distribution of a soil sample (Gee & Or, 2002). Particle-size analysis is defined as a measurement of the size distribution of the individual (primary) particles in a soil sample, according to texture fractions in a given classification scheme (Baver et al, 1972; Gee & Or, 2002). The size limits of the three main fractions of soil particles sand, silt and clay are given by diameter ranges, according to different scales. Silt consists of medium-sized particles, with diameters from 0.02 to 0.002 mm (ISSS) or from 0.05 to 0.002 mm (USDA), and the clay fraction contains the smallest soil particles, with diameters below 0,002 mm or 0,2P on both scales (Gee & Or, 2002)
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