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Soil acidity
Soil acidity affects availability of nutrients and toxic elements in the soil which can limit plant growth and agricultural production.
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Soil acidity in agriculture can contribute to:
- increased nitrate pollution of groundwater and reduced water quality;
- reduced plant yields through reduction in the availability of nutrients or an increase in the availability of toxic elements (eg aluminium);
- reduced farm income, land values and domestic/export earnings;
- reduced plant species options for agriculture;
- reduced vegetative cover, leading to accelerated run-off and erosion;
- degradation of the clay minerals of soil (i.e. hard setting) resulting in reduced fertility;
(National NRM M&E Framework, 2004b; Beeston, et al. 2005).
Soil acidification (the accumulation of acid in the soil) is a natural process that may accelerate under agricultural management. Cropping is generally more acidifying than growing pastures (National NRM M&E Framework, 2004b; NLWRA, 2001).
Desired outcome
To maintain soil acidity at a suitable level for agricultural production.
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This objective is implicit within the NLWRA Australian Agriculture Assessment 2001 report (NLWRA, 2001) and the National NRM M&E Framework (2004a).
Indicator
Topsoil pH (from ASRIS dataset- NLWRA, 2001)
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Soil acidity can be measured by pH. Soils with a lower pH are more acidic as shown in Figure 1.
Figure 1: Soil pH range (pH measured in 0.1 M CaCl2) (Source NLWRA, 2001).pH is a logarithmic scale ranging from 0 to 14 (figure 1). A neutral pH is seven, and each pH unit below seven (e.g. six) is ten times more acidic. Conversely, soils with pH values above seven become progressively more alkaline (NLWRA, 2001).
Soil condition is a matter for target in the National NRM M&E Framework and soil acidity/acidification is a recommended indicator (National NRM M&E Framework, 2004a&b).
Other possible indicators
Proportion of farms that have significant soil acidity problems (ie ABARE Resource Management Surveys).
Lime sales may be used as a surrogate for soil acidification (ABARE surveys or ABS AgStats).
The advantages and disadvantages of using these indicators are discussed under ‘data’.
Summary measure
The proportion of land with topsoil pH above 5.5 (ie suitable acidity).
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Although any soil below pH 7 is considered acid, a pH of 5.5 or less is considered a useful lower threshold of acidity below which there is some likelihood of soil degradation and production loss, while a pH below 4.8 may cause significant limitations on plant growth (NLWRA, 2001; National NRM M&E, 2004b). Highly alkaline soils (pH above 8.5) are also considered undesirable for plant growth (see alkalinity component).
Results
To expand view, click image
Figure 2. Summary measure for Soil acidity
The summary measure (Figure 2) shows the extent to which the desired outcome is being achieved on a scale of 0 to 1. The summary measure for the year 2001 is just above 0.5 based on the indicator values displayed in Map 1. Table 1 and 2 give results by GRDC Agro Ecological Zones.
To expand view, click image
Map 1: Topsoil pH for land under grains cropping.
Table 1: Area (in 000’s Ha) and percentage of land with suitable acidity for grains cropping in GRDC Agro Ecological Zones
| Regions | GRDC Agro Ecological Zones | Cropped land with pH >5.5 (000’s Ha) | Total cropped area (000’s Ha) | Performance measure (% of cropped land with pH >5.5) |
| Northern | Qld Central | 1,590 | 1,619 | 98 |
| NSW NorthEast-Qld SouthEast | 3,299 | 3,801 | 87 | |
| NSW NorthWest-Qld SouthWest | 1,533 | 1,576 | 97 | |
| Southern | NSW Vic Slopes | 854 | 3,137 | 27 |
| NSW Central | 1,328 | 1,656 | 80 | |
| SA Midnorth-Lower Yorke Eyre | 2,181 | 2,183 | 100 | |
| SA Vic Bordertown-Wimmera | 3,636 | 4,113 | 88 | |
| SA Vic Mallee | 3,270 | 3,271 | 100 | |
| Tas Grain | 0 | 304 | 0 | |
| Vic High Rainfall | 380 | 1,248 | 30 | |
| Western | WA Central | 334 | 5,897 | 6 |
| WA Eastern | 0 | 1,557 | 0 | |
| WA Mallee and Sandplain | 303 | 1,187 | 25 | |
| WA Northern | 61 | 2,214 | 3 | |
| Total | All Zones | 17,178 | 32,144 | 53 |
Source- NLWRA 2001 Australian Soil Resources Information System
The data suggest that soil acidity is an issue in almost half of the grains cropping area. Soil is inherently patchy and the amount of soil affected within any location will vary considerably. Soil acidity is not a major issue in the Northern Region and the North and West zones of the Southern Region which have performance measures of 80% or higher. Soil acidity is of greatest concern in the Western Region and the south-east of the Southern Region where performance measures drop below 30%. Although performance measures for the four WA zones are less than 25%, over 90% of their land has pH >4.8, ie moderately acid or better (Table 2). These results are consistent with Beeston et al. (2005).
