THE MEASUREMENT OF SOIL STRUCTURE - SOME PRACTICAL INITIATIVES

Soil structure has many aspects, and it is difficult to measure and characterize, particularly in soils where structure changes with wetting and drying. There is a need for soundly based methods robust enough to be used routinely on a range of soils. Such methods should also provide parameters for use in soil, hydrological and agronomic process models to enable the effect of soil structure to be simulated. This paper describes five methods and presents some results of indices which are proving useful in soils studies. An index of aggregate breakdown is derived from the proportion of sizes < 0.125 mm formed in the soil surface under simulated rain. This index closely relates to infiltration and erodibility in the field. Disc permeameters, operated at a range of tensions, offer a convenient way to identify the various size classes of larger pores that are available to transmit water into soil under low energy wetting. The presence of many pores in the 0.7-3.0 mm range is linked with farmer observations of better soil structure in the field. It has been used to compare treatments and provide soil hydrologic parameters in soils that do not seal or are protected from heavy rain. In Vertisols, characterizing structure degradation and also the propensity for self-mulching present difficulties. Indices derived from clod shrinkage studies can discriminate between different treatment histories and different field soil structure. The proportion of oriented clay can also be used to diagnose soil structural damage by farming operations. Self-mulching is an important phenomenon in some Vertisols which is not, in general, well understood. Further work is needed to validate and improve each of the indices reviewed. Studies using the methods outlined could provide sufficient understanding of structural processes for them to be modelled. Key criteria for use in system simulations and quantitative land evaluation might then be measured (or modelled) by simplified, routine methods.