Chapter 5 - Effects of Compaction on Soil Microstructure

Abstract

Microstructure encompasses all aspects of soil structure that are revealed when soil structure is examined at a magnification of x5 or greater. The microstructure plays an important part in determining the properties and behaviour of soils as it regulates and influences many processes in the soil, such as physical processes, decomposition and transport processes, and the functioning of soil organisms and plant root systems. The architecture of the microstructure can be divided into several levels: (1) the macroaggregation; (2) the macroporosity in aggregates and non-aggregated materials; (3) the packing of primary particles in the groundmass; (4) the arrangement of clay particles. Changes in microstructure due to compaction occur at all these levels. They can be subdivided into changes in microporosity and changes in particle arrangement in the groundmass. During compaction, either by cultivation activities or by field traffic, voids are reduced in size and changed in shape, they may become deformed or disrupted, or disappear completely. Generally, the larger voids are reduced in size and more smaller voids, lacking interconnections, result. Consequently, soil properties and soil behaviour change considerably. Soil fauna and plant rooting are mainly responsible for the establishment of continuous void systems. Compacted conditions in the subsoil can be removed temporarily, but often recompaction occurs within a few years, resulting in reduced permeability to water and air, due to changes in the pore size distribution and pore continuity. Compacted soil conditions can also be due to disintegration of soil aggregates at the surface, which can lead to the formation of compact, laminated crusts which hamper infiltration and germination, or can result in filling and blocking voids below the surface. Consequently, total porosity is reduced and most of the continuous voids disappear. When the soil is subject to a load, such as by farm machinery, changes occur also within the groundmass. The arrangement of the primary particles collapses, whereby fine material will be squeezed between larger sand and silt grains. Mechanical deformation, such as that occurring during tillage, causes shear failure characterized by realignment of particles. Changes in the arrangement of particles and degree of realignment can be quantified and linked with the exerted load or shear stress and/or with changes in soil properties, such as, for example, permeability. Microscopic studies provide considerable insight into how soils compact, and they explain why and when soil properties and behaviour change. Already a number of processes can be analysed but more research is needed before all processes involved will be fully understood.