A healthy soil has effective nutrient cycling, good water infiltration and storage, and provides food and shelter for soil organisms. Soil health is affected by and contributes to land condition; that is, the capacity of a landscape to respond to rain and grow pasture. Best management practices for soils are those that manage pastures for sustainable long-term forage production.
The productive capacity and resilience of soils are determined by their inherent physical, chemical and biological characteristics and by how these characteristics are managed. Understanding the strengths and vulnerabilities of particular soil/land types, allows for better management decisions regarding grazing and infrastructure development to be made.
Landholders generally recognise differences in soil properties and behaviour across their property. The most critical aspect of understanding soils is recognizing and identifying soil properties beneath the surface and knowing the consequences if the surface soil is disturbed.
Land resource mapping and information (land types, soil types, and regional ecosystems) provides information on soil properties and in most extensive grazing situations this information is all that is required for planning grazing management and property development. Where sown pastures are being developed, soil testing should be undertaken to assess if the soil is suitable for the proposed pasture species and the fertility is adequate. In forage cropping and intensive pasture systems, soil tests are commonly used to assess the availability of soil nutrients and plan fertiliser application. When undertaking soil tests, it is important to used accredited laboratories and follow the collection instructions to ensure representative samples are obtained.
This module has five key areas to address soil characteristics and their implications for grazing management.
Key area 1 – Soil types
Extensive mapping of land systems and soil types across Australia has been undertaken. The scale of this mapping is variable, and so the ability to obtain specific information on soil types within individual properties is variable.
Land type mapping is a good starting point for identifying the soil types on a property. ‘Land types’ are areas of grazing land characterised by consistent patterns of soil, vegetation and topography. They are typically identified by their native vegetation, for example: poplar box flats, spotted gum ridges, or mountain coolibah open woodland; however their productivity is underpinned by their underlying soil.
Land type descriptions include an overview of the principal soil type and references to where more detailed soil information and mapping can be found. NRM groups and state agencies can provide land and soil type mapping and supporting information.
There are many soil classification systems that can be used to determine different soil types. Soils may be broadly grouped into three categories:
Uniform soils – behave in a similar manner from the surface through to the bottom of the soil profile. They may be uniform sands, uniform loams or uniform clays.
Gradational soils – generally have a lighter surface and gradually increase in clay content throughout the profile. The distinction between soil layers is difficult to see on visual inspection.
Texture contrast soils – display an abrupt change between the surface soil and the subsoil characteristics e.g. sandy topsoil and clay subsoil.
Key area 2 – Soil physical properties
The physical properties of a soil largely determine the ability of the soil to capture rainfall, store water and supply it to the pasture or forage. Soil physical properties therefore influence how effectively pastures can grow and the resilience of the landscape. Understanding the strengths and vulnerabilities of particular soil/land types, allows for better management decisions regarding grazing and infrastructure development to be made.
Key area 3 – Soil chemical properties
Key area 4 – Soil biology
The living and dead biological components of the soil drive many of the processes that keep soils productive and resilient. About 85% of the biological component is made up of decomposing organic matter (plant and animal), 10% is live plant roots, and around 5% is in the form of living micro- and macro-organisms (soil biota). The microbiota includes bacteria, fungi, green and blue-green algae, protozoa and nematodes. The macrobiota includes earthworms, termites, dung beetles and other insects. In fertile soils, the biomass of micro-organisms alone can exceed 20 t/ha.
Soil organisms help maintain soil fertility and health by regulating nutrient cycling, maintaining soil structure and interacting with plants in the ecosystem. Healthy populations of soil organisms require adequate supplies of plant organic matter, which is their main source of food.
Soil organic carbon levels are determined by the balance between the amount of plant material grown and returned to the soil and the rate at which the soil organic matter is broken down by microbes. Soil organic carbon tests can provide a guide to the soil’s organic matter status.
High ground cover and soil organic matter levels have many benefits including: increased rainfall infiltration; reduced runoff; greater soil water holding capacity; protection from excessive soil temperatures and reduced evaporation. These significantly increase rainfall use efficiency to grow plants and increase soil organic carbon levels.
Soil organic carbon and the population of soil organisms are maximised by growing productive pastures, avoiding overgrazing and maintaining high levels of ground cover. Much of the extra organic material will break down to benefit soils (support soil microbes, supply nutrients, maintain structure) but the remainder can improve soil carbon levels. However, soil type and climatic conditions determine how much carbon can be stored. In lighter soils, carbon is more easily broken down and the process is more rapid where there is more rainfall and heat.
Key area 5 – Fertiliser use
In higher rainfall intensive pasture systems and where forage cropping is undertaken, fertiliser application is often a critical part of management. Soil testing provides the information needed for deciding whether fertiliser is needed, which fertilisers to use and application rates. Test strips can be used to assess the response of pastures and forage crops to fertiliser application.
Application method, timing of application and proximity to waterways and wetlands need to be carefully considered to minimise the risk of fertiliser loss and the movement of nutrients off-site. Fertiliser application is regulated in certain catchments such as those flowing into the Great Barrier Reef. Up-to-date records need to be kept of the fertilisers stored on a property and of fertiliser use – where, when and how much.
Fertilisers should be stored so they are kept dry and free from contamination and cannot leach into waterways.