6.2 Soil Structure and Friability

Soil consists of five major components: mineral matter and organic matter that will generally bind together to form aggregates, air and water that will be held in the soil pores and soil organisms.

Soil structure describes how the basic soil materials (sand and clay) are arranged into soil aggregates and the pore spaces between them (see Figure 1).

Structural stability indicates how these aggregates and the pores between them are affected by wetting, rainfall and other destructive forces, eg traffic.

For soils other than sands, soil structure has an important bearing on:

• permeability of the soil to water and air,

• root penetration and of seedling emergence,

• resistance to the erosive forces of water and wind.

A well structured soil resists the erosive forces of water and wind, does not crust, allows seedlings to emerge unimpeded and can be worked over a wide range of moisture contents with minimal damage.

The proportions of sand, silt and clay have a major influence on soil structure. In sandy soils the particles do not bond together strongly and therefore have little structure, but are free draining. As the clay content of the soil increases from loam to clay loams to clays, the particles become held together more strongly and generally structural strength increases. This bonding is due to the high level of chemical activity exhibited by clay particles.

For production, good soil structure relies on the formation of small soil aggregates which do not break down when wetted, with good pore spaces between the aggregates. In the right conditions, soil particles will cluster together and become stabilized by organic matter, fungal hyphea and polysaccharides produced from root, fungi and bacterium exudants and organo-metallic complexes. In the wrong conditions, aggregates will not form, or will break apart when wetted, reforming into large, dense and impermeable clods with few pore spaces.

Therefore, soils with poor structure will probably be low in clay or organic matter and high in minerals, such as sodium, that reduce the ability of clay particles to bind together. Management practices that break down organic matter cause soil structural degradation. This is indicated by hard setting soil surfaces and crusting in tilled soil.

It should be remembered that organic matter not only helps to hold particles together, but it also helps hold them apart, creating pore spaces. 

A leading builder in the Minlaton district during the 1940s always said, when challenged about the strength of a mortar mix, “mortar’s job is to keep bricks apart not hold them together.”

Soil pores allow the movement of water, air and microbes as well as providing pathways of least resistance for root growth. The porosity (volume of pores) of a soil will vary with the texture of the soil, but in a well structured soil, pore spaces will represent 30-50% of the soil.

It must be remembered that management can radically influence organic matter levels and, therefore, soil structure and can modify the effects of soil texture and indications given by soil colour.

Structural quality of a particular soil type can be assessed by measuring soil strength, the size and stability of aggregates and by looking at pore density and root growth.

6.2 Soil Structure and Friability

[ Back ] [ Next ]