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5 Land

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2.2 Soil

Understanding the current state and condition of Australian soils requires an appreciation of their diversity and capability to support different forms of land use. It also requires an appreciation of human impacts, not only in recent years and decades, but also on longer timescales of centuries and millennia. This is because the effects of some forms of land use are long lasting, some rates of change are very slow, and remediation can take decades.

2.2.1 Baseline

The environmental baseline adopted throughout much of this report is that preceding European settlement (1750). However, this is problematic for soils, because there is limited evidence about the physical, chemical and biological conditions at this time. There is generally a much better understanding of soil changes associated with land clearing and conversion to land uses such as agriculture and forestry. Most assessments of soil change presented here relate to the condition before clearing, unless otherwise stated. Even then, it is instructive to start with the context set by Aboriginal land management.

At the time of Aboriginal arrival more than 50 000 years ago,¹⁶ the vegetation in many parts of Australia was quite different from now. Many of the widespread species were those that we now associate with dry rainforest environments—eucalypts did not have their present dominance.¹⁷ Although fire was a natural feature of the landscape, it appears to have been both less frequent and less widespread. The Australian megafauna were common at this time, becoming extinct 40 000–50 000 years ago. While there has been great debate over the cause of megafauna extinction, the loss of large herbivores from the Australian landscape and the subsequent changes in vegetation and fire had an undoubted effect on soil properties and processes. There is still much to learn, but it is clear that Aboriginal impacts on Australian soil must have been profound.

There is a clearer understanding of the impact of European land use. The resulting soil degradation, particularly in the 100 years after 1850, was extreme in some regions. Scott¹⁸ provides an instructive historical account for the Murray–Darling Basin. Rates of soil change were dramatic, with severe erosion, organic matter loss and nutrient depletion commonplace across large areas. Fragile soils were cleared of vegetation, and land management practices were crude. Today’s controls on clearing and more enlightened land management have made this gross form of land degradation uncommon. Although major threats to soil health remain in many regions, these are less visible, very persistent and widespread. We only have a rudimentary understanding of baselines and current rates of change.

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2.2.2 A framework for understanding soil

The major soil types in Australia are summarised from the Australian Soil Classification¹⁹ in Table 5.2. A generalised map of the major soil types (orders) is provided in Figure 5.2.

Table 5.2 Australia’s main types of soil

Soil order	Simplified description	Proportion of Australian soil (%)
Anthroposols	Soils formed by humans	No data
Calcarosols	Soils dominated by carbonate	9.2
Chromosols	Neutral to alkaline soils with a sharp increase in texture with depth	3.0
Dermosols	Structured B horizons (having a concentration of silicate clay, iron, aluminium and organic material) and gradational to minor changes in texture with depth	1.6
Ferrosols	High iron levels and gradational to minor changes in texture with depth	0.8
Hydrosols	Wet soils	2.2
Kandosols	Strongly weathered earths with minor changes in texture with depth	16.5
Kurosols	Acid soils with sharp increases in texture with depth	1.0
Organosols	Organic soils	0.1
Podosols	Soils with accumulated organic matter, iron and aluminium	0.4
Rudosols	Minimally developed soils	14.0
Sodosols	Soils with sodic subsoils, which are often alkaline, and with a sharp increase in texture with depth	13.0
Tenosols	Slightly developed soils	26.3
Vertosols	Cracking clays	11.5

In this chapter, we use the hierarchical stratification of Australia’s landforms from the Australian Soil Resource Information System (ASRIS).^a The ASRIS mapping hierarchy divides Australia into three physiographic divisions, which are further subdivided into 23 provinces and 220 regions. These broadscale mapping units have similar geological origins and a characteristic suite of soils and landforms. Even then, a diversity of soils and land management systems often occurs within each region. It is therefore only possible to reach general conclusions about the state of the soil for each region—there are always local exceptions.

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2.2.3 Key indicators of soil condition

Many physical, chemical and biological processes occur in soils, and they operate at different rates across the landscape according to the climate, land use and soil type. The following processes are key indicators of soil condition:

• carbon dynamics
• acidification
• soil erosion.

These processes have major environmental and economic consequences for Australia. Other aspects of soil change are also important, but they are dwarfed by the significance of these three.

A fourth key process affecting soil condition in Australia is dryland salinity. A large proportion of Australia’s agriculture is undertaken in areas with a rainfall of 450–800 millimetres per year. In their natural condition, these landscapes had minimal deep drainage (generally less than 20 millimetres per year), and natural stores of salt brought in by rain and dust had accumulated in the soil in many regions. The removal of native vegetation changes the hydrological cycle, because trees and shrubs intercept significant quantities of rain—often 10–20% of rainfall fails to reach the soil surface. When vegetation is removed, more water either infiltrates or runs off the surface. If the original vegetation has been replaced by more shallow-rooted species that use less water (e.g. annual crops and pastures), even more water passes through the soil. This may lead to rising groundwater levels and, in some cases, dryland salinity.

Dryland salinity has been one of Australia’s most costly forms of land degradation. The assessment by the National Land & Water Resources Audit (NLWRA)²⁰ provides a comprehensive overview. Assuming no changes in water imbalance, the NLWRA expected dryland salinity to increase from 5.7 million hectares to 17 million hectares by 2050. However, the millennium drought appears to have halted the spread of dryland salinity in most of the worst affected regions, especially in south-west Western Australia. Chapter 4: Inland water, Box 4.3 summarises the current status of salinity and its impact on inland waters.

