Soils are an essential component of all ecosystems and contribute significantly to Australia’s environmental, economic and social wellbeing. Australia’s natural assets and agricultural production rest on healthy soils. Soil Science Australia has estimated that all the ecosystem services that soils deliver are valued at $930 billion per year, making soils Australia’s most valuable natural asset (Soil Science Australia 2019). However, Australian soils and the agricultural landscape are degraded and face a number of challenges, including erosion, acidification, salinisation, sodification, soil carbon loss, contamination, and urban and industrial expansion (McKenzie et al. 2017). Climate change is placing further pressure on soils through increased frequency of droughts and extreme weather events, and increasing average temperatures. All cause serious soil loss and damage (e.g. Grace et al. 2006, Rabbi et al. 2015, Borrelli et al. 2020, DAWE 2021t).
Soils are layered mixtures of mineral and organic particles on the land surface, formed over very long time periods from the weathering of rocks, transport of sediments and interactions with living organisms. A decline in the amount and health of soil directly affects its ability to provide important ecosystem services that support our natural environment and agricultural industries. Soil degradation is not easily reversible, and rehabilitation can take many decades; the full range of biodiversity may never be recovered.
The types of ecosystem services provided by healthy, biodiverse soils include (Baer & Birgé 2018):
- regulating, cycling, storing and providing water and nutrients
- supporting the production of food and fibre from native vegetation, agriculture and forestry
- supporting habitat for plants, animals and microbes, living above and below ground
- detoxifying pollutants from the landscape, and biological pest control
- reducing amounts of greenhouse gases in the atmosphere by storing carbon in organic matter.
Soils may also harbour bacteria, viruses and fungi that can cause disease (Baumgardner 2012), or persistent organic and other pollutants that can be toxic to living organisms (Jayaraj et al. 2016, Rodríguez-Eugenio et al. 2018) (see Landfill and land contamination). These toxic compounds include agricultural herbicides and pesticides, such as glyphosate and neonicotinoids. For example, the persistent residue and highly toxic nature of neonicotinoids to insects led to those toxic compounds being banned for outdoor use in Europe since 2018(Goulson & signatories 2018).
The different properties of soil need to be understood for effective and sustainable management. Soil properties include (Dominati et al. 2010):
- inherent properties, such as texture, depth, clay type, stoniness and biology
- managed properties, such as mineral nutrients, including phosphate and nitrogen, soil organic matter, biological activity, and bulk density.
These soil properties influence the appearance, productivity and health of the soil – for example, the biological activity of soils is essential for soils to perform their natural functions of maintaining structure, cycling nutrients and decomposing toxins (Doran & Zeiss 2000). The manageable properties of soils are the most important for land managers, as these can be modified by applying supplemental inputs (e.g. irrigation, fertiliser) or adopting sustainable management practices to minimise degradation (e.g. tillage, vehicle tracking) (see Soil capital assets).
Soils can be changed or degraded by various processes (Figure 10), many of which are tied to land use (FAO 2017) (see Land use), including:
- changes to native vegetation as a result of clearing, fire, grazing and cultivation, which alter the soil’s physical, chemical or biological properties
- intensive agriculture practices (e.g. inputs such as irrigation, fertiliser, herbicides and pesticides, mechanical preparation, and heavy machinery), which can degrade the function or quality of the soil through acidification, topsoil loss, nutrient imbalance, compaction, reduced biological activity and organic carbon loss (see Carbon below the ground in soils)
- processes that decrease the quantity of soils, including erosion (see case study: Vegetation cover as a national indicator of soil health and erosion risk), and processes that degrade the quality of soils, such as salinisation and acidification
- disruption of biological activity in soils, which can precipitate excessive mineralisation of organic matter leading to the loss of structure and nutrients.
- In vulnerable dryland regions, human-induced soil degradation is an important contributing factor in desertification, defined as a persistent or irreversible reduction in the capacity of ecosystems to supply ecosystem services for decades (Becerril-Piña & Mastachi-Loza 2019).
Soil classification
- While little information has been documented about Indigenous peoples’ knowledge of soil, traditional ecological knowledge systems recognise soil as being associated with different ecosystems. For example, different tree communities are associated with different soil types.
Australian scientists have developed a classification of soils to give consistency in communication and to ensure data collected for different purposes can be combined or compared. The third edition of Australia’s soil classification by Isbell & National Committee on Soil and Terrain (2021) incorporates new knowledge accumulated over 5 decades to derive a soil taxonomy. At the highest level (order), the taxonomy now comprises 15 groups with the inclusion of a new order – Arenosols – for deep sands. Following recent development of a Soil Data Federation System and new digital soil-mapping technologies, work is underway to refine the map of Australian soil orders (Figure 11) (see case study: The Australian SoilDataFederator: a TERN initiative delivers open-access soil data for all). To date, more than 65,758 soil profiles from across Australia have been classified at the family level.
