Australian Natural Resources Atlas

Natural Resource Topics

Australian Agriculture Assessment 2001

Australian agriculture assessment 2001
National Land and Water Resources Audit, 2001

Appendix 2. Australian Soil Resources Information System

pH (topsoil, subsoil)

Soil pH is a measurement of the relative acidity or alkalinity of the soil and provide a guide to the overall chemical balance of the soil. The pH scale is divided into 14 points. Seven is the neutral point and each number below seven indicates ten times more acidic while each number above seven indicates ten times more alkalinity. On the pH scale:

Methods used to measure soil pH affect results with the main difference being between field kits (approximating pH as measured in water) and laboratory measurements (usually reporting pH in CaCl2 solution).

Plants do not grow well outside the range of 4.5 - 8 since soil pH determines the availability of soil nutrients to plants:

Plant preference for certain types of soil means that they are often starved of important nutrients or damaged by unwanted minerals:

Strongly acid soils have pH in water below 5.5 or pH in CaCl2 below 4.5. Agricultural problems associated with preventing legume nodules forming; release of toxic levels of aluminium reduced availability of phosphorus; and some trace element deficiencies.

Acid soils are commonly treated by the addition of lime to neutralise the acidity (see Soil acidification section of the Australian Agriculture Assessment 2001 report).

How does it vary and what is it related to?

Soil pH is determined partly by the chemical composition of material from which the soil has developed, and partly by how much the soil has been leached. Soils derived from limestones or basalts are generally quite alkaline. Soils derived from quartz-rich rocks (sandstones, granites) may be acid. Acid soils develop in areas where rainfall is high, since water percolating through the soil washes away soluble bases such as calcium.

pH of the soil may also change as a result of farming practices. Cropping tends to increase the acidity of soils in the long term, since ploughed soils are more prone to leaching and nutrients are removed in crops. Prolonged use of fertilisers also increases soil acidity. Acidification of improved pastures due to fertiliser application is very common in Australia (see Soil acidification section)

Acid sulfate soils, with very low soil pH values, can develop in particular environments where pyrite occurs in the soil. Disturbance of the soil leads to oxidation and release of sulfuric acid. Acid sulfate soils occur in coastal marine environments, saline discharge areas and mine tailings.

pH
< 4.3 extremely acid
4.3 - 4.8 highly acid
4.8 - 5.5 moderately acid
5.5 - 7.0 mildly acid
7.0 - 7.7 mildly alkaline
7.7 - 8.5 moderately alkaline
> 8.5 highly alkaline

How and why does it vary across Australia?

The digital map of pH shows that, as expected in light of their carbonate content, Calcarosols in South Australia, Victoria and eastern West Australia have alkaline pH (> 7). Vertosols in the Murray and Darling alluvial plains and Queensland also have alkaline to neutral pH. Forested areas and coastal areas with high rainfall have the lowest pH.

Tables A18 and A19 show variation in topsoil and subsoil pH for different land use types. These show that:

How can this map be applied?

Distribution of soil pH can be used to assess soil acidity known as surface acidity when it is in the topsoil or ploughed layer (roughly 0 - 30 cm depth) - see Figure A22. Subsurface acidity (see Figure A23) occurs below the ploughed layer (30 - 60 cm). It can have as much effect on reducing yield as surface acidity but is much more difficult and costly to correct.

Level of uncertainty

The models for pH are amongst the most reliable of the point-based models.

Error diagnostics

Error diagnostic Topsoil Subsoil
Number of points used 24319 12193
R2 0.677 0.605
Relative error 0.51 0.54

Soil points with pH measurement for topsoil are well distributed and the model is reliable in most areas. It performs best in the Murray-Darling Basin, Victoria and southern and central Queensland, and is weakest in South Australia, Northern Territory, Moreton (Queensland) and coastal New South Wales.

The subsoil model is less reliable (due to only half the number of points being available). It is most reliable in the Murray-Darling Basin, Tasmania and Queensland and is weakest in South Australia, northern New South Wales and the Northern Territory.

Table A18 pH of topsoil by percent of land use type.

< 4.3 4.3 - 4.8 4.8 - 5.5 5.5 - 7.0 7.0 - 8.5 > 8.5 Total land use
class area (ha)
(% of land use class)
Conservation and natural environments 8 23 35 17 16 1 54 814 300
Production from native environments 2 12 30 42 13 2 184 376 200
Cropping 1 6 41 33 19 0 22 241 000
Grazing modified pasture 1 19 39 29 11 0 18 482 600
Horticulture 2 27 23 22 26 0 351 500
Irrigated cropping 2 4 22 66 6 0 948 800
Irrigated modified pasture 0 9 19 65 7 0 1 079 900
Total area* 282 294 300

Table A19 pH of subsoil by land use type.

< 4.3 4.3 - 4.8 4.8 - 5.5 5.5 - 7.0 7.0 - 8.5 > 8.5 Total land use
class area (ha)
(% of land use class)
Conservation and natural environments 10 14 25 36 15 0 54 814 300
Production from native environments 2 4 15 52 27 0 184 376 200
Cropping 1 3 9 54 33 0 22 241 000
Grazing modified pasture 0 5 26 50 19 0 18 482 600
Horticulture 6 17 19 31 28 0 351 500
Irrigated cropping 0 2 12 42 44 0 948 800
Irrigated modified pasture 1 5 8 38 48 0 1 079 900
Total area* 282 294 300

* Area of river basins containing intensive agriculture

Distribution of pH of the topsoil within the river basins containing intensive agriculture. Distribution of pH of the subsoil within the river basins containing intensive agriculture.

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