Soil: Understanding

2016

Understanding the current state and condition of Australian soils requires an appreciation of their diversity and their ability 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 impact of land-use change is long-lasting, soil formation is very slow, and remediation can take decades. Most states and territories now explicitly include a section on soil in their own SoE and NRM report cards. However, there is currently no standard set of indicators for monitoring soil condition, and each jurisdiction uses its own set. The ratings, symbols and reporting regions used are not standardised either between states and territories or with the Australian Government.

Baseline

The environmental baseline adopted throughout much of SoE 2011 was the international pre–industrial revolution baseline (1750). However, for soil, this is problematic because there is limited evidence about the soil’s physical, chemical and biological condition at that time, although there is an 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 in the 2011 assessment, unless otherwise stated.

A new generation of large-scale soil mapping (see Box LAN9) will inform national mapping and monitoring of carbon, biodiversity, agricultural impact and ecosystem functions in general.

Box LAN9 Soil and Landscape Grid of Australia

The new Soil and Landscape Grid of Australia provides gridded data on soil and landscape attributes, along with estimates of uncertainty. Data are provided for 90 metre pixels, and are available for 6 soil depths (0–5 centimetres, 5–15 centimetres, 15–30 centimetres, 30–60 centimetres, 60–100 centimetres, 100–200 centimetres) down to a maximum of 2 metres. The data are in easily accessible raster file formats, which can be downloaded or viewed through the CSIRO data access portal. Attributes available are as follows:

Soil attributes

Landscape attributes

  • bulk density
  • organic carbon
  • clay
  • silt
  • sand
  • pH (water)
  • pH (calcium chloride)
  • available water capacity
  • total nitrogen
  • total phosphorus
  • effective cation exchange capacity
  • depth of regolith
  • depth of soil
  • coarse fragments
  • slope (per cent)
  • slope (per cent), median, 200 metre radius
  • slope relief classification
  • aspect
  • relief, 1000 metre radius
  • relief, 300 metre radius
  • topographic wetness index
  • topographic position index
  • partial contributing area
  • multiresolution valley bottom flatness
  • plan curvature
  • profile curvature
  • Prescott index
  • solar radiation (SRAD), net radiation, January
  • SRAD, net radiation, July
  • SRAD, total shortwave, sloping surface, January
  • SRAD, total shortwave, sloping surface, July

The Soil and Landscape Grid draws together historical and new data generated from sampling, new laboratory measurement techniques, remote sensing and modelling (Grundy et al. 2015). Funded through the Terrestrial Ecosystem Research Network, it is the result of national collaborative research involving CSIRO; the University of Sydney; Geoscience Australia; and Australian, state and territory government agencies. It adds further value to hundreds of millions of dollars worth of investments in soil surveying during the past 50 years.

A framework for understanding soil

The major soil types in Australia are summarised using the Australian Soil Classification in Table LAN3. A generalised map of the major soil types (orders) is provided in Figure LAN16.

In this report, we use the hierarchical stratification of Australia’s landforms from the Australian Soil Resource Information System (ASRIS). The ASRIS mapping hierarchy divides Australia into 3 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. Therefore, it is only possible to reach general conclusions about the state of the soil for each region—there are always local exceptions.

Table LAN3 Australia’s main types of soil

ASC order

Simplified description

Percentage of Australian soil

Anthroposols

Soils resulting from human activities

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

ASC = Australian Soil Classification

Key indicators of soil condition

A healthy soil has biological, chemical and physical properties that promote the health of plants, animals and humans, while also maintaining environmental quality (Soil Quality n.d.). The notion of ‘soil health’ reflects that soil is not an inert growing medium, but a living, dynamic environment, full of microbial and macroinvertebrate life. Many physical and chemical processes that occur in soils are mediated by biological processes, which operate at different rates across the landscape according to the climate, land use and soil type. Results from the current Biomes of Australian Soil Environments (BASE) project (Bioplatforms Australia 2014) should provide useful data for understanding how soil microbial diversity supports healthy soil function.

Soil health is the condition of the soil relative to a set of benchmarks that encapsulate healthy functioning. The key indicators of soil condition in agro-ecosystems are:

  • carbon and nutrient content
  • acidity (pH) and acidification trend
  • soil structure and porosity
  • topsoil thickness
  • secondary salinity.

The processes that control these indicators are interrelated to some extent; for example, a soil’s thickness, structure, porosity, and carbon and nutrient content determine its susceptibility to erosion.

The carbon and nutrient content reflect the soil’s fertility, and its ability to support vegetation and other biota. Native plants are adapted to the natural soil pH, whether it is acid or alkaline. Most crops, however, have a preference for slightly acidic soils (pH 5.5–6.5). Higher acidification can lead to a decrease in crop biomass and protective cover, a concomitant decrease in soil organic carbon and nutrient content, and eventually erosion that results in thinner topsoil.

In Australia, naturally saline or sodic soil occupies 27 per cent of the continent. Anthropogenic secondary soil salinity can result from changes in landscape hydrology due to land clearing, or irrigation with low-quality water and inadequate drainage. The removal of native vegetation changes the hydrological cycle, because trees and shrubs intercept significant quantities of rain—often 10–20 per cent of rainfall fails to reach the soil surface. If the original vegetation has been replaced by shallower-rooted species that use less water (e.g. annual crops and pastures), more water passes through the soil. This may lead to rising groundwater levels and, in some cases, secondary dryland salinity. If the salts are sodium carbonates, problems associated with soil sodicity will result; the soil will become more dispersible and therefore erode more easily.

Metcalfe D, Bui E (2016). Land: Soil: Understanding. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/land/topic/2016/soil-understanding, DOI 10.4226/94/58b6585f94911