The outlook outlined by the NLWRA²⁰ will need further moderation if current projections are correct for a drying of southern Australia. However, the long-term outlook for more recently cleared land in the northern Murray–Darling Basin and central Queensland is unclear. Large areas are yet to reach a new hydrological equilibrium after clearing.

Given the effects of drought over the reporting period, this report does not provide an update on previous SoE reports or the NLWRA assessment²⁰ regarding salinity. However, close surveillance of groundwater systems is essential, particularly in regions that returned to wetter conditions in 2010–11. A key requirement for understanding the state of dryland salinity in Australia will be to maintain the groundwater monitoring network established under the National Action Plan for Salinity and Water Quality.

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2.2.4 Carbon dynamics

The management and monitoring of soil carbon has become a matter of national and international importance. Soil carbon can be a significant source or sink for GHGs depending on how land is managed. The management of soil carbon is also central to maintaining soil health and ensuring global food security.²¹ However, there are complex trade-offs between reducing GHG emissions and producing food.

At the global scale, the amount of carbon contained in terrestrial vegetation (550±100 petagrams [1 billion tonnes]—Pg) is of a similar order to that in the atmosphere (800 Pg). However, the organic matter in soils is two to three times this amount. Approximately 1500–2000 Pg of carbon is in the top metre, and as much as 2300 Pg in the top three metres.²²

The carbon content of soil is a key indicator of its health, and is a master variable that controls many processes (e.g. nutrient cycling, development of soil structure, water storage). The carbon derives primarily from plant materials created through the capture of atmospheric carbon dioxide via the process of photosynthesis. These organic materials are cycled through the soil and used by organisms as a source of energy and nutrients. A significant amount of carbon dioxide is returned to the atmosphere as a result of respiration. Increasing soil organic carbon leads to an increase in:

• energy supply for biological processes
• direct nutrient supply to plants (particularly nitrogen, phosphorus and sulfur)
• capacity to retain and exchange nutrients
• aggregation of soil particles and stability of soil structure
• water storage and availability to plants
• beneficial thermal properties
• pH buffering (helping to maintain acidity at a constant level).

The maximum equilibrium carbon content for a soil at a given location is determined by environmental factors such as rainfall, evaporation, solar radiation and temperature. For example, soil carbon content is generally larger in cool, wet environments. A lack of nutrients and a limited capacity to store and supply water in a soil can reduce this potential maximum, as can other constraints to plant growth (e.g. toxicities). Within these constraints, the actual amount of organic carbon contained in a soil will be determined by the balance between carbon inputs and losses, which are strongly influenced by land management and soil type. Agricultural practices that alter rates of carbon input (e.g. plant residues, compost, mulch) or loss (e.g. removal of crops, cultivation) change the stock of soil organic carbon.

Soil carbon stocks in Australia

Soil carbon stocks are low in many Australian agricultural systems. Conversion from native vegetation to agriculture typically reduces soil carbon by 20–70%. ^23-24 This reduction is often associated with declining soil health and significant emissions of GHGs. It is generally acknowledged that more conservative forms of land management can increase soil carbon stocks, and have a significant impact on national and global emissions. This opportunity is the motivation behind many schemes around the world that aim to restore soil carbon stocks, including the Australian Government’s Carbon Farming Initiative. ^15,25

Carbon resilience and land management

There are different types of carbon in soils. It is useful to recognise three primary fractions:²⁶

• particulate organic carbon (POC): organic carbon associated with particles larger than 0.05 millimetres (excluding charcoal carbon)
• humus organic carbon (HUM): organic carbon associated with particles smaller than 0.05 millimetres (excluding charcoal carbon)
• resistant organic carbon (ROC): organic carbon found in the soil particles smaller than 2 millimetres having a chemical structure similar to charcoal.

The three primary fractions have contrasting dynamics. POC can be readily increased in a soil, but it also breaks down quickly. In contrast, ROC takes much longer to increase unless it is added via an amendment such as biochar.

The relative amount of each fraction in a soil determines the resilience of the soil’s carbon stocks. Figure 5.3 shows a typical pattern of soil carbon loss for each fraction after conversion from native vegetation to a cropping system in southern Australia. The system is converted back to pasture after 33 years of cropping, to restore soil carbon. This results in a quick increase in POC, but only a small increase in HUM and ROC. After 10 years, the pasture is converted back to a cropping system (year 43), and it experiences a quick decline. It only takes 9 years to return to the pre-pasture low point. In the first period, this amount of decline took 18 years (years 15 to 33) because the soil had proportionally more HUM. The key message is that, even under improved systems of land management, carbon stocks can be less resilient than those developed over long periods under native vegetation.

Sanderman and Baldock²⁸ recently reviewed replicated Australian field trials with timeseries data, providing an important new insight into carbon dynamics in agricultural systems. They concluded that, although the implementation of more conservative land management practices will lead to a relative gain in soil carbon, absolute soil carbon stocks may still be on a trajectory of slow decline (Figure 5.4). Their results showed that many soils used for agriculture are somewhere between scenario A and B.

