Abstract

Compacted and well-structured sites, on a Vertisol used for irrigated cotton production, were compared using a range of procedures for assessing soil structural form. The techniques considered were: visual/ tactile assessment using the SOILpak score, shear strength, penetration resistance, core bulk density and air-filled porosity, clod shrinkage analysis, and image analysis. All of the methods under consideration distinguished compacted soil from well-structured soil, including core bulk density, which had previously been thought of as a poor measure of soil structure in Vertisols. The simple SOILpak scoring procedure was shown to have great potential as a tool for land managers. Soil factors known to directly affect root growth, mechanical impedance and aeration, correlated well with the indirect procedures. The cheaper methods (SOILpak score and core bulk density determination) related most strongly to shear strength at the reference water content (plastic limit). The SOILpak score was more strongly related to air-filled porosity than clod shrinkage analysis or image analysis; shear strength had the same predictive power as the SOILpak score for estimating air-filled porosity. However, an advantage of using resin-impregnated soil monoliths and the SOLICON image analysis system is that the samples are big enough to represent a large proportion of the root-zone. Staining of the soil with a Rhodamine dye solution provided images that were too coarse for the routine determination of SOLICON parameters, but it is a useful and rapid procedure for highlighting those macropores that are connected with the soil surface. Clod shrinkage parameters underestimated soil air-filled porosity where compacted aggregates were interspersed with finely aggregated soil.