Cotton root growth in a compacted Vertisol (Grey Vertosol) - I. Prediction using strength measurements and 'limiting water ranges'

The shear strength of a Vertisol under a broad range of compaction conditions has been related to 'non-limiting water range' (NLWR), 'partially limiting water range' (PLWR), and 'least-limiting water range' (LLWR) estimates for the growth of cotton roots. These factors indicate the soil water content range that land managers should aim to maintain so that root growth limitations caused by excessive hardness and poor aeration are minimised. The proportion of macropores available for root extension as the bulk soil becomes too anaerobic and/or hard for their growth has been quantified via a re-assessment of published data that relate oxygen flux density to air-filled porosity. A shear vane was shown to be better than a penetrometer for distinguishing compacted and non-compacted soil under a broad range of soil water contents on a Vertisol used for irrigated cotton production. Published critical limits for cotton root growth based on penetrometer data are of limited value. Nevertheless, these data have been converted for use with a shear vane. The NLWR and PLWR estimates have also been related to the SOILpak score, core bulk density, a clod shrinkage parameter, and a SOLICON image analysis factor.