Analysis by Sanderman et al.²⁴ of major management options for sequestering carbon in agricultural soils (Table 5.3) highlights the inevitable tradeoff between agricultural production (i.e. carbon exports in the form of crops, fibre and livestock) and carbon sequestration (capture and storage) in soils.

Table 5.3 Summary of major management options for sequestering carbon in agricultural soils

0 = nil; + = low; ++ = moderate; +++ = high; C = carbon; H = high; L = low; M = medium; SOC = soil organic carbon
Management	SOC benefita	Confidenceb	Justification
1 Shifts within an existing cropping/mixed system
a. Maximising efficiencies i. Water use ii. Nutrient use	0/+	L	Yield and efficiency increases do not necessarily translate to increased C return to soil
b. Increased productivity i. Irrigation ii. Fertilisation	0/+	>L	Potential trade-off between increased C return to soil and increased decomposition rates
c. Stubble management i. Eliminate burning and grazing	+	M	Greater C return to soil should increase SOC stocks
d. Tillage i. Reduce tillage ii. Direct drilling	0 0/+	M M	1) Reduced till has shown little SOC benefit 2) Direct drill reduces erosion and destruction of soil structure, thus slowing decomposition rates; however, surface residues decompose with only minor contribution to SOC pool
e. Rotation i. Eliminate fallow with cover crop ii. Increase ratio of pasture to crops iii. Pasture cropping	+ +/++ ++	M H M	1) Losses continue during fallow without any new C inputs—cover crops mitigate this 2) Pastures generally return more C to soil than crops 3) Pasture cropping increases C return with the benefits of perennial grasses (listed in 2.c, below) but studies lacking
f. Organic matter and other offsite additions	++/+++	H	Direct input of C, often in a more stable form, into the soil; additional stimulation of plant productivity (see 1 above)
2 Shifts within an existing pastoral system
a. Increased productivity i. Irrigation ii. Fertilisation	0/+	L	Potential trade-off between increased C return to soil and increased decomposition rates
b. Rotational grazing	+	L	Increased productivity, including root turnover and incorporation of residues by trampling, but lacking field experience
c. Shift to perennial species	++	M	Plants can use water throughout the year; increased below-ground allocation but few studies to date
3 Shift to different system
a. Conventional to organic farming system	0/+/++	L	Likely highly variable, depending on the specifics of the organic system (e.g. manuring, cover crops)
b. Cropping to pasture system	+/++	H	Annual production, minus natural loss, is now returned to soil; active management to replant native species often results in large C gains

Source: Sanderman et al.²⁴

Assessment of state and trends in soil carbon across Australia

A group of experts in soil carbon and land resource assessment was convened to provide an assessment of the state and trends in soil carbon across Australia. The assessment summary provides ratings for regions where the most significant issues are apparent. The ratings for all physiographic regions are available on the SoE website.^b A more comprehensive assessment of soil carbon across a range of agricultural systems will be available at the end of 2012 when a major national collaborative study is completed. However, it is possible to draw the following conclusions from the currently available evidence:

• The time since clearing is a key factor determining current trends. For example, large parts of Queensland are on a declining trend because widespread clearing for agriculture was still occurring in the 1990s.
• Few regions have increasing soil carbon stores.
• Regions with intensifying systems of land use (e.g. northern Tasmania) have decreasing stores.
• Most regions with a projected drying climate have declining trends.
• The savanna landscapes of northern Australia have significant potential for increasing soil carbon stores,²⁹ but this requires changes in grazing pressures and fire regimes.

Some of the extensive cropping lands in southern Australia with weathered and naturally infertile soils are rated as good (i.e. 30–70% loss) or very good (i.e. <30% loss) because they had small carbon stores at the time of European occupation and have not changed substantially (although soil biodiversity has undoubtedly changed). Many of these soils have also benefited from the addition of fertiliser and the correction of trace element deficiencies.

