Soil Strength

Soil Colour | Soil Particles | Bonding and Aggregation | Porosity | Changing Soil Structure | Soil Strength

What gives soil its strength?
The mechanical strength of soil is an important concept in considering (and predicting) soil behaviour.

We use strength to represent the reaction of a soil to an applied force. High-strength soils resist deformation (compaction especially), break-up (shearing and shattering), and slippage. However, high-strength soils also resist root penetration and exploration.

Strength is imparted to a soil by virtue of:

cohesive forces between particles; and
frictional resistance met by particles that are forced to slide over one another, or move from interlocked positions.

Why is soil strength important?
A healthy soil has complex strength requirements. It wants to be a stable structure to support vegetation and a certain level of traffic, and protect the vast array of pores and fissures. But at the same time it wants to support cultivation by the farmer, earthworks by soil fauna and the exploration of roots, water and gasses. It wants robust friability. It does not want to mimic concrete.

Soil specialists have developed the term consistence (see following) to help deal with this complex need.

How is soil strength affected by water content?
Clay type and content influence strength through their role in cohesion. Water content significantly modifies their cohesion. As water content increases – cohesion decreases. This is because increasing water content causes greater separation of clay particles (and thus easier slippage) and further, causes softening of soil cements.

The following graph illustrates the consequences:

Compactibility of soil can be used as an example of how moisture content, strength and management can interact. More severe compaction occurs with traffic on wet soils than dry soils. One passage of a tractor over wet soil, caused an equivalent reduction in infiltration rate to four passages over dry soil. The dry soil has much greater resistance to structural change because it has a higher strength.

This is demonstrated by the by the ‘critical moisture content curve’, where for a given compactive force, the compactive effect increases as moist content increases up to a point where the soil becomes so wet that compactions drops off. The ‘critical moisture content’ for compaction occurs at the peak of the curve – this is most undesirable moisture content for trafficking on the soil. The critical moisture content of any soil will depend on numerous factors including soil texture and organic matter content.

Further information is available elsewhere on this site – see compaction and traffic.

Thus water content of the soil, via its effect on soil strength will affect:

Structural stability
Erosion
Tillage
Root penetration
Trafficability
Susceptibility to landslip

What is consistence?
Consistence is a useful way to approximate to soil strength by testing the amount of force required to just cause the breaking or deforming of a 20 mm piece of soil (as ped or aggregate) when a "compressive shearing force" is applied between thumb and forefinger. The standard ratings used applied to a dry sample of soil, are:

Loose	No force required. Separate particles such as loose sands
Very weak	Very small force almost nil
Weak	Small but significant force
Firm	Moderate or firm force
Very firm	Strong force but within the power of thumb and forefinger
Strong	Beyond the power of thumb and forefinger. Crushes underfoot on a hard flat surface with small force
Very strong	Crushes underfoot on a hard flat surface with full bodyweight applied slowly
Rigid	Cannot be crushed underfoot by full body weight applied slowly

Increasing moisture content will cause quite a change of response to a given application of thumb force – hence if you are using this to compare changes in paddock soil condition, make sure you do it at a standard moisture content – and "dry" is the easiest to standardise.

Consistence is a useful indicator of changing soil condition.

How does the strength of soil affect productivity?
Soil strength will influence aggregate stability and soil structure.

High soil strength, depending on where in the soil profile, can:

Inhibit seedling emergence (soil crusts and plough layers have a high structural strength)
Limit root penetration

Low soil strength soils are:

Susceptible to compaction (both by machinery and stock)
Susceptible to erosion (both wind and water)

How do you measure soil strength?
Soil engineers use a range of methods to measure soil strength. The usual way is to apply a shear force and record the force required to cause the soil structure to break or fail. But as soil managers we are probably interested in deformations (e.g. disaggregation, compaction, or plough smearing) more than in block failure.

Penetration resistance is a useful concept as it can be used to indirectly measure soil strength and readily pick-up variation within soils. Further, it can assess the magnitude of crusting, compaction and the occurrence of plough pans. A probe is pushed into soil and the resistance to its entry is the penetration resistance. It is convenient to think of it as what a root (or an adventurous worm) might encounter as it explored a body of soil. In fact, there is a strong negative relationship between the rate of root elongation and penetrometer resistance.

A simple penetrometer indicates soil strength and moisture content

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