Plant and Soil 149: 149-158, 1993. © 1993 KluwerAcademic Publishers. Printedin the Netherlands. PLSO 9748 Field evaluation of laboratory techniques for predicting the ability of roots to penetrate strong soil and of the influence of roots on water sorptivity S.A. MATERECHERA, A.M. ALSTON, J.M. KIRBY 1 and A.R. DEXTER 2 Department of Soil Science, Waite Agricultural Research Institute, The University of Adelaide, Glen Osmond 5064 South Australia. 1CSIRO Division of Soils, G.P.O. Box 639 Canberra ACT 2601, Australia and 2Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK Accepted 25 August 1992. Accepted in revised form 10 December 1992 Key words: compaction, field evaluation, plant species, root penetration, sorptivity Abstract The ability of two laboratory screening techniques to predict the abilities of roots of eight crop species to penetrate a compacted soil were evaluated and compared in a field experiment. A soil tilled to remove the effects of mechanical resistance was planted with the same species to serve as a control. Depth of root penetration, root density and the influence of the roots on the sorptivity of water were measured. Roots of all species penetrated deeper in the deep tilled than compacted soils. There were differences in the ability of roots of the species to penetrate the compacted soil. Generally dicotyledonous species had more roots penetrating to depth in both the compact and deep tilled soils. Within the main species classifications, lupin and safflower (dicotyledons) and oats and barley (monocotyledons) had the highest penetration into the compacted soil. Water sorptivities in the deep tilled soils were higher than those of the compact soil. Soil from planted treatments had higher sorptivities than soil which had not been planted. This is attributed to biopores left by the roots. Sorptivities of soils which had dicotyledonous species were generally higher than those of monocotyledons. The soil planted with safflower produced the highest sorptivity in the compacted layer (0.1-0.3 m). A comparison of the accuracy of the two laboratory screening methods in predicting the field penetration of roots suggest that the method involving mechanical stress was better than that involving osmotic stress. Relative root diameter was found to be a better indicator of the penetration ability of roots than relative root elongation. Introduction There is extensive evidence to show that soil compaction limits root penetration and may restrict the ability of the plant to absorb water from the subsoil, particularly in semi-arid en- vironments (Holloway, 1991; Voorhees, 1992). When roots cannot penetrate deeply because of compaction, crops are susceptible to drought and consequent yield reduction occurs (Hhkansson et al., 1987; Oussible et al., 1992). Deep tillage is one way to treat soil compaction to promote deep rooting (e.g. Coventry et al., 1987). How- ever, crop responses to deep tillage have often been variable and the soil may recompact to densities equal to or even greater than before (Mead and Chan, 1988). A potential alternative to deep tillage which