Soil Shrinkage Curve Research Articles

THERESA: II. Thickness variations of vertisols for indicating water status in soil and plants

THERESA (Transferts Hydriques Evalués par le REtrait des Sols Argileux) is a method for estimating the water content of vertisols in the field by measuring the vertical deformation of the solid. Deformation is controlled by changes in the water content of the clay matrix, and only the matric water component of the total water content can be determined with this method. The relationship between matric water storage or reserve ( Sm and Rm(i)) calculated from deformation data and three indicators of water status in the soil and plants were studied, so as to find out more about the actual meaning of this parameter in terms of water uptake by plants. The three indicators used were: total water storage and reserves ( S and R( i)) which control the amount of water available, relative transpiration ( Ep ETM ) which is the outcome of the balance between water availability and plant requirements, and the growth rate of an organ ( ΔL Δt ) which indicates how water stress affects plant growth. Evaluating matric water contents from changes in the thickness of soil layers (measured with THERESA transducers) was tested in three settings: in grassland from 1985 to 1988 and during two sugar cane growth cycles from 1989 to 1991. By measuring vertical solid displacements and the gravimetric water content, the soil shrinkage curve in the field and changes in the thickness of soil layers were available. These were then used for removing the bias in water storage calculation introduced when the changes in bulk density and thickness are ignored. Matric water contents measured with THERESA corresponded to a low availability domain. Relative transpiration ( Ep ETM ) and stem growth rate ( Δ Δt ) decreased significantly as soon as the matric reserve was drawn from. These results give confirmation that matric water contents measured by THERESA are an appropriate indicator for rationing water and for monitoring irrigation in swelling clay soils.

Indexes of residual shrinkage to quantify the comparative effects of zero and mechanical tillage on a vertisol

Differences in the soil physical condition of the surface layer of a Vertisol (near Warwick, Queensland) under a zero versus mechanical tillage trial were investigated by using indices derived from a limited part of the soil's shrinkage curve. Shrinkage data were generated by coating initially air-dry clods in SARAN resin, then oven-drying them in six temperature increments to 105�C. Values of both the slope and y-axis intercept of a straight-line fit to the data were significantly greater in the mechanically tilled treatment. This showed that mechanical tillage led to decreased bulk density (increased porosity) and caused the soil to have greater shrinkage potential.

Shrinkage Curve Indices to Quantify Cultivation Effects on Soil Structure of a Vertisol

AbstractDifferences in selected parameters and derived variables of models fitted to soil shrinkage curves were used to investigate soil structural degradation of an irrigated Pellustert in northern New South Wales, Australia. Significant differences between clods taken from the same depths in land prepared for cotton (Gossypium hirsutum L.) when “dry” and “wet” were found in specific clod volume at zero water content, the slope of the line in the normal shrinkage zone, the airfilled specific pore volume throughout normal shrinkage, and the range in water content in the structural shrinkage zone. These indicators showed that this soil prepared wet exhibited increased bulk density, decreased porosity, loss of structural pores, and tended to have weak structural development.