Soil chemistry

Healthy soils are essential for productive farms, biodiversity and clean water. 

They provide:

• the foundation for food and fibre production
• nutrition for plants, animals and people
• habitat for most of the world’s living organisms
• carbon storage and climate regulation
• water storage and filtration.

Physical, chemical and biological aspects of soil are closely linked. Improving one often benefits the others. For example, better structure promotes plant growth, which increases biological activity and nutrient cycling.

How soil forms

Soil develops slowly as rock breaks down and mixes with organic material from plants and animals.
Over time, weather, climate and biology transform rock into the soil that supports all land-based life.

Five main factors influence soil formation:

• Parent material – the type of rock, such as basalt, granite or sandstone, determines how much sand, silt and clay the soil contains.
• Living organisms – plants, insects, mosses and bacteria release organic acids that break down rock and release minerals.
• Climate – rainfall and temperature affect how fast rocks weather. In high-rainfall areas like the Northern Rivers, nutrients have been leached from the soil over thousands of years.
• Time – the longer a soil is exposed to wind, rain and biological activity, the more its minerals and texture change.
• Topography – position in the landscape influences depth and fertility. Ridge soils are usually shallower, while valley soils are deeper, higher in clay and more fertile.

Soil composition

All soils contain a mix of:

• Mineral particles (sand, silt and clay)
• Organic matter
• Water
• Air

How these elements combine determines a soil’s texture, structure, porosity, chemistry and colour.

Soil texture

Texture describes the relative proportions of sand, silt and clay.

Texture is determined by the soil’s parent material and cannot be changed by management; however, it affects how:

• easily water moves through soil
• well nutrients are retained
• plants establish roots.

Learn how to use the ribboning method to determine soil texture.

Soil structure

Soil structure describes how sand, silt and clay particles are arranged into clusters called aggregates, or peds.

Organic matter and soil organisms bind these particles together, creating pores that store air and water.

A well-structured soil is friable – it crumbles easily, allowing air and water to move freely and roots to grow deeply.

Structure strongly affects soil aeration, drainage, nutrient availability and biological activity.

Managing structure

Practices that improve soil structure

• Increase soil organic matter
• Grow perennial plants and green manure crops
• Support soil biological activity

Practices that damage structure

• Excessive tillage.
• Heavy traffic when soil is wet.
• Overgrazing.
• Declining organic matter.

Soil porosity

Porosity is the amount of space between soil aggregates.

In a healthy topsoil, about half the total volume (50%) is pore space and is filled with air or water depending on moisture levels.

High porosity improves water infiltration and storage, supports healthy roots and promotes biological activity.

Clay content

The chemical fertility of a soil is shaped by its parent material, clay type and formation processes.

Clay minerals influence a soil’s ability to hold and exchange nutrients — known as its cation exchange capacity (CEC).

Soils with higher clay content often have greater nutrient-holding potential, but can also compact more easily, reducing aeration and nutrient availability.

Sodicity and acidity in soil

Soil constraints like acidity and sodicity can significantly impact pasture and crop productivity. 

These chemical limitations affect soil structure, nutrient availability, and water movement—ultimately reducing yields and carrying capacity.

• Soil acidity develops more rapidly on high-producing or lighter soils and can interfere with legume nodulation and species sensitive to low pH.
• Sodicity affects soil structure and water dynamics, making it harder for plants to establish and grow. Many soils in the Murray region have sodic subsoils, and some also have sodic topsoils.

These constraints can often be managed with:

• lime to correct acidity
• gypsum to improve sodic soils.

Choosing the right product and approach depends on your soil type and cost-effectiveness. Ongoing trials under the National Landcare Program are exploring the best strategies to reduce these constraints and improve pasture performance.