Soil stability

What is soil stability?

Soil stability refers to the ability of a soil to maintain structural integrity on wetting.

Soil consists of mineral 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. The structure of a soil depends on i) the presence of soil aggregates that help determine its overall productivity and capacity to store both water and nutrients; and ii) the proportions of sand, silt and clay.

Structural stability indicates how these aggregates and the pores within and between them are affected by rainfall, wetting, and soil management. A well structured soil resists the dispersive and erosive forces of water and erosion by wind, does not crust, allows seedlings to emerge unrestrained and can be worked over a wide range of moisture contents with minimal damage.

Soils with poor structure are sometimes low in clay, but poor soil structure is most common in relatively high-clay soils that have a dense or ‘massive’ structure that is impenetrable to root growth and these soils are difficult to work. Poorly structured soils are also often low in organic matter and high in exchange cations such as sodium that decrease the ability of clay particles to bind together. In a well structured soil, pore spaces represent 50-60% of the total soil volume.

Soil instability leads to the collapse of soil aggregates, creating layers which are hard when dry and waterlogged when moist. There are two main types of soil instability:

• Slaking
• Dispersion

Slaking

On wetting, porous aggregates rapidly absorb water and soil may swell, becoming soft and trapping air internally. With further wetting, weak aggregates collapse and break into smaller particles. This process is known as slaking.

Slaking is due to low soil organic matter and is often associated with intense rainfall hitting dry, bare soil and occurs within minutes. It can lead to blocking of soil pores normally associated with gas exchange and water movement. In naturally hardsetting soils that are unable to re-form aggregates, a crust will form when the soil surface dries.

Dispersion

Dispersion causes micro-aggregates to collapse and release clay, silt and sand particles. Smaller particles move to block soil pores, which slows internal drainage and contributes to an increased risk of waterlogging. Typically, dispersion is associated with sodic soils.

A simple way of checking for sodic, or dispersive soils, is to take 2-3 pea-sized samples of DRY SOIL and put them in a shallow container of rainwater. When in contact with water, the water turns milky as the clay disperses. It may take a number of hours (up to 20 hours) for clay, sand and silt particles within soil aggregates to separate in naturally dispersive soils.

In sodic soils, an excess of exchangeable sodium (Na⁺) attached to clay particles causes the clay in some soils to swell excessively, leading to the closure of pores within and between the aggregates. Sodic soils also have a tendency to disperse, especially when cultivated and/or impacted by rain or surface run off. Once dry, the dispersed soil particles settle in a solid mass causing a hard-setting soil crust.

Approximately 25% of Australian soils by land area are sodic and tend to degrade under bad management. The condition is commonly associated with either previously saline landscapes from which salts have been subsequently leached, but sodium has remained, or in saline soils which are often sodic by nature. This effect may be masked while the dissolved salts remain because of favourable ionic effects, however, once these salts are leached the soil becomes unstable when wet.

Sodic clay surface soils disperse in water, reducing soil strength, and making them more prone to erosion. Sodicity is most obvious in the soil surface when clay dispersion leads to crusting – but sodium concentrations are generally greatest in the subsoil where they may result in very low permeability of clay layers, thereby restricting drainage and leaching.

How to recognise soil instability

Soil crusting

One of the biggest signs of poor soil stability is soil crusting.

Soil crusting is a natural process caused by wetting and drying of weakly aggregated soil. Many surface crusting problems in agricultural soils result from the impact of rainfall on unprotected surfaces (no soil cover). Crusts are recognised as a hard surface layer up to 1 cm thick which can often be lifted off from the soil surface.

Hard-set topsoil

Soil structure breakdown caused by rapid wetting can lead to hardsetting and usually affects the entire topsoil (horizon A).

Once wet, finer particles become suspended in soil water, aggregates breakdown and soil structure collapses. As the soil dries it hardens without restructuring, becoming massive or cemented with few or no cracks, and reduced pore space.

What are the effects of poor soil stability or soil instability on plant growth?

The net effects of soil instability are often poor plant establishment, and decreased soil water entry resulting in poor plant growth and reduced crop yields.

Features of poor soil stability include:

• Surface compaction and sealing, ponding of rainfall
• Poor/patchy seedling emergence (coleoptile unable to penetrate surface crust)
• Poor/patchy crop and pasture growth
• Shoot distortion (twisted, bent, thickened)
• Excessive surface water and runoff
• Restricted gas exchange under wet conditions
• Dispersive soils prone to water and wind erosion
• High exchangeable sodium percentage (ESP)

Impacts of soil crusting and hardsetting on production

The effects of both crusting and hardsetting on seedling emergence are particularly evident when deep planting below about 10 cm, resulting in uneven germination and shoot distortion.

Hardsetting soil may also constrain the timing of cultivation leading to production losses associated with delayed sowing.

Poor soil workability

The cost of operations and machinery wear increase, and difficulties can be experienced in preparing an even seed bed. Sodic soils are prone to both crusting and hardsetting and gave rise to the term “Sunday soils” (too wet to seed Saturday, and too dry to seed Monday).

Reduced germination and seedling vigour

Surface compaction restricts germination of seedlings and may result in root and shoot distortion. Soil crusting does not slow germination but prevents the hypocotyl emerging through the hard surface layer. This often causes thickening of the hypocotyl (a condition referred to as ‘big shank’ or ’thick-legged‘ cotton), resulting in reduced seedling vigour.

Low rates of water infiltration

Structural decline results in decreased porosity, slowing infiltration of water into the soil profile particularly on soils with little or no organic cover. In summer rainfall cropping regions, for example, the infiltration rate can be halved due to the impact of high energy raindrops on the soil surface.

Decreased plant-available water capacity

Due to the decrease in water infiltration, less water enters and is stored in the soil, resulting in less plant available water and greater run-off. A strong negative relationship exists between exchangeable sodium percentage and plant-available water capacity, reducing crop yields due to high soil strength, compaction, and low-permeability (lack of air) that affects root growth.

What factors affect soil stability?

How to increase and maintain soil stability