Indicator: Soil acidity

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What the results tell us for the ACT

There is insufficient information available to provide a detailed analysis of the extent of soil acidification or identify any trends in the ACT. The available data are patchy but provide a starting point for discussion. Currently there are two known sources of broadscale data. These are the Land Capability Survey reports and broad brush data collected by Jenkins (2000) in the preparation of the Regional soil landscape map.

The figures in Table 1 were extracted from Land Capability Survey reports undertaken between 1989 and 1993.

Table 1. Area and percentage of land tested with pH below 5.5 (figures relate to topsoil)
Location pH and % below pH 5.5 Area surveyed (hectares)
Tidbinbilla Nature Reserve pH above 5.5  
Abattoir Holding Paddocks 6% below 5.5 1 000
Pine Ridge pH 5.5 1 200
Huntly pH 5.5 5 000
Woolshed Creek pH 6.0 3 000
West Belconnen 30% less or = 5.5 1 200
Bulga Creek pH above 5.5 3 855
CSIRO Gungahlin 2% below 5.5 1 000
Guises Creek pH mostly below 5.5 5 000
Woden Creek 10% below pH 5.5 1 000
Paddy's River pH above 5.5 10 000
Jerrabomberra 5% below 5.5 2 800
Pialligo pH above 5.5 1 682

Data collected by Jenkins (2000) in the preparation of the Regional soil landscape map are shown in Table 2 below.

Table 2. Examples of recorded acid topsoils in the ACT
ACT pH Locations where acid soils have been recorded
4.5 – 5.0 Majura, Gungahlin, Hall, Coppins Crossing area,
Lower Molonglo River Corridor, Murrumbidgee River Corridor, Woden Valley
5.0 – 5.5 North Shore Lake Burley Griffin, East Belconnen

Both sets of data (Table 1 and Table 2) demonstrate the existence of strongly acid (pH 5.0–5.5) and very strongly acid (pH 4.5–5.0) soils in the ACT. Nevertheless, they must be viewed with caution since they are not comprehensive and many of the soils in the ACT are inherently acidic. Further studies are required to establish the full extent of acid soils in the Territory and to determine whether they are caused by degradation or are the result of natural processes.

A more detailed study was carried out for the Reedy Creek Landcare group in 2000 and involved taking samples across a transect of the catchment. Samples were taken at 0–10 centimetre depth and 10–20 centimetre depth.

Table 3. Reedy Creek Soil Sample Summary – Preliminary field data
Site pH (Ca Cl2)
0-10 cm
pH (Ca Cl2)
10-20 cm
pH (H2O)
00-10 cm
pH (H2O)
10-20 cm
1 5.8 6.1 6.9 6.9
2 5.9 6.2 6.9 6.9
3 5.6 5.9 6.7 6.9
4 5.9 5.9 6.6 6.9
5 5.6 6.0 6.6 6.6
6 5.0 5.1 6.5 6.2
7 6.5 6.0 7.1 6.9
8 5.6 6.1 6.7 7.3
9 7.1 6.1 8.2 7.5
10 5.6 5.8 6.6 6.9
11 5.9 5.5 7.1 6.9
12 6.1 6.3 7.4 7.3
13 6.2 6.6 7.4 7.5
14 6.9 7.3 7.8 8.2
15 4.9 5.5 6.2 6.6
16 5.2 5.3 6.6 6.6
17 6.0 6.1 7.1 7.1
18 5.8 6.2 6.9 7.1
19 5.0 5.6 6.2 6.6
20 5.4 5.5 6.8 6.6
21 5.4 6.0 6.8 7.4

Table 3 shows that generally soil acidity is not a major problem in the catchment but that some sites are more acidic, possibly related to landuse and soil type. Anecdotal evidence indicates that soil acidity has improved in parts of the catchment as a result of liming treatment. More detailed laboratory analysis of the samples in this study will be undertaken.

