Abstract
ABSTRACT: The least limiting water range is a soil physical quality indicator, which is useful to predict the optimum water range for plant growth in a given soil and to study the effects of soil use and management over this optimum water range by integrating the effects of available water, penetration resistance and air filled porosity. This study tested six equations to fit water retention and penetration resistance surface responses used to determine the least limiting water range and present a simple algorithm written in the open source software R for fitting, calculation and visualization of the least limiting water range. Five soils from Brazil and Canada, under different use and management conditions were used to test the functions. The results show that the three water retention surface responses had good statistical properties for fitting water retention and that two of the penetration resistance surface responses were adequate to fit the data, while one failed to achieve convergence in two instances. The open source code performed as well as the commercial statistical package SAS for fitting the penetration resistance and water retention equations.
Highlights
Over the last decades, there has been a search for indicators of soil physical quality able to integrate soil physical parameters in a single index that can be measured and correlated with crop production factors (Dexter, 2004; Letey, 1985; Silva et al, 1994)
The results show that the three water retention surface responses had good statistical properties for fitting water retention and that two of the penetration resistance surface responses were adequate to fit the data, while one failed to achieve convergence in two instances
Least Limiting Water Range (LLWR) is sensitive to changes in soil air filled porosity (AFP) and soil resistance to penetration (SR) and integrates the traditional concept of available water defined as the water content between field capacity (FC) and permanent wilting point (PWP) in a particular soil
Summary
There has been a search for indicators of soil physical quality able to integrate soil physical parameters in a single index that can be measured and correlated with crop production factors (Dexter, 2004; Letey, 1985; Silva et al, 1994). The Least Limiting Water Range (LLWR) is an indicator of soil physical quality and has been used and validated under different crop production systems (Lima et al, 2012; Silva et al, 1994; Wu et al, 2003; Zou et al, 2000). In the implementation of the method by Silva et al (1994), AFP, SR, PWP and FC are defined as a function of soil bulk density (Db), thereby creating an index that, once parameterized, can be used for monitoring soil physical quality by measuring soil bulk density alone. The objectives of this research were to: i) evaluate the fitting properties and statistical quality of three soil water retention and three soil resistance to penetration equations to calculate LLWR, and ii) present a code developed in the open source software GNU R (R Development Core Team, version 3.0.1 “Good Sport”) for fitting the relevant equations and creating a LLWR plot
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