Soil is one of agriculture’s fundamental resources. Without it, agriculture would not exist as we know it. However, Australia’s soils are old and weathered, and some of our agricultural practices have degraded soils so that they become less productive and cause environmental degradation.
In recent years there has been increasing interest in the concept of soil health, which considers all aspects of soil, that is, physical structure, chemical components and biological life, rather than looking at each of these separately. A soil does not have to be agriculturally productive to be healthy. However, many agricultural practices can make soils less healthy than they were in their natural state.
By managing structure, nutrients and biology in the soil, farmers can work soils within their capability so that the soils are able to recover from agricultural disturbance without being degraded.
To maintain and improve soil health, farmers need to plan for the long term and:
NSW Agriculture has a soil health research team at Wollongbar, which is looking at soil health in orchards and developing techniques for improving soil biological life. This team has:
In June 2000, NSW Agriculture held a soil health workshop at Wollongbar to look at all the issues involved in soil health. A copy of the proceedings is available from Wollongbar. Further soil health workshops were held in Tamworth in February 2002.
There are several soil issues facing agriculture today:
Salinity is a significant issue for both soil and water quality in NSW. Increasing salt concentration in soils threatens some of the most productive farmland in NSW.
Saline soils interfere with the osmotic gradient that exists between plants and the soil environment. When the concentration of salt in the soil increases, many plants are unable to take up the water they require and the nutrients that are dissolved in this water. This directly impacts on crop production and can lead to plant death and crop failure.
Salt occurs naturally in all soils. It is the concentration of these salts in the top layers of the soil horizon that directly impacts on agriculture in NSW. Salt is concentrated in soil by water movement in the soil profile. In Australia this is mostly due to rising watertables. Watertables can rise within reach of plants’ roots and the soil surface due to the following:
In NSW 1.3 million hectares is at risk from rising watertables. The cost of lost production and damaged infrastructure in the Murray Darling basin alone due to soil salinity is estimated to be $100 million per year.
When soils become acid, some nutrients such as aluminium and manganese become more available, while other nutrients such as phosphorus and molybdenum become less available. This disrupts plant growth, which directly impacts on the productivity of crops and pastures grown in agriculture. The lack of productivity also contributes to:
If no action is taken to slow or correct soil acidification, the acid leaches into the subsoil where it cannot be reached with current farming methods. This means the soil is permanently degraded.
Some soils are naturally acid due to either the type of rock they are derived from or the regular heavy rainfall that leaches nutrients from the soil. Soils can be made acid by:
NSW has 13.7 million hectares of agricultural land seriously affected by acid soils, with another 5.7 million hectares at risk. The cost of soil acidity in lost agricultural production is estimated to be between $90 million and $225 million per year.
Acid sulfate soils are caused by the oxidation of coastal muds that contain iron pyrite. When exposed to air the pyrite produces sulfuric acid and iron, and these have severe impacts on soil and water quality, as well as plant and animal life.
Sodic soils are the 'sleeping giant' of soil management problems facing agriculture, as almost half of NSW soils are sodic and tend to degrade under bad management.
Sodic soils are so-called because they have sodium ions attached to clay particles. When these soils are wet, the particles disperse and move away from each other, which is seen as cloudy water. When the water evaporates, these individual particles settle in a solid mass, causing waterlogging and hard-setting soil crusts.
It is difficult for roots to move through these soils. Gypsum can help ameliorate these soils, but this is not economical for large areas.
Soil structure is the arrangement of individual soil particles - sand, silt, clay and organic matter - into aggregates or peds. Peds store moisture and nutrients, while the spaces between them allow water and air to flow through the soil. Soil structure can be affected by:
Structural degradation in the Murray–Darling Basin is estimated to cost $145 million annually in lost production.
NSW Agriculture has worked on ways to reduce structural degradation, including:
NSW Agriculture has produced several SOILpak advisory guides to help farmers minimise soil degradation and improve soil structure.
Compared with other soil facets, little is known about soil organisms and how their functions affect the nature of soil. However, it is recognised that they play a vital role in maintaining soil health and determining many soil characteristics. These organisms include earthworms, nematodes, protozoa, fungi and bacteria.
Living soil organisms will:
The linkages and connections between organisms in the soil are thought to be incredibly complex. The 'food web' should be considered as one part of a larger ecological food web covering the whole landscape. The interconnectedness and complexity of the soil 'food web' means any disruption to one of the functional groups or to a species within the group will have far-reaching and significant effects, even beyond the soil sphere itself.
Any appraisal of the health of a soil must take into account the wellbeing and interactions of the living communities that exist within it.
Soil organisms break down organic matter, releasing nutrients for uptake by plants and other organisms. The nutrients stored in the bodies of soil organisms are also released when the organisms die or are preyed upon by other organisms, thus preventing nutrient loss by leaching.
Soil organisms may also excrete nutrients in plant-available forms. Some organisms, such as vesicular arbuscular mycorrhiza (VAM), actively help plants to take up nutrients such as phosphorus and zinc.
Soil organisms can protect plants from diseases and pests in several ways:
Soil organisms help to maintain the stability of soils as well as enabling proper aeration and drainage. For example, excretions from microbes act as ‘binding agents’, which maintain soil structure, and earthworms are important in soil mixing. These help to prevent erosion, waterlogging and compaction problems.
In addition to the previous points, which will result in better plant growth, some soil living organisms secrete substances that act as growth promotants.
The following factors are important in ensuring the maximum benefit for the organisms that live in the soil:
Many agricultural practices can harm the organisms that live in the soil. There are management actions that can be employed to minimise harmful practices:
The cultivation of agricultural soils releases carbon dioxide into the atmosphere, contributing to the world’s greenhouse gas problems. Agriculture may be able to play a part in reducing greenhouse gas emissions by storing carbon so that it is not released into the atmosphere. This process is known as carbon sequestration and can occur in several ways:
However, there are several concerns about sequestering carbon, including: