Subsamples of a granular material contain a finite number of grains. Practical methods of subsampling do not select these grains on an entirely random basis. Both of these features contribute to the observed variability between subsamples. As an example, the theoretical coefficients of variation attributable to statistical variation between 10-g random subsamples taken from four different ( < 2 mm) air-dry soils have been calculated. These are calculated to give CVs of between about 0.002 and I per cent between particle size analysis results. In an actual comparison between four different subsampling methods, CVs of between 4 and I8 per cent were observed in the proportion of coarse sand (2-0.2 mm) in 15-g soil subsamples. Rotary subsampling and chute splitting were found to be more reliable methods of subsampling than cone and quartering or random spoon sampling. Variability between subsamples is shown to depend not only on subsampler design, but also on the precise details of the procedure adopted, and the physical properties ofthe sample itself.

Summary The hydraulic resistivity of Vicia faba L. roots grown in soil was estimated from steady state measurements of transpiration rate and leaf and soil water potentials. Root and stem axial resistivities, estimated from xylem vessel radii, were negligible. Root radial resistivity was estimated to be 1.3 • 1012 sm -I This root radial resistivity value was used to estimate root resistance to water uptake for a field crop of Vicia faba. Previously published results were used for root distribution and soil water contents at the drained upper limit (DUL) and the lower limit (LL) of extractable soil water. Soil resistance to water uptake was estimated from single root theory, using the steady rate solution. At the DUL, root resistance was about l0 s times greater than soil resistance. At the LL, soil resistance exceeded root resistance for depths less than 0.3 m, but for depths greater than this soil resistance was smaller than root resistance. Estimates of possible uptake rates at given leaf water potentials indicated that overall soil resistance had a negligible influence upon uptake, even at the LL. The reliability of this result is examined in detail.It is concluded that over the complete range of extractable soil water contents soil resistance per se would not have limited water use by this crop. This conclusion may also be valid for a wide range of soil and crop combinations.

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