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
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.
Figure 1: Soil aggregation is essential for good
Source: Hi-Fert, Plant Nutrition and Soil Fertility, 1997
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
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
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.
The root soil interface (Rhizosphere ) is an area of intense
Source: A Rovira
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
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.
(Click for larger image)
Ideal proportions of soil pores, their size, Function and
Source: A Class – CRC for Soil & Land
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
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