Assessment summary 5.1—state and trends of soil carbon

Physiographic region	Summary	Assessment grade				Confidence
		Very poor	Poor	Good	Very good	in grade	in trend
1 Paroo Plain and Warwick Lowland	Rangelands with extensive grazing and soil erosion caused by wind particularly on sandplains
2 Warrego Plains	Rangelands with minor opportunity cropping
3 Tenterfield Plateau	Land used for grazing of modified and natural pastures and nature conservation. Significant losses of soil carbon, particularly in the east
4 Clarence Lowlands	Infertile coastal lowlands used for forestry, grazing, cropping and some nature conservation
5 Cobar Plains	Significant declines in soil carbon, mostly associated with historically poor management. Overgrazing by feral goats is causing further decline, despite improving land management
6 Barrier Ranges	Surface content of carbon declining due to grazing and prior clearing
7 Gunnedah Lowlands	Declining trend due to intensification of cropping
8 Macleay Barrington Fall	Area used for nature conservation and production forestry, with some grazing. Possible decline due to logging
9 Merriwa Plateau	Mixed farming on fertile Ferrosols and Vertosols
10 Condobolin Plains	Sodosols and Vertosols used for cropping and grazing
11 Bathurst Tablelands	Grazing of modified and natural pastures dominates. General improvement in land management practices
12 Hawkesbury Shoalhaven Plateaus	Diverse landscape with natural conservation, forestry, grazing, horticulture and urban land uses. Fire regime and land management practices are most likely causing a decline
13 Cumberland Lowland	Mostly urban and industrial land use
14 Werriwa Tablelands	Relatively infertile lands used for grazing. Suspected slight decline
15 Monaro Fall	Forestry, nature conservation and grazing. Land management is improving
16 Australian Alps	Mostly used for nature conservation. Controls on grazing and reduced erosion stabilised early losses, but the increased intensity and extent of fires are likely to be causing a slight decrease, particularly in Organosols
17 Mallee Dunefield	Cropping, grazing and nature conservation with irrigated agriculture along the Murray River. Improved farming practices have improved soil condition in some areas, but soil carbon levels are probably still declining
18 Wimmera Plain	Mainly cropping and grazing. Former grazing lands now used for nature conservation may still be experiencing declining carbon content. Changing farming practices to no-till may reduce soil carbon loss in some areas, especially on heavier soils. Soil carbon is still likely to be declining in this region
19 Riverine Plains	Dryland cropping, irrigated agriculture and grazing, with the latter dominant in the west. Slight declines under dryland cropping systems, although farming practices may halt this
20 West Victorian Plains	Grazing, cropping and expanding plantation forestry. Areas converted from pasture to cropland are probably declining, as are soils used for continuous cropping
21 Midlands Plain	Dryland cropping, grazing and increasing irrigated cropping. Intensification of cropping is causing the decline
22 Lakes Plateau	Nature conservation reserves where wildfire and grazing have initiated sheet erosion over large areas, with very limited potential for recovery
23 West Tasmanian Ridges	More frequent and/or hotter fires in conservation reserves are causing losses, especially in Organosols. Northern areas dominated by production forestry, with no potential for soil carbon sequestration
24 East Tasmanian Hills	Production and plantation forestry, with minor declines due to erosion. Irrigated cropping in the south-east and north-east is causing declines
25 North West Ramp	Decline is associated with irrigated cropping
26 Roe and Carlisle Plains, Coonana–Ragged and Bunda Plateaus	Mainly grazing of native vegetation. Shift from perennials to annuals and possible increase in fire frequency may lead to nominal decline
27 Southern Goldfields Plateau	Soil carbon decline restricted to pastoral areas
28 Swan Plain	Urban areas and intensive agriculture. Declines due to tillage. Irrigated pasture systems for dairy may be stable or increasing
29 Woodramung Hills	Low input cropping and grazing. Drying trends have compounded effects of clearing and cropping on soil carbon
30 Murchison Plateau, Leemans and Yaringa Sandplains, Carnegie and Glengarry Hills, Augustus Ranges	Areas with extensive grazing of native vegetation, with declines in more heavily grazed areas. Drying trends are an additional factor
31 Carnarvon Plain	Nature conservation, extensive grazing and small areas of intensive irrigated horticulture. Episodic removal of surface soil carbon in horticultural areas
32 Fitzroy Plains	Extensive grazing of native vegetation
33 Daly Basin	Small areas of intensive agriculture likely to have declining carbon. Remainder is used for extensive grazing
34 Whirlwind Plain and Birrundudu Plain	Extensive grazing with small areas of more intensive development on better soils. Possible minor decreases in soil carbon due to high seasonal stocking rates
35 Barkly Tablelands	Extensive grazing on clay plains. Some impacts on soil carbon due to grazing
36 Toowoomba Plateau	Ferrosols used for cropping and pasture, with increasing agroforestry. Slow increases occurring under pasture and agroforestry
37 Central Uplands	Partially cleared grazing country. Soil carbon is likely to be declining where cleared recently, otherwise stable
38 Atherton Tableland	Fertile land with high rainfall. Diverse land uses, with soil carbon now increasing under pastures and tree crops. Stable under rainfed crops but still decreasing under small crops
39 Garnet Uplands	Recently intensified land use after clearing. Soil carbon likely to be declining

Recent trends	Improving	Stable	Confidence	Adequate high-quality evidence and high level of consensus
	Deteriorating	Unclear		Limited evidence or limited consensus
				Evidence and consensus too low to make an assessment
Grades	Very good: Carbon stocks have either remained the same or increased (<30% loss or increase)
	Good: Carbon stocks have changed in some areas (30–50% loss)
	Poor: Carbon stocks have decreased (50–70% loss)
	Very poor: Carbon stocks have decreased substantially (>70% loss)

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2.2.5 Soil acidification

Soil acidification is an insidious process that develops slowly. If not corrected, it can continue until the soil is irreparably damaged. Acidification affects about half of Australia’s agriculturally productive soils. The severity and extent of acidification are increasing in many regions, due to inadequate treatment, intensification of land management, or both.

Soil acidification is of greatest concern in situations where:

• agricultural practices increase soil acidity (e.g. use of high-analysis nitrogen fertilisers, large rates of product removal)
• the soil has a low capacity to buffer the decrease in pH (e.g. infertile, light-textured soils)
• the soil already has a low pH.

