J.M. Kirby

Changes to the physical properties of soils puddled for rice during drying.

Abstract Puddling is used to prepare soil for irrigated rice ( Oryza sativa L.) throughout SE Asia creating a soft mud often over a plough pan. Whilst these conditions are favourable for the rice crop, they are less so for any dryland crop, such as mungbean ( Vigna radiata (L.) Wilzek) or soybean ( Glycine max L. Merr.), grown in the dry season (DS) after rice harvest leading to low or erratic yields. The physical properties of soils puddled for rice were measured as they changed in the period after draining flood water for rice harvest. The experiments were run at four sites in Indonesia and the Philippines for 4–6 weeks, during which they were kept free from weeds. Apart from one site, where there were heavy rain showers, the sites received no effective rainfall during the experiment. Soil moisture content and hydraulic potential in the upper 40xa0cm were measured regularly in five replicate plots. Evaporation from the soil was measured using mini-lysimeters. Strength properties were measured using a penetrometer. Cracks were measured using an intercept technique. Hydraulic conductivity was calculated using a modification of the instantaneous profile technique. The sites behaved in a similar fashion, with initial loss of water by drainage, followed by loss by evaporation from the surface. However, the low conductivities limited the upwards supply of water for evaporation so that evaporation from the soil surface decreased. This resulted in strong drying of the upper 5–10xa0cm, but much smaller decreases in moisture content lower down. The strength of the upper layers increased as they dried. The low conductivities suggest that waterlogging due to heavy rain is likely to cause problems for dry season cropping. However, the preservation of water in the sub-soil may allow flexibility in the sowing dates for dry season crops to avoid periods of heavy rain or shortage of labour.

Drying of some Philippine and Indonesian puddled rice soils following surface drainage:: Numerical analysis using a swelling soil flow model

Abstract Heavy clay soils used for paddy rice production in SE Asia are usually puddled to grow the rice crop. Soil conditions following puddling and drying might be unsuitable for following dry season crops, such as mungbean or soybean, which often produce poor or erratic yields. We analysed water movement in the drying process in order to better understand the drying behaviour in order that we may generalise the findings, and to examine the prospects for using a swelling soil flow theory in predicting that behaviour. A numerical solution to a general, material coordinate-based, one-dimensional flow equation for swelling soils was used to analyse experimental results from four sites in Indonesia and the Philippines. The soils at the four sites were a Ustic Epiaquert, a Typic Ustropept, an Aeric Tropoquept and a Chromic Epiaquert. In the experiments, evaporation, cracking and changes to moisture ratio (ratio of volume of moisture to volume of solid) and void ratio were monitored for several weeks following surface drainage. Hydraulic properties were also measured. The numerical predictions of changes to moisture ratio compared well to the measured results. Predictions of cracking were based on assumptions about the ratio of vertical to lateral shrinkage, and reasonable comparisons were obtained with the measurements. The analysis showed that the measurement of soil evaporation by mini-lysimeters provided a good estimate of the overall soil evaporation at the wet end but underestimated evaporation from dry soil. The analysis also showed that the measurements of void ratio in the experiments were sometimes in error, because of difficulties in accurate volume sampling of very weak, wet soils. It is shown that in these swelling systems it was better to use material coordinates and moisture ratios rather than spatial coordinates and volumetric moisture contents.

A recording system for measuring in situ soil stresses due to traffic

Abstract An inexpensive, reliable and rapid data capture system is described for the in situ measurement of stresses imposed on the soil by agricultural traffic. Data were recorded at rates of 200 readings s −1 from each of eight earth pressure cells in the field. Recorded data adequately defined stress waveforms underneath vehicles for most field operations. Rates of up to 1500 readings s −1 on each of eight channels or 12 000 readings s −1 on one channel can be used if necessary. Stresses recorded in the field under several agricultural vehicles are presented.

Cohron sheargraph data: interpretation using critical state soil mechanics

Abstract Critical state soil mechanis was used to explain the differences in stress paths recorded in sheargraph tests in weak, strong and medium-strong soils. It was projected that the cohesion of both peak and ultimate strength lines would be near zero for weak and medium-strong soils, but the peak cohesion would be high for strong soil. The friction angle of the peak strength line was projected to be lower than that of the ultimate line for strong soils and less than or equal to that of the ultimate line for weak and medium-strong soils. The projections about the type of stress path and the relationship between cohesion and friction for peak and ultimate strength lines were tested by performing field sheargraph tests in a Vertisol. It was found that the fraction angle of the ultimate line was higher than that of the peak line, which suggests that in loose to medium dense soils it may be more appropriate to use the ultimate line for tine design procedures that employ the friction angle in calculations. The different forms of stress path recorded in field sheargraph tests were associated with different regimes of soil behaviour in laboratory shear, in which the height change during shear was measured. Tests with strong soil resulted in sheargraph tests that showed brittle stress paths with well defined shear stress peaks, and in laboratory shear tests that showed shear accompanied by expansion. Tests with weaker soil resulted in sheargraph stress paths without well defined peaks, and in laboratory shear accompanied by compression. Shear accompanied by expansion is desirable in tillage for seedbed preparation and therefore the sheargraph may be useful in identifying the right soil conditions for tillage.

Mechanical properties of a grey clay used for rice production in Australia

Abstract Soil strength and compression properties, and the speed dependence of these properties, were measured on a cracking grey clay used for rice production in Australia. Samples from several tillage treatments of a field experiment and laboratory-remoulded samples were examined. The soil showed deformation behaviour consistent with the critical state concept of soil mechanics. In particular, the soil compressed with shear when subjected to test normal stresses that were similar to the pre-consolidation stress and expanded with shear at test normal stresses that were low compared to the pre-consolidation stress. The pre-consolidation stress of the soil was small (about 30 kPa). At faster rates of shear, the soil was stronger and expanded more than soil sheared at slower rates. Shear and consolidation tests showed that samples taken from a sheeps foot roller treatment were stronger, less compressible and denser than the control, whereas samples taken from a rotary hoe treatment were weaker, more compressible and looser than the control.

Calibration of a sheargraph

Abstract A sheargraph transmits normal and shear stresses to the soil via a spring which is compressed and rotated. The compression and rotation are recorded directly on conventional, rectilinear graph paper. However, the forces involved in compression and torsion of the spring in a sheargraph are not independent of one another. As a result, lines of equal normal or shear stress are not parallel to the axes of rectilinear graph paper. Calibration equations for the relationship between normal and shear stress and the compression and rotation of the spring contain interaction terms. This paper describes a general form for these equations which is shown to fit experimental calibration data reasonably well. For a spring which is rotated during a test in the direction of coiling (i.e. wound up), as is the case for the sheargraph used in this paper, assuming independence of shear and normal stresses leads to an underestimate of the soil shear strength compared with the correct calibration. Conversely, a spring which is rotated opposite to the direction of coiling (i.e. unwound), assuming independence of shear and normal stresses leads to an overestimate of the soil shear strength compared with the correct calibration. Tests on a sand (which had a linear failure envelope) and a silt loam (which had a curved failure envelope) demonstrated a closer agreement with shear box results when the correct calibration was used.