Land: Agricultural soil acidification
- Authors
- Reviewer
- Key findings
- Indicators and summary of status
- Importance
- Pressure and condition
- Response
- References
Authors
Philip Bloesch and Phil Moody, Department of Natural Resources and Water
Reviewer
John Armour, Department of Natural Resources and Water
Key findings
- Soil acidification is an endemic process in every agricultural system.
- Soil acidification is particularly significant in agricultural systems