The process of acidification considered in this report is distinct from that associated with acid sulfate soils. Such soils occur primarily in coastal settings, and naturally contain iron sulfides that severely acidify when oxidised. This can occur through drainage of coastal wetlands, or exposure due to drought, as was the case in the Lower Lakes of South Australia during the millennium drought.

The main onsite effects of acidification include:

• loss or changes in soil biota involved in nitrification (fixing nitrogen, a key nutrient, within the soil)
• accelerated leaching of plant nutrients (manganese, calcium, magnesium, potassium and anions)
• induced nutrient deficiencies or toxicities
• breakdown and subsequent loss of clay materials from the soil
• development of subsoil acidity
• reduced net primary productivity and carbon sequestration
• erosion as a result of decreased groundcover that may follow acidification.
• The potential offsite effects include:
• mobilisation of heavy metals into water resources and the food chain
• acidification of waterways as a result of leaching of acidic ions
• increased siltation (where there are fine sediments suspended in the water and deposited on the floor) and eutrophication (where a high concentration of nutrients typically triggers excess growth of algae) of streams and water bodies.

Soil acidification in Australia

In 2001, the NLWRA estimated that soil acidity affected 50 million hectares of surface layers and 23 million hectares of subsoil layers of Australia’s agricultural zone. The estimated annual value of lost agricultural production due to soil acidity was $1.585 billion, about eight times the estimated cost of soil salinity at that time. More recent studies have confirmed the scale of the problem (e.g. Lockwood et al.2003;³⁰ Wilson et al.³¹), but there have been only a few detailed quantitative investigations.^32-33

Ultimately, soil acidification restricts options for land management, because acid-sensitive crops and pastures cannot be grown. It also looms as a major constraint on Australia’s capacity to increase carbon in agricultural soils.

It is relatively straightforward to reverse short-term soil acidification through the application of lime. However, it is much harder to reverse the problem if the acidification has advanced deeper into the soil profile, because incorporating lime at depth is prohibitively expensive. Prevention rather than cure is essential.

While rates of lime application appear to be increasing, they still fall far short of what is needed to arrest the problem. Western Australia has one of the best programs for combating acidification in Australia, but the rates of lime application are still much lower than what is needed to avoid irreparable damage (Figure 5.6). Around $400–500 million is already being lost in annual productivity in the state.³⁴

A similar situation exists in South Australia (Figure 5.7). The average quantity of lime sold annually over the past decade (113 000 tonnes) is only 53% of that needed to balance the estimated annual soil acidification rate.³³

Assessment of state and trends of soil acidification across Australia

A group of experts in soil acidification and land resource assessment was convened to provide an assessment of the state and trends of soil acidification across Australia. Assessment summary 5.2 provides ratings for regions where the most significant issues are apparent. The ratings for all physiographic regions are available on the SoE website.

Australia does not have an organised monitoring system for soil acidification, which accounts for the significant uncertainty in many regions. However, the following conclusions can be drawn from the evidence available to the expert group:

• Soil acidification is widespread in the extensive farming lands of southern Australia (Figure 5.8).
• Rates of lime application are well short of those needed to arrest the problem.
• Acidification is common in intensive systems of land use (tropical horticulture, sugar cane, dairying).
• Acidification is limiting biomass production in some regions, but the degree of restriction is difficult to estimate.
• Trends in the tropical savannas are uncertain. If acidification is occurring, it will be a difficult problem to solve.
• Carbon losses are most likely occurring across regions in poor condition, and soil acidification is a major constraint on storing carbon in soils in the future.

