Guideline Index

Chapter 4: Soil Properties

4.4 Biological Properties

In this section:

4.4.1 Living Organisms

Many living organisms are found in healthy soil, from large creatures, such as earthworms, to the smallest bacteria. Soil organisms help to decompose organic matter. The burrowing habit of the larger organisms incorporates the organic matter into the soil and also creates large pore spaces that aerate the soil and allow faster water infiltration. The smaller organisms, such as bacteria, actinomycetes, fungi, yeasts, algae and protozoa, further decompose the organic matter, which releases nutrients in a form that plants can use.

Living organisms are an important fraction of the soil. Their presence is encouraged by high organic matter levels, adequate soil moisture, and good drainage and aeration.

In a healthy soil, the domestic animal weight above the ground surface is substantially exceeded by the weight of the organisms living in the soil. For example, the earthworms alone can weigh from 100 kg/ha to more than 1500 kg/ha, depending on the suitability of the soil for earthworm survival (Brady & Weil 1999). A normal population of fungi weighs between 1000 and 15,000 kg/ha (Brady & Weil 1999). Please refer to Chapter 5 for further information.

4.4.2 Organic matter

Organic matter is anything that is living or the remains of a living thing. However, in the context of soil composition, organic matter is a build-up in the soil of decayed plant and animal residues.

Organic soils, such as peats, contain from 20% to as much as 95% organic matter. Mineral soils contain anywhere from a trace to 15% or 20% organic matter. Organic matter is composed of about 57% organic carbon.

Australian mineral soils contain up to 10% of organic matter, but most range from 1% to 7%. However, the influence of organic matter on soil properties, and consequently on plant growth, is much greater than this small portion might indicate.

The benefits of organic matter in the soil include improving soil structure and increasing the nutrient and water holding capacity of the soil. Organic matter also provides a food supply for soil biology. Soils with low organic matter can have ‘poor’ structure, hold little water, and erode or leach nutrients easily. The exception is cracking clay soils where clay minerals have the main effect on structure. Soils with high organic matter levels have ‘good’ structure, good water-holding capacity, and reduced erosion and nutrient leaching.

Organic matter plays a key role in nutrient cycling in the soil – Refer to Chapter 5 for more information.

When the organic matter is fully broken down, one of the things that is left is humus. Humus ranges in colour from brown to black, and the intensity of its colour is influenced by climate (rainfall and temperature) rather than by the amount of organic matter in the soil.

Humus has some useful qualities in that it adsorbs nutrients, adsorbs much higher quantities of water than clay can, and improves soil structure due to its low plasticity and good cohesion. Thus, organic matter also plays an essential role in maintaining a loose, friable soil structure.