Abstract
ABSTRACT This study tested the hypothesis that the limiting values of S-index, proposed in the literature can not be used to determine the degradation condition of soils under soybean cultivation in the state of Pará, Brazil. The objective was to determine limiting values of S-index using soil physical attributes and validate it with soybean root growth, in soils with different textural classes. For the experimental design, the following treatments were established: five compaction levels for sandy loam and sandy clay loam soils and three levels for clayey and very clayey soils. The following physical parameters were analysed: particle-size distribution, soil bulk density, critical soil bulk density, degree of compaction, soil-water retention curve, S-index and relative root length of soybean. The limiting values of S-index varied according to soil textural class and were equal to 0.037, 0.020 and 0.056 for sandy loam, sandy clay loam and clay, respectively. The S-index does not apply to soils with clay content > 71%, because it does not vary with the degree of compaction of the soil or the root growth of the evaluated crop.
Highlights
Material and MethodsThe S-index was proposed by Dexter (2004a, b) as an indicator of soil physical quality with sensitivity to identify changes in its physical properties and, its state of degradation (Dexter, 2004a)
Calculated from the slope of the soil water retention curve at its inflection point, the theory for the index is based on the fact that most soil physical properties are controlled by characteristics of its structure, which in turn is defined by the pore size distribution in the matrix
With the increase in degree of compaction (DC), there was a significant decrease in θs in the classes sandy loam (17% of clay) and sandy clay loam (21% of clay), with no variation of θr and small variation for the parameters α and n, which are responsible for the shape of the curve
Summary
Material and MethodsThe S-index was proposed by Dexter (2004a, b) as an indicator of soil physical quality with sensitivity to identify changes in its physical properties and, its state of degradation (Dexter, 2004a). Many authors show that S-index is a useful tool to estimate various soil physical properties, such as porosity, bulk density, resistance to root penetration, organic matter, among others. These authors suggest that S values allow the direct comparison between different soils and the effects of treatments and management conditions (Dexter & Czyz, 2007; Asgarzadeh et al, 2010; Delgadillo et al, 2011). Dexter (2004a) and Dexter & Czyz (2007) proposed that an adequate root growth requires S-index values > 0.030, values between 0.020 and 0.030 promote low root growth and, for S < 0.020, there is no root development, regardless of the type of soil and plant species. Soils with limitations to soybean root growth will pose limitations to less tolerant crops
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