Assessment summary 5.2—state and trends of soil acidification

Physiographic region	Summary	Assessment grade				Confidence
		Very poor	Poor	Good	Very good	in grade	in trend
1 Toowoomba Plateau	Naturally fertile land with widespread intensive use. Insufficient lime application
2 Maryborough Lowland	Soils under pine plantations are acidifying, but improved practices for sugar cane and horticulture appear to be effective
3 Atherton Tableland	Improved practices across diverse land uses, but localised subsoil acidification under banana cropping
4 Garnet Uplands	Recently intensified land use after clearing
5 Tenterfield Plateau	Grazing of modified and natural pastures, with widespread declines in pH
6 Clarence Lowlands	Coastal lowlands with a variety of soil types. Significant decline in pH on the floodplains due to intensive agriculture
7 Cobar Plains	Cropping and grazing of modified and natural pastures in the east, with declines in pH evident
8 Mitchell Slopes	Diverse landscape, but pH has declined markedly, especially on soils with a long history of mixed farming
9 Merriwa Plateau	Fertile Ferrosols and Vertosols, mostly used for mixed farming. Slow decline in pH likely due to management systems
10 Goulburn Corridor	Minimal evidence, but declines in pH are expected due to lack of liming
11 Hunter Valley	Mined areas and Hunter River floodplains are mostly managed sustainably for acidification, but declines in pH are expected on the Singleton Valley floodplains
12 Riverine Plain	Diverse region, with declines in pH in irrigation districts and in soils used for dryland cropping and grazing of annual pastures, especially in the south
13 Hume Slopes	Mostly mixed farming. Widespread and significant declines in soil pH
14 Condobolin Plains	Declines in pH restricted to irrigation districts
15 Bathurst Tablelands	Diverse soils used for grazing of modified and natural pastures
16 Hawkesbury Shoalhaven Plateau	Substantial declines in pH in soils used for agriculture
17 Cumberland Lowland	Declining pH in soils used for vegetable production and intensive agriculture
18 Werriwa Tablelands	Declining pH in soils used for grazing of native and improved pastures
19 Tinderry Gourock Ranges	Declining pH in soils used for grazing of natural and modified pastures
20 Monaro Fall	Declines in pH in soils used for grazing
21 Monaro Tableland	Declining pH in soils used for grazing of natural and modified pastures
22 Australian Alps and East Victorian Uplands	Grazing lands outside conservation reserves are showing declines in pH
23 West Victorian Uplands	Areas used for grazing of dryland annual pastures are acidifying
24 West Victorian Plains	Diverse lands with evidence of acidification restricted to poorly drained Sodosols and Vertosols used for dryland cropping and the grazing of annual pastures
25 Lincoln Hills, Eyre Dunefield and Yorke Peninsula	Mainly dryland agriculture. Rates of liming are not sufficient to balance rates of acid addition for most soils. Generally stable pH in Calcarosols
26 Flinders–Lofty Ranges	Diverse lands with stable pH in the arid north. Declines in pH are occurring in the temperate central area under dryland cropping and viticulture. Lands in the cool temperate south used for viticulture, horticulture and grazing are acidifying
27 Mallee Dunefield	Mainly cropping in rotation with pastures. Trend to more cropping. Declining pH in surface soil layers over calcareous subsoils. Significant declines in pH in horticulture areas along the Murray River
28 Wimmera and Millicent Plains	Diverse soils used for dryland cropping, grazing, some irrigation and forestry in the south. Declining trend in pH due to insufficient lime use
29 Warren–Denmark Slopes, Leeuwin Peninsula and Donnybrook Lowland	Forestry, intensive agriculture and dryland cropping. Significant declines in pH in intensively used areas
30 Albany Esperance Sandplain	Grazing and cropping systems are acidifying, particularly on lighter soils
31 Avon Plateau and Northam Slopes	Diverse soils. Widespread surface and subsurface soil acidity. Inadequate application of lime
32 Darling Range	Mostly forested, but areas use for cropping and grazing are acidifying
33 Swan Plain	Urban areas and intensive agriculture, with declines in pH in the latter
34 Dandaragan Tablelands	Declines in pH restricted to areas used for cropping and grazing systems
35 Greenough Hills	Significant acidification of both surface and subsurface horizons on noncalcareous soils used for cropping
36 Woodramung Hills	Significant acidification and low pH in the western part of the region used for cropping
37 Carnarvon and Top End Coastal Plains	Declining pH in agricultural areas used for intensive irrigation and horticulture
38 Daly Basin	Declining pH in areas used for horticulture and more intensive pastoral development

Recent trends	Improving	Stable	Confidence	Adequate high-quality evidence and high level of concensus
	Deteriorating	Unclear		Limited evidence or limited consensus
				Evidence and consensus too low to make an assessment
Grades	Very good: Current management adequate—low level of monitoring required
	Good: Needs management and monitoring, otherwise returns will be threatened
	Poor: Urgent amelioration. Yields and returns are compromised, returns currently threatened
	Very poor: Beyond recovery (economic). Yields no longer economic. Current system untenable with limited options

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2.2.6 Soil erosion

The rate of soil formation is typically very slow. The fastest rates occur in dune sands in moist environments, where weakly developed soils can develop over decades or centuries. In river alluvium, a strongly developed soil (Chromosol) can develop in 20 000–30 000 years. Soil formation from weathering rock is much, much slower and varies with the environment and rock type. An average of 1 millimetre per 1000 years or slightly less is typical.

Water erosion

Current rates of soil erosion by water across much of Australia now exceed soil formation rates by a factor of at least several hundred and, in some areas, several thousand. As a result, the expected half-life of soils (the time for half the soil to be eroded) in some upland areas used for agriculture ranges from less than a century to several hundred years.

The latest assessment³⁵ concluded that soil erosion by water in Australia is still at unsustainable rates, but there are large uncertainties about the time until soil loss will have a critical impact on agricultural productivity. Environmental impacts of excessive sedimentation and nutrient delivery on inland waters, estuaries and coasts are already occurring (see Chapter 4: Inland water).

Up to 10 million hectares of land have less than 500 years until the soil’s A horizon (effectively the more fertile ‘topsoil’) will be lost to erosion. Most of this land is in humid subtropical Queensland. Integrated studies of soil formation and erosion using a variety of techniques will be needed to better understand the extent, severity and significance of the problem. However, it is clear that a concerted program of soil conservation is essential to control this chronic form of land degradation across large areas of Australia.

The key to controlling soil erosion by water is the maintenance of a protective cover on the soil surface (e.g. living plants, litter, mulch). Other soil conservation practices—such as contour banks, filter strips and controlled traffic—are important, but secondary to the maintenance of cover.

Land-management practices have improved significantly during the past few decades, due to better grazing practices, adoption of conservation tillage, enforcement of forestry codes and soil conservation measures in engineering (e.g. relating to road construction and urban development).

