Why Do Soils Consolidate And Need Loosening?

Why do soils consolidate?

All soils consolidate over time. They do so naturally, under the combined influences of moisture and gravity. Soil consolidation/density is greater with depth because of the pressure exerted by the weight of soil above any particular point. This pressure is called overburden pressure and it is calculated from the knowledge of the bulk density of soil and its moisture content above the depth of interest.

Whilst the overburden pressure increases with depth soil density does not. Soil density reaches a maximum for any particular soil type when the friction between soil particles (called internal friction) can withstand the overburden pressure imposed by the soil above.

These natural consolidation processes are substantially enhanced by compaction caused by tractors, harvesters, cultivation implements and grazing animals. All of these add substantially increased overburden pressure to the soil and create abnormally compact densities beneath their tracks.

Cultivated Soils Consolidate
A consolidated soil with very compact subsoil and a shallow and unstable topsoil

How does water affect consolidation?

The influence of soil moisture on the consolidation process is twofold:  it adds weight to the soil, increasing the overburden pressure; and it reduces the friction between particles.  This interaction explains why saturated or flooded soils become very dense, which is most often noticed only when the soil has dried.

Water plays another very influential role in soil consolidation.  When water is added to soil as rainfall or irrigation it is absorbed into soil pores by capillary forces within and between aggregates or clods.  This absorption process weakens and breaks bonds between soil particles within aggregates or clods and builds pressure on air trapped inside them.  The weaker bonds and pressure of entrapped air cause aggregates and clods to break apart, and the smaller aggregates and individual particles settle into a denser matrix with a substantially reduced porosity.

The dispersed waterlogged surface of the soil in the previous photo

What are the consequences of consolidation on plants?

As soil consolidates the largest pores are lost first, its density increases and the volume of pore space (water holding capacity) declines (Figure 1).  Also, as pore sizes become very much smaller, plant extractable water and water transmission properties decline very rapidly (Figure 2).

The consequences of soil consolidation for plants are:

  • Less water infiltrates and what does moves very slowly over less distance
  • Less water drains from the soil
  • Less water is available for plants to extract
  • There is insufficient air for plants roots and soil organisms to respire (oxygen depletion)
  • Penetration resistance is too large for roots to proliferate.

As a result, plant roots concentrate in a shallow depth of soil at the surface, and plants grow more slowly, are prone to droughting and yield less.

What are the advantages of a deep, stable, loose rooting zone?

The advantages of a deep, loose seedbed are the reverse of the degradation caused by consolidation:

  • More water infiltrates and moves quickly over large distances
  • More water drains quickly to deeper soil layers
  • Less water evaporates from the soil
  • More water is available for plants to extract
  • There is ample air for plants roots and soil organisms to respire
  • Roots grow and proliferate more easily
  • Increased root growth reinforces the loose soil and minimises consolidation
  • Increased root growth produces larger plants and yields

How can a loose, deep seedbed be created and maintained?

A patented machine called a Deep Seedbed Conditioner (DSC) has been developed and tested over many years, on many different soils, in rainfed and irrigated farming systems. The DSC machine can be used on the broad-scale, flat seedbeds of rainfed cropping, and on the bed-and-furrow cropping systems used for irrigation and/or waterlogging prevention.

The DSC machine slices through roots at a depth of 250 to 300 mm and lifts, open and drops soil without displacing it, and the preserved root systems reinforce the loose soil minimising or even preventing consolidation.

Details of recommended manufacturers and field-scale research data supporting all the claims of improved soil conditions and productivity can be found by visiting the website of Maximum Soil and Water Productivity Pty Ltd at https://maxswp.com.au

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