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Table 2: Area (in Ha) in each pH class for grains cropping in 14 GRDC zones
Regions GRDC Agro Ecological Zones Extremely acid Highly acid Moderately acid Mildly acid Mildly alkaline Moderately alkaline Highly alkaline Total cropped area pH ≤4.3 pH 4.3-4.8 pH 4.8-5.5 pH 5.5-7 pH 7-7.7 pH 7.7-8.5 pH ≥8.5 Northern Qld Central 300 300 28,400 1,463,400 123,100 3,900 - 1,619,400 NSW NorthEast-Qld SouthEast 8,100 31,000 462,900 2,864,700 433,800 - - 3,800,500 NSW NorthWest-Qld SouthWest - 200 42,700 1,509,700 21,700 1,600 300 1,576,200 Southern NSW Vic Slopes 1,400 528,200 1,753,900 853,400 100 - - 3,137,000 NSW Central 300 11,500 316,200 1,189,200 124,700 13,600 - 1,655,500 SA Midnorth-Lower Yorke Eyre - - 2,300 576,200 787,300 764,200 53,100 2,183,100 SA Vic Bordertown-Wimmera 78,100 240,700 157,900 2,269,200 1,293,400 73,300 200 4,112,800 SA Vic Mallee - - 1,700 649,400 1,813,400 780,300 26,600 3,271,400 Tas Grain 500 83,600 220,000 200 - - - 304,300 Vic High Rainfall 77,200 627,000 163,800 378,600 1,600 - - 1,248,200 Western WA Central - 187,100 5,375,700 333,300 1,100 - - 5,897,200 WA Eastern - 600 1,555,800 300 - - - 1,556,700 WA Mallee and Sandplain 2,100 30,200 852,200 302,200 - 400 - 1,187,100 WA Northern
13,500
179,900
1,959,800
60,500
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-
-
2,213,700
Total All Zones 181,200 1,920,000 12,864,900 10,986,900 4,477,100 1,633,400 80,200 32,143,700 Source- NLWRA 2001 Australian Soil Resources Information System
Data
Data quality:
Spot observations were collected and analysed by State and CSIRO soil agencies. These samples were tested for pH in soil laboratories, with the resulting pH value (and method) recorded (location, depth, horizon). pH in water (method 4A1) was standardized to pH in calcium chloride. Input sample locations have variable positional accuracy most locations are expected to be within 100m of the recorded location and vertical information is expected to be within 20 mm of the recorded depth below surface. Input sample attribute accuracy was provided at one decimal point and is deemed to be + - .1 pH.
Additional environmental parameters were used to model and predict soil pH at untested sites and provide a modelled surface layer. The other parameters used in the production of the modelled surface have a range of positional accuracy ranging from + - 50 m to + - kilometres. This contributes to the loss of attribute accuracy in the modelled surface. The modelled predictive surface is a 250m grid and has a locational accurate of about 1m. To provide consistency the dataset was resampled, increasing the cell size to 0.01 deg. The positional accuracy has not been altered in this process. Uncertainty surfaces for the modelled data are available. For more information see the Australian Soil Resources Information System or the metadata (see Lineage).
The attribution of landuse is bounded by the quality issues related to the NLWRA Landuse map. See lineage for more information.
Lineage:
The acidity map for Australia was compiled by NLWRA. The dataset presents a surface of predicted pH (X 1000) of layer 1 (A Horizon - top soil) surface for the intensive agricultural areas of Australia. Spot observations were collected and analysed by State and CSIRO soil agencies and then used to model predicted soil pH at untested sites (NLWRA, 2002). The map was further classified into seven different categories using Figure 1 (More information- Soil Acidity metadata). The NLWRA Landuse map was used to identify areas that were cropped for grains (More information- Landuse Map metadata). Thereafter using the identified cropped area, the area of land under each pH category was extracted for each GRDC zone (More information- GRDC Zones metadata) (Table 1).