An ability to monitor land cover provides a key input to assessments of erosion risk across the landscape. Three significant developments in this regard have occurred since the last SoE report:

• Ground-based monitoring of management practices and land cover has been under way for long enough to identify trends.
• New data on land-management practices are available from the Australian Bureau of Statistics (ABS).
• Remote sensing has advanced to the point where monitoring of bare soil and surface cover is possible. This will eventually lead to more reliable estimates of erosion rates.

The ground-based surveys and ABS data reveal a pattern of:

• more careful grazing and maintenance of effective land cover at critical times of the year
• improved adoption of conservation practices, especially across the cropping lands of southern Australia
• an associated large decline in the amount of tillage in farming systems (Figure 5.9).

Figure 5.10 shows two images of Australia derived from remotely sensed data. The images show the proportion of bare soil and surface cover that is either photosynthetically active (i.e. growing vegetation) or inactive (e.g. crop residues, plant litter). Figure 5.10a shows the Australian continent in 2006, during the millennium drought. Figure 5.10b shows the same seasonal period in 2011, when the drought had broken. Large reductions in the area of bare soil are evident, especially in the north, east and south-east of the continent. The intense rainfall and floods associated with the breaking of the drought resulted in widespread erosion, especially in south-east Queensland. A full timeseries showing seasonal averages for the past decade is available on the SoE website^d, illustrating the pervasive effect of fire, especially across northern Australia.

Wind erosion

Climate is by far the strongest determinant of wind erosion. Land management can either moderate or accelerate wind erosion rates. Unravelling these two influences has been difficult, but the millennium drought provided an excellent opportunity to gauge the effectiveness of improvements in land management that have occurred in recent decades.

It has been well documented in historical accounts of land degradation in Australia that wind erosion was very active during the drought periods of the late 19th and early 20th centuries (e.g. Ratcliffe³⁷). While these anecdotal reports present dramatic images of huge dust storms engulfing rural towns, and sand drifts burying fence lines and blocking rural roads, until now it has never been unequivocally established whether the ‘dust bowl years' of the 1940s were due to extreme drought, poor land management or both.

The millennium drought resulted in large dust storms and other wind erosion activity. Two extreme dust storms hit eastern Australian cities on 23 October 2002 and 23 September 2009. Wind erosion has environmental impacts at the source where soils are eroded (onsite wind erosion), and much greater economic and human health impacts downwind from the source where air quality is reduced (offsite wind erosion). The extreme dust storms increased public awareness of both these impacts, and also raised the question of whether this recent period of wind erosion was more or less active than that of the 1940s, and whether changes in land management have played a role.

McTainsh et al.³⁸ have analysed wind erosion activity during the 1940s and 2000s. They used archived meteorological data to calculate the dust storm index (DSI) for both periods.^39-40 The DSI, which provides a measure of the frequency and intensity of wind erosion activity, is the accepted measure of wind erosion activity, and has been used in past SoE reports⁴⁰ and reports on the rangelands.⁴¹ Wind erosion activity for the 1940s and 2000s was compared using the highest DSI year for each available station within each decade (decade maximum DSI) (Tables 5.4 and 5.5; Figure 5.11).

Overall, mean onsite wind erosion in the 1940s was almost six times higher (mean DSI = 11.4) than in the 2000s (mean DSI = 2.0), and the mean maximum DSI for the 1940s was four times that of the 2000s (Tables 5.4). There were also significant regional differences (Table 5.4). Wind erosion was much higher in the Mulga, Riverina and central Australia than in the South Australian and Western Australian rangelands, and the decrease in wind erosion in the 2000s is much more pronounced in the east and centre of the continent. For example, in the Mulga, the mean 1940s DSI is 21.3, compared with only 0.5 for the 2000s. Similarly, in the Riverina region, the mean decadal DSI decreases from 17.7 to 2.9. The 1940s erosion rate in central Australia is large, but the decrease in the 2000s is less.

Table 5.4 Dust storm index at six onsite wind erosion regions for the 1940s and 2000s

Region and station	1940s mean DSI	1940s mean DSI range	2000s mean DSI	2000s mean DSI range	1940s maximum DSI	2000s maximum DSI
Qld: Mulga Charleville	21.3	19.0–23.6	0.5	0.5–0.6	39.0	1.7
NSW: Riverina Wagga Wagga	17.7	16.9–18.5	2.9	2.89–2.93	51.5	9.2
Central Australia: Alice Springs	13.7	11.3–16.2	3.5	3.4–3.5	37.3	15.0
Southern SA: Ceduna	6.0	5.95–5.98	2.5	2.3–2.6	17.6	7.8
WA: Southern Rangelands Kalgoorlie	5.4	2.19–8.6	1.6	1.4–1.8	21.6	5.9
WA: Northern Rangelands Port Hedland	4.4	3.4–5.5	1.3	1.1–1.4	20.0	7.4
Mean	11.4		2.0		31.2	7.8

DSI = dust storm index; NSW = New South Wales; Qld = Queensland; SA = South Australia; WA = Western Australia

Source: McTainsh et al.³⁸

The offsite wind erosion record at 11 coastal cities (Table 5.5 and locations in Figure 5.11) during the 1940s demonstrates that wind erosion across the inland had a very significant impact on the coast. This is especially so from Brisbane to Adelaide. Although the south-east coast DSI levels in the 1940s are around 40% of those inland, the mean DSI in the 2000s is less than 5% of the 1940s value (Table 5.5). There has been a large decrease in dust storms reaching the coastal cities.