Acid soils are of particular significance to agricultural production and it would be helpful for rural lessees to know the pH status of their soils so that they can take remedial action if necessary. While resources are unlikely to be available for the ACT Government to carry out the necessary testing, it may be feasible to explore funding to buy kits to enable farmers and community groups to conduct soil pH tests.

Agricultural production in the ACT is mainly fine wool production, based on low inputs – low outputs on native pasture. It has been assumed (but not confirmed) that this system has maintained the balance without causing a decline in pH. However, where introduced pastures have been established there is an increase in the risk of acidifying soils.

About the data

Soil acidity data reported here were measured in the laboratory using a 1:5 soil:water solution, while the New South Wales Agriculture data are reported as pH (calcium chloride).

| More information about soil pH measurements |

References

Jenkins, B.R. (2000) Soil Landscapes of the Canberra 1:100 000 Sheet Report, Department of Land and Water Conservation. Sydney.

Jenkins, B.R. (2000) Soil Landscapes of the Canberra 1:100 000 Sheet Map (Topsoil Acidity) Department of Land and Water Conservation. Sydney.

Data from these two reports are spatial and reliability is moderate since the collection was broadscale although the techniques employed were accurate.

Land Capability Survey Reports (1989–1993). The reports were developed by various authors and funded through the National Soil Conservation Program. A range of 13 different land uses were nominated for Land Capability analysis.

Further information is available by contacting Mr Bill Woodruff, Parks & Conservation, Environment ACT, PO Box 1065, Tuggeranong ACT 2901, phone 02 6207 2195

Data from this series of reports are spatial and reliability is moderate only since the surveys were not comprehensive and the method of determining pH is not recorded.

Reedy Creek Catchment Study

Further information available from John Rees, Reedy Creek Catchment Group Coordinator, PO Box 484, Fyshwick ACT 2609.

Data from this study are spatial and reliability is low (preliminary data).

Description: What does 'soil acidity' measure?

Which data are collected?
  • area and proportion of land with a decline in pH to below pH 5.5
  • area and proportion of land with potential acid sulphate soils
Why do we report this indicator?

Soil acidity is a widespread and generally under-reported environmental problem in the Region, and in much of Australia. Although soil acidification is a natural process, soils can become more acidic more quickly under some landuses and land management strategies.

Many soils in the Australian Capital Region are naturally acidic, showing marked changes through the soil profile, for example a brown loamy topsoil (A horizon) will often overlay a red or yellow clay subsoil (B horizon). These soils are a thought to be a product of millions of years of weathering and leaching that has removed nutrients and smaller particles from topsoil, which tends to acidify soils. Many native plants in eastern NSW are adapted to growing on acid soils.

Agricultural landuses, which remove even more nutrients through crops or livestock, are one of the main ways in which soils are further acidified in rural Australia. The result can be loss of agricultural productivity, to the point where some types of agriculture (e.g. cropping) are not possible. This indicates a decline in the health of the land in State of the Environment reporting.

Other activities which can cause soils to acidify include mining, disposal of industrial waste, and exposure of potential acid sulfate soils. Disturbance of potential acid sulfate soils can have adverse effects on biodiversity, and on the condition of marine, estuarine and stream waters. Because of this, it is important to identify and manage potential acid sulfate soils appropriately.

Measuring soil acidity

Acidity is reported in terms of pH. Soils of a pH less than 7.0 are considered to be acidic, soils of pH 7.0 are considered neutral, and soils of a pH greater than 7.0 are alkaline. Although different plants require different soil pH levels to thrive, most agricultural crops and pastures will start to suffer if the soil acidity falls below a pH of 5.5.

Note that there is sometimes confusion over reporting of soil pH values as they can be reported as pH (water) or pH (calcium chloride) values. pH determination (1:5 Soil:CaCl2) is considered to approximate average soil solution calcium and salinity levels. It is generally 0.5 to 1.0 units less than pH 1:5 soil:water, and so it is important to check which method was used to measure soil pH when assessing laboratory or field measurements.

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