Other possible sources of data: Other datasets that could have been used for acidity included:- The Digital Atlas of Australian Soils mapped soils at a broad scale- 1:2 million and provides 'soil landscape' polygons for which the soil type can be assumed to be the dominant soil (though there may be as many as 20 or more soil types in each polygon). The soils were classed and estimated pH ranges are given for each class, whereas the NLWRA (2001) measured samples’ pH directly. For these reasons the Digital atlas was not used. An advantage of the Digital Atlas is that its scope is the whole of Australia, whereas the NLWRA (2001) dataset is only for the intensive landuse zone.
- ABAREs 2001-2002 Resource Management Survey (2002) measured the proportion of farms that experienced significant soil acidity problems reported at the Statistical Division (SD) level for Australia. This data set provides a measure of the impact of soil acidity rather than a measure of soil acidity itself and reporting at SD level means the spatial specification is very coarse. This is ‘presence-absence’ data for a farm, so provides a less accurate estimate of the area of the problem. The survey is based on farmers opinion, which may vary due to various factors, and is a less direct measure of soil acidity.
- Lime sales are monitored through ABS AgStats and ABARE surveys. Limes sales can be used as a surrogate indicator for soil acidity/acidification, but have disadvantages relative to direct measures of pH. Lime sales are influenced by the distance from market (ie transport cost), environmental conditions, fertiliser use and financial performance of farms. On the other hand, lime sales provide a time series of information that is not currently available for soil pH.
Responses
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The main responses to acidic soils in grain cropping are (NLWRA, 2001, NRM M&E 2004):
- Use of lime to ameliorate acidic surface and subsoils
- Controlling acid additions through better management of nitrogen cycles (fertilisers/legumes), and carbon cycles
- Use of plant tolerance to reduce the effect of acidic soil conditions
For acid-sulphate soils breaking the acid scalds to leech acid and salts will enhance revegetation. Mulching scalds and acid soils and revegetating with acid tolerant plants will tie-up acid soils (Woodhead, et al. 2000).
More information on management practices
-Minimum tillage and direct drilling
Interactions with other components
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Changes in this component are linked to changes in ‘soil nutrients’, ‘surface water acidity’, ‘surface water other’ ‘ground water acidity’ and ground water other’ (acid water can activate some elements in water). This component affects ‘individual income’ directly through costs associated with adding lime and fertilisers.
External drivers
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The price of lime. Distance from lime source. Fertiliser use. Commodity prices. Crop selection/choices. Individual income. Climate variation (eg Drought).
References
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ABARE (2002) 2001-02 Resource Management Survey: Natural Resource Management on Australian Farms report, ABARE, Canberra.
ABARE (2005) Australian Farm Surveys, ABARE, Canberra. see Website- http://www.abareconomics.com/interactive/farm_surveys/, Updated June 2005
ABS (2001) ABS Agricultural Statistics, ABS, Canberra. see Website - http://www.abs.gov.au/ausstats/abs@.nsf/b06660592430724fca2568b5007b8619/a89f51dcb5e2e31bca2568a900139429!OpenDocument, Updated 8 December 2006.
ASRIS (2005) Australian Soil Resources Information System. CSIRO, see Website- http://www.asris.csiro.au/index_ie.html, Created 2004, Updated 15/11/2005.
Beeston, G., Stephens, D., Nunweek, M., Walcott, J. and Ranatunga, K. (2005) GRDC Strategic Planning for Investment based on Agro-ecological Zones- Final Report, BRS, WA Agriculture.
NLWRA (2001) Australian Agriculture Assessment 2001 report, NLWRA, Canberra. Website- http://audit.ea.gov.au/ANRA/land/land_frame.cfm?region_type=AUS®ion_code=AUS&info=soil_acid Accessed 22/3/2006.
National NRM M&E Framework (2004) Soil condition: soil acidity, NRM Australian Government. Website- http://www.nrm.gov.au/monitoring/indicators/soil/acidity.html, Updated, Wednesday, 22-Dec-2004, Accessed 23/9/2005.
