Soil compaction caused by animal treading in grazing of pastures has not been considered a serious problem in Australian soils. However, recent circumstantial evidence suggests that in northern Victoria compaction does occur. In an experiment conducted at the Hamilton Pastoral Research Station (western Victoria) in 1973, grazed pastures with various stocking rates showed increases in bulk density and bearing capacity of the soil, and decreases in hydraulic conductivity occurred with increasing stocking rate. Some change in pasture composition was also noted.

Most studies of plant roots growing in soil involve destructive sampling or involve estimates of root growth only at a soil‐viewing surface interface where growth conditions are different from those in bulk soil. This research tested the feasibility of using preferential attenuation of thermal neutrons by roots to overcome these two disadvantages of current methods. Radiographs were obtained of soybean [Glycine max (L.) Merr.] and corn (Zea mays L.) roots growing through bulk samples of unsaturated soil. Plants were grown at 22 C in loamy sand soil wetted to 9% volumetric water content. The soil was contained in either 2.5‐ or 5.0‐cm thick aluminum boxes. At irregular intervals, the sample containers were transferred into a 6° diverging 0.04eV (thermal) energy neutron beam obtained from the Ames Laboratory Research Reactor, U. S. Energy Research and Development Administration, Iowa State University, Ames, Iowa. The samples were exposed to the neutron beam for 8 to 10 min. This exposure allowed about 5 × 109 neutrons/cm2 to strike an indium collector plate attached to the rear of the container. After the collector plate was removed from the neutron beam, photographic film was attached to the collector plate for 1 ½ hours; then the film was processed. The preferential neutron scattering by the roots allowed elongation rates of soybean radicles or seminal roots of corn to be determined easily through either 2.5‐ or 5‐cm thick soil samples. Small lateral roots of either species were less clearly distinguishable and resolution must be improved before elongation rates can be measured for laterals with a diameter smaller than 0.33 mm. Plants were not harmed visibly by the thermal neutron fluxes used in these experiments.

A field experiment was made to determine water uptake patterns of soya beans and water use and to compare relative effectiveness of roots at various depths in the profile.Depth of water extraction by the root systems increased with rooting depth. Water uptake rates decreased with soil water content at all soil depths and the soil water content at which roots extracted almost no water increased with depth. The maximum rate of uptake of the deep roots was greater, per unit of length, than shallow roots. INTRODUCTIONWater percolates below normal plant rooting depth annually in many areas of the world. Stewart et al. (1975) estimate that 28 mm or more of water annually percolates beyond the rooting depth of maize in about half of the continental United States. Even where deep percolation occurs, plants periodically suffer water stress because their evaporation exceeds precipitation and stored water that is available in the normal rooting depth. Studies by LaRue, Nielsen & Hagan (1968) and by van Bavel, Brust & Stirk (1968) show that some water moves upward into the root zone from below, but that quantities axe relatively small when compared with total evaporation. If normal rooting depth could be increased, water that percolates beyond the rooting zone might then be used to reduce the intensity and duration of the stress.A programme was begun during 1973 to determine the feasibility of deepening the rooting zone of soya beans (Olycine max (L.) Merr.) to reduce plant water stress. Objectives of the present study were to determine (a) water extractions patterns, (6) total water use, and (c) relative effectiveness of water uptake per unit of length of roots, at various depths in the soil profile.

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