Soil health
- Evaluating the health of soils from first principles requires measurable indicators that cover physical, chemical and biological dimensions related to functional processes, and that change rapidly in response to management. Raghavendra et al. (2020), for example, identified 15 key indicators for monitoring soil health. The most commonly used indicators are soil organic carbon and pH (acidity), followed by phosphorous, water-holding capacity and infiltration, and bulk density. Indicators of microbial activity and diversity are among the most sensitive to land-use and management practices, but require specialised methods of measurement that are often expensive to implement or lack clear, actionable interpretation (Raghavendra et al. 2020, Fierer et al. 2021). Although among the most promising indicators of soil health and function, soil biology is underrepresented in soil-quality assessments (Bünemann et al. 2018, Lehmann et al. 2020, Thiele-Bruhn et al. 2020).
- A comprehensive review of factors influencing soil health in Australia and regional priorities for management is provided by McKenzie et al. (2017). A proxy of soil health can also be gained through identifying which soil orders are in cleared and regrowth or modified areas in Australia (Figure 12).
- Our understanding of soil biology has been advancing through sampling and detecting the DNA signatures of biota that occupy different soils – for example, through the Australian Microbiome project within Bioplatforms Australia. The microbiome consortium has developed a comprehensive database of microbial diversity across a range of Australian terrestrial soils, which provides an important baseline for understanding the role of biota in healthy, functioning soils (Bissett et al. 2016). The soil microbiome is a mega-diverse and functionally important ecosystem, which is closely coupled to the above-ground vegetation community (Bowd et al. 2021), and underpins the success of land-restoration efforts (Yan et al. 2020).
- Other ways to monitor soil health relate to the extent of adoption of regenerative management strategies that support restoration of soil function. These strategies include managing groundcovers and specific sequences of crop rotation; avoiding tillage, or using mulch tillage strategies and direct mulching; nutrient management within crop requirements (Raghavendra et al. 2020); integrated pest and weed management, including biological control methods; and vehicle management to minimise surface compaction (McPhee et al. 2020). The Australian Government Department of Agriculture, Water and the Environment, as well as state and territory agencies, work in collaboration with peak industry bodies to actively support adoption of regenerative practices in agriculture and forestry. Wider adoption of regenerative farming practices has the additional benefit of supporting resilience to drought. However, when the potential for carbon sequestration becomes saturated, the risk of nitrous oxide emissions increases, and these unintended consequences need to be better understood (Ranganathan et al. 2020).
Many of the commonly observed physical and chemical indicators of soil health have informed digital soil mapping (Grundy et al. 2020, Kidd et al. 2020), which has been disseminated through the Terrestrial Ecosystem Research Network’s (TERN’s) Soil and Landscape Grid of Australia (CSIRO & TERN 2021a). A further innovation is the Australian Cosmic-Ray Neutron Soil Moisture Monitoring Network (CSIRO & TERN 2021b), which uses cosmic rays originating from outer space to measure average soil moisture over an area of about 30 hectares to depths in the soil of 10–50 centimetres. Satellite monitoring of groundcover is also being used to understand where risks of soil loss due to wind and water erosion may be merging and how to balance farm management decisions to maintain sufficient groundcover during early onset of drought (see case study: Vegetation cover as a national indicator of soil health and erosion risk). Rainfall and land use are also useful indicators of soil erosivity for evaluating how risks may change in the future with projected climate extremes (Borrelli et al. 2020). These tools are helping to assess the current status and trend of soil health in Australia.
Some agricultural systems have become so well adapted to soils of lower condition that the full extent of land deterioration may be masked. For example, acid-tolerant wheat varieties can mask declines in soil acidity, because crops continue to provide reasonable yields (Amjad et al. 2014). A potential indicator of this decline may be the performance of rotation crops – an acidifying system becomes more restricted in the rotational crops that will perform well (particularly legumes) (McMillan & Small 2016). Plant biomass provides an aggregate indicator of the potential for change in soil organic carbon, for which the flows rather than the stocks are the important indicator of soil function (Macdonald et al. 2020).
In the future, Australia’s capacity and ability to monitor important changes in the state of soil and its function will be increased by the activities under the Commonwealth Interim Action Plan for the National Soil Strategy (see Retaining and restoring natural capital assets). Ideally, similar to vegetation condition, changes in soil health over time will be measured relative to a reference state specific to each soil class, for example, aligned with the framework developed by Román Dobarco et al. (2021).
Overall, Australian soils are deeply weathered, old and infertile, which makes them vulnerable to degradation. Ongoing clearing and unsustainable agricultural practices continue to impact soil health. Declines are seen where there is current or recent clearing and land conversions. Erosion continues in areas of reduced groundcover.
Related to United Nations Sustainable Development Goal targets 2.4, 12.4, 15.3
Decades of land-use intensification have altered the structure and function of vulnerable soils, requiring significant inputs to maintain productivity. There are local areas of regenerative and restorative practices.
Although this zone has generally lower-intensity land use, there is still significant damage to soils from unsustainable grazing practices and climate change extremes that reduce groundcover and expose soils to erosion.
Pressures on soils are variable across this zone, with a decline in soil fertility in areas of grazing and clearing. This can be compounded by climate change extremes that reduce groundcover, thereby exposing soils to degradation from erosion.