Western Australian soils
Western Australia (WA) has many different soil types across its diverse, complex, natural and modified landscapes. Our soils are biologically and chemically diverse, highly variable in texture and distribution and often nutrient-deficient. They have differing degrees of capability for food production and other land uses.
To understand soil health, it is important to consider:
- Soil function as a medium for plant growth, regulator of water supplies, recycler of raw materials, habitat for soil organisms, and landscaping and engineering medium.
- Soil physical characteristics: Macro structure - presence of compaction, aerobic and non-aerobic status / ratios
- Chemical and nutrient status – relative to plant and microbial needs, including pH
- Biological activity - quantity and diversity of microbiota and flora
- Landscape degradation (as a function of soil and landscape functions) including vulnerability to erosion and dryland salinity
- Paddock history are influencers of soil biology, structure and the type and severity of land degradation
- The physical characteristic of soil (micro flora, biota, exudates and humates is generally described as having three mineral component parts
- WA soils often have little silt because our soils are ancient. Water repellence is a by-product of organically- (plant and animal) derived hydrophobic residues (from decomposition processes) that coat the soil particles
- Sandy soils are also at greater risk of soil acidity and nutrient leaching.
- Agricultural processes can result in the removal of nutrients from the soil and increased rates of soil acidification.
Soil health challenges in Western Australia
- Acidifying soils
- Dryland salinity
- Soil loss from wind and/or water erosion
- Compaction and structural decline
- Off site impacts including leaching
- The heat of bushfires can kill soil biology. Plant matter is converted to ash and charcoal- which increases the soil fertility in the short term with the increased availability of a range of nutrients (including potassium and lime) and associated soil disturbance encouraging weed species infestations.
- Water repellance
- Extended periods of waterlogging and inundatio
Soil organic matter
Organic matter is the biological component in soil that includes living macro and micro biota and plant material, but also dead / decomposing components and associated plant exudates and animal waste.
All soils have capacity for increasing and storing soil organic matter (SOM). Some soils will accumulate carbon content faster because of texture, pH, moisture availability and temperature. Others are often slower to accumulate carbon where the soil is acid and there is little clay present.
Soil Organic Matter (SOM) comprises about 58-60% organic carbon. Soil organic carbon (SOC) has the components of humic and fulvic acids in particulate forms that adhere to clay particles. The remaining mass comprises water and other soil nutrients such as nitrogen, sulphur, and potassium.
In successive dry years, where there may also be lower vegetative ground cover, they may not support a progressive increase or retention of soil carbon.
Benefits include providing nutrients and improving:
- soil structure
- drainage
- cation exchange capacity
- water retention
Improving soil health
Improving the health and condition of soil, supports the health of crops and pastures, animals and the environment, and can include practices such as:
Groundcover
Maintaining diverse vegetative ground cover and increasing total plant biomass (and increasing organic matter inputs) is important for soil surface protection to build and retain soil organic matter and improve water holding capacity. Roots can also support mobilising carbon deeper into the soil profile.
Increased plant diversity and increased plant cover year-round also promotes soil biological activities. Plants and plant residues on the soil surface can moderate temperature extremes by keeping soil cool in hot weather and insulating against heat loss in cold weather. The cover provided by plants and plant residues also minimise water loss from the soil that assists in temperature regulation. Soil temperatures of 40 C or above slow the growth of microorganisms in the soil. If soil temperatures reach 60 C or more, microorganisms die. The optimal temperature range for soil microbial activity is 10 C to 24 C.
Depending on soil colour and texture, bare and exposed soil surfaces can be hotter than air temperature by as much as 15 C.
Minimum or no-tillage can minimise surface soil compaction, improve sub surface soil porosity, and increase water infiltration. Conventional cultivation breaks down soil aggregates and exposes previously protected organic matter to microbial activity and increases decomposition. It can also change soil biota and flora populations and disrupt soil fungi.
Liming to manage soil acidity for plant growth will promote the activity levels of the soil micro flora and biota. Liming can also support the availability of different soil nutrients.
Reduced chemical input minimising the use of pesticides and fungicides can support improved soil microbial activity. Extending rotations can reduce the need for herbicides and fungicides.
Benefits of improving soil health
- Reduced input costs – efficient nutrient use
- Increased rainfall infiltration and water holding capacity
- Improved biomass and agricultural productivity
- Increased opportunity for carbon capture
- Supported ecosystem services including increased soil biodiversity and reduced leaching of useful nutrients into waterways and groundwater
- Resilience to climate/seasonal variability
- Reduced off-site impacts of landuse.
The soils history and most significant non biologic elements will influence the soil biology, structure and degree and forms of degradation.
Soils that are sandier have larger pores between the particles and promotes soil fungi. Changes in soil organic matter levels will be observed sooner in sandy soils.