Total root length per unit ground area (La) is often considered to be directly related to the amount and rate of water uptake. Experiments were conducted to compare the water use of spring wheat, barley, lupin (L. angustifolius) and field pea on four differing soil types in drought-stressed conditions. The La values of cereals were consistently five to ten times as large as those of grain legumes, whereas the aboveground biomass was sim~iar and never greater than twice that of the grain legumes. Growing-season water loss (WL) from the soil profile was very similar for wheat and lupins, despite this big difference in root length. Soil evaporation may have been greater under lupins, but when crop water uptake was compared for the period when leaf area was greatest, rates of change in soil water content within the root zone were still similar and were not well correlated with La. Specific root water uptake (Ur) was consistently greater for lupin than wheat. Maximum rooting depth was better correlated with WL than was La in all cases. Higher Ur values in lupin and pea may be related to their large and abundant metaxylem vessels, which give much lower axial resistance than in cereals. These results provide strong evidence for genotypic variation in root morphology, density and root extension between dicotyledenous and monocotyledenous species. They also indicate that La is not necessarily the root morphological characteristic most responsible for efficiency of water uptake in drought-stressed environments.

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We report experiments comparing several legumes with the aim of distinguishing genotypic from environmental differences in those root parameters which influence water use and nutrition. Temperate crop and pasture legumes were grown on three xeric psamment soils at latitude 29�S. in Western Australia to evaluate the relative root characteristics of different species. Sites had 428, 298 and 155 mm rainfall in the (winter) growing season. Maximum root depth was significantly different (P < 0.001) between genotypes, but non-significant between sites. Genetic control for maximum root depth is thus implied. This has significance for water use and plant production of these low water-storage sandy soils. Lupinus spp. extended their roots to an average 190 cm, Pisum sativum L. to only 65 cm, Triticum aestivum L. to 113 cm. The pasture species ranged from Vicia benghalensis L. (75 cm) and Trifolium spp. (69 cm) to Medicago spp. which averaged 92 cm. Lupins and wheat had less than 50% of their total root length (cm-1) in the top 20 cm, whereas other species all had over 70% of roots in that layer. Pasture legumes with high root density in that layer tend to dry sandy topsoils frequently, reducing nutrient availability and hence vegetative growth. Root density averaged only 0.045 for lupins, compared with 1.0 cm-2 for clovers. All total root lengths were low at maturity, compared with more favorable environments. Nevertheless, they were calculated to be adequate for water and mobile nutrient extraction from these sandy soils.

In the fourth consecutive year of a long-term tillage trial in a loamy sand in Western Australia above- and below-ground growth of a wheat crop (cv. Gamenya) was monitored. The grain yield was nearly 20% greater in the crop sown after ploughing than in those crops sown by direct drilling. A similar trend had been noted in each preceding year. Cumulative water use was 15 and 9 mm less with zero and minimum tillage than with ploughing. The ploughed treatment had lower soil strengths, not only in the seedbed but also to 50 cm in the subsoil, until 8 weeks after seeding. These lower soil strengths coincided with more-rapid wetting of the subsoil under ploughing, and faster root extension rates. Dry matter production was significantly greater in the ploughed crop from the initial sampling date, and direct-drilled crops were not able to compensate in the later part of the season, despite using slightly more water during that period. The slower wetting of the profile under direct drilling was a persistent feature over several seasons in this soil type. It was considered responsible for the increased soil strength and slower root extension rates. The implications of direct-drilled sandy soils for crop productivity, and drainage control of recharge soils in saline catchments, are discussed.

After 3-8 years' continuous cropping with conventional cultivation and direct drilling, five Australian soils were examined for changes in structural stability attributable to tillage methods. Some increase in organic carbon was found in four direct drilled soils, and the proportion of carbon in the greater than 2 �m fractions was also higher in these soils after short ultrasonic treatment. Direct drilled soils had significantly greater stability with at least one of the stability tests used, but no soil showed consistently greater stability to five different tests. One direct drilled soil was more stable than its ploughed equivalent after alkaline and neutral salt extraction of metal ions and associated humic substances. One direct drilled soil was more dispersed by polysaccharide extraction. Three direct drilled soils retained greater stability after selective extraction of trivalent metal ions complexed to humic fractions. Structural improvement takes place in Australian soils after several years of direct drilling, but may be at a slower rate and to a lesser extent than has been reported for wetter environments.