Table 5.5 Dust storm index at 11 offsite locations for the 1940s and 2000s

Location	1940s mean DSI	1940s mean DSI range	2000s mean DSI	2000s mean DSI range	1940s maximum DSI	2000s maximum DSI
Cairns	2.3	1.7–2.9	0.0	–	7.0	0.1
Townsville	1.1	0.8–1.3	0.3	–	8.0	1.2
Rockhampton	3.6	–	0.2	–	21.0	0.5
Brisbane	3.8	0.17–6.8	0.1	0.05–0.15	21.0	1.2
Sydney	3.3	0.7–5.9	0.3	–	16.0	2.4
Canberra	7.8	–	0.1	–	15.0	0.2
Melbourne	5.0	4.3–5.6	0.1	–	11.0	0.4
Adelaide	8.2	7.0–9.4	0.6	–	29.2	5.2
Perth	1.3	–	0.0	–	4.0	0.1
Broome	0.2	–	0.3	–	1.3	0.6
Darwin	8.1	7.9–8.2	0.2	–	20.1	1.1
Mean	4.1		0.2		14.0	1.2

DSI = dust storm index; - = data not available

Source: McTainsh et al.³⁸

As noted above, previous national SoE reports have provided updates on the DSI across the continent. Maps for the continent from 2001 to 2009 are shown in Figure 5.12.

Assessment of state and trends in erosion across Australia

Figure 5.13 and Assessment summary 5.3 provides an assessment of soil erosion by wind and water across Australia. The assessment draws heavily from the NLWRA,⁴² Bastin et al.,⁴¹ Leys et al.,⁴³ Bui et al.³⁵ and McTainsh et al.³⁸

Assessment summary 5.3—state and trends of soil erosion by water and wind

Component	Summary	Assessment grade				Confidence
		Very poor	Poor	Good	Very good	in grade	in trend
102. Peninsular Province	Land management in the grazing and cropping lands is improving, although localised erosion continues at unsustainable rates
103, 104. Burdekin and Fitzroy Provinces	Land management in the grazing and cropping lands is improving, although erosion continues at unsustainable rates
105, 106. New England-Moreton and Macquarie Uplands Provinces	Management of surface cover has improved in most areas, but erosion from less fertile grazing lands is significant
107. Kosciuszkan Uplands Province	Diverse lands experienced severe gully and sheet erosion after clearing. Increasing extent of wildfire is likely to increase erosion
108. Tasmanian Uplands Province	Unsustainable rates of erosion under intensive agriculture in the north, and widespread hillslope erosion in the west and south-west due to wildfire
201. Carpentaria Lowlands Province	Hillslope erosion primarily in the east and south associated with clearing and grazing
202. Central Lowlands Province	Large and diverse province, with widespread wind erosion in the centre and west. Hillslope associated with grazing by stock and feral animals. Rate of erosion has slowed
203. Murray Lowlands Province	Extensive wind erosion associated mainly with dunefields. Increased surface cover has slowed previously severe erosion
301. North Australian Plateaus Province	Sheet and gully erosion in areas cleared for grazing or agriculture. Vulnerable to severe erosion due to intensity of rainfall and erodibility of the soils
302. Kimberley Province	Fast rates of sheet and gully erosion have slowed, but remain unsustainable in areas with poor surface cover
303. Carpentaria Fall Province	Erosion rates have slowed, particularly in the south-east of the province
304. Barkly-Tanami Plains Province	Sheet and gully erosion in areas cleared for grazing or agriculture. Vulnerable to severe erosion due to intensity of rainfall and erodibility of soils
305. Central Australian Ranges Province	Extensive wind erosion, with gully and sheet erosion in areas subject to grazing by stock and feral animals
306. Sandland Province	Gully and sheet erosion in areas subject to grazing by stock and feral animals
307. Pilbara Province	Gully and sheet erosion in areas subject to grazing by stock and feral animals
308. Western Coastlands Province	Gully and sheet erosion in areas subject to grazing by stock and feral animals
309. Yilgarn Plateau Province	Extensive wind erosion, particularly in the north
310. Nullarbor Plain Province	Erosion rates are relatively stable, unless there is a loss of surface cover due to grazing or fire
311, 312. Eyre Peninsula and Gulfs Ranges Provinces	Diverse lands with a history of unsustainable rates of wind and water erosion. Land management practices are improving

Recent trends	Improving	Stable	Confidence	Adequate high-quality evidence and high level of consensus
	Deteriorating	Unclear		Limited evidence or limited consensus
				Evidence and consensus too low to make an assessment
Grades	Very good: Current management adequate—low level of monitoring required
	Good: Significant erosion at the time of clearing (>5 tonnes/ha/year). Rates are 1-5 tonnes/ha/year. Needs management and monitoring, otherwise the system of land use will be threatened in the long term
	Poor: The majority of the landscape has been eroded to the extent that plant growth has been affected and, in agricultural systems, yields and returns are compromised. Erosion rates are unsustainable (>5 tonne/ha/year)
	Very poor: Current rates of erosion are unsustainable, with large areas reaching exhaustion within 50 years

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