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

The shear strength of a Vertisol under a broad range of compaction conditio ns has been related to 'non-limiting water range' (NLWR), 'partially limiti ng water range' (PLWR), and 'least-limiting water range' (LLWR) estimates f or the growth of cotton roots. These factors indicate the soil water conten t range that land managers should aim to maintain so that root growth limit ations caused by excessive hardness and poor aeration are minimised. The pr oportion of macropores available for root extension as the bulk soil become s too anaerobic and/or hard for their growth has been quantified via a re-a ssessment of published data that relate oxygen flux density to air-filled p orosity. A shear vane was shown to be better than a penetrometer for distin guishing compacted and non-compacted soil under a broad range of soil water contents on a Vertisol used for irrigated cotton production. Published cri tical limits for cotton root growth based on penetrometer data are of limit ed 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 sc ore, core bulk density, a clod shrinkage parameter, and a SOLICON image ana lysis factor.