Soil physics

If soil is physically healthy, plants grow better, water is used efficiently, and the soil can recover from stress.

Healthy soil physical characteristics ensures:

• efficient water infiltration and retention.
• strong root growth and plant support.
• active soil biology and nutrient cycling.
• resistance to erosion, compaction, and waterlogging.

Healthy soil structure and porosity equals happy plants and productive fields.

Soil structure

Soil structure describes how sand, silt, and clay particles are arranged and held together.

Well-structured soil crumbles easily (friable), has low bulk density, and high porosity.

Poorly structured soil may form clods or dust, restricting root growth, water movement, and biological activity.

In other words, a soil that crumbles easily and isn’t too hard is good for roots, water, and soil life.

Factors influencing soil structure: 

• organic matter
• soil biota (earthworms, roots, fungi)
• clay content
• management practices such as compaction or cultivation.

Soil breaks into units called aggregates (or peds) that contain organic matter and pore spaces. 

• macropores (>0.08mm) allow water infiltration and root passage,
• mesopores (0.08–0.03mm) store plant-available water
• micropores (<0.03mm) store water mostly unavailable to roots.

Larger pores help water drain and roots grow; medium pores hold water plants can use; tiny pores mostly store water that’s hard for plants to reach.

Aggregate stability

Keep organic matter high and avoid salty soils. Your soil will stay crumbly and healthy instead of collapsing into dust or mud.

• Aggregate stability is the soil’s ability to resist disintegration under stress (rain, machinery).
• Stable aggregates maintain pores for water and air, while unstable aggregates may slake or disperse.
• Organic matter and fungal networks strengthen aggregates, while excess sodium (sodicity) can collapse structure.

Soil strength

Timing matters; avoid working soil when it’s too wet or too dry to prevent damage and compaction.

Soil strength is the resistance to deformation, influenced by moisture. Dry soils resist deformation but may restrict root growth. Wet soils are weaker and more prone to compaction.

Bulk density and compaction

Heavy, compacted soil makes it hard for roots to grow and water to move — don’t drive on wet fields!

Bulk density measures soil weight per unit volume. It is inversely related to porosity. Sandy soils typically have higher bulk densities (1.3–1.7 g/cm³), while well-structured clays range from 1.1–1.6 g/cm³. Bulk densities above 1.6 g/cm³ can restrict root growth, water movement, and biological activity. Compaction is greatest in wet soils or under heavy machinery.

Soil porosity

Soil porosity refers to the spaces within and between aggregates and soil particles. The amount of pore space determines the amount of water a given volume of soil can hold. In a well structured soil the porosity of the top 10-15cm will be 50%. 

For a given volume of dry soil the mineral and organic matter component occupies half the space and the remainder is air. When wet, these air filled spaces can potentially hold water. Porosity may range from 30-60%, sandy soils have lower porosity and clay soils tend to have higher porosity.

The soil profile is composed of a range of pore sizes of differing origin which have different functions.

Pore size and pore size distribution can change regularly within a soil. Natural wetting and drying of soil, biological activity and artificial (mechanical) soil processes create and destroy pores and change other attributes of soil aggregates. 

Possible effects of soil processes on pore size distribution.

Practical tips

Treat your soil gently, feed it organic matter, and let roots and earthworms do some of the hard work.

• Avoid working wet soils or driving heavy machinery on them.
• Grow diverse plants with strong roots to naturally create macropores.
• Maintain organic matter through mulches, compost, or cover crops.
• Minimise tillage and use controlled traffic to reduce compaction.
• Monitor bulk density and observe soil response after rain.
• Encourage soil fauna like earthworms to create natural pores.