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Understanding Soil
Terminology
What is soil structure ?
Soil structure is the aggregation of soil particles (sand, silt, clay and organic matter) into granules, crumbs or blocks. It is the shape that the soil takes based on its physical, chemical and biological properties; it is very important since (along with texture) it affects drainage and aeration capacity of the soil.
Without structure a soil collapses and compacts, resulting in a number of problems for landscaping and civil projects. It can be considered as the “framework” of a Soil Profile.
What is a soil profile ?
Soils develop over long periods of time and generally become deeper and develop distinct layers or horizons. A soil profile is made up of three layers: topsoil; subsoil and parent material.
Topsoil
This thin layer (usually less than 30cm) is normally the most fertile because of the organic matter that has accumulated from plant and biological activity. In this layer, the majority of the feeder roots are present which take up nutrients.
Subsoil
Usually lighter in colour as it does not contain as much organic matter as topsoil. This layer can be composed of varying proportions of clay, sand, silt and stones.
Parent Material
This is the original material from which the soil is developed. This layer has deposits of sand, gravel, pebbles, boulders and rock in various mixtures. Why is structure important ?
Virtually all soils need an open structure through the soil profile in order to function effectively as a growing medium. In particular, soil structure influences the main soil and plant root functions: aeration; drainage; root development. Without structure, soils will suffer from anaerobism, waterlogging and nutrient lock-up and, ultimately, plants will die!
What is compaction ?
Compaction destroys soil structure because it increases the density of the soil by packing the particles closer together; this causes a layer within the soil profile that is impregnable to plant roots, water and air. It can occur in the topsoil and the subsoil layers.
Topsoil is easier to restore than subsoil but the latter cannot be ignored as rooting alone will not break up or restructure a compacted subsoil.
Main causes of compaction
- misuse of heavy equipment
- trampling or trafficking over soils
- handling soils when wet and plastic
- stockpiling soils inappropriately
Danger signs of compaction
Anaerobism and waterlogging are the biggest soil-related causes of plant failures in landscaping projects but both can be avoided with the right ground preparation.
How you can detect compaction
- ponding of water on the surface
- resistance experienced when pushing a soil probe or spade into a soil
- black, anaerobic layer with a sour odour in a turf-rooting layer
- uneven plant growth
- yellowing leaves
- waterlogging
- poor root development and shallow root systems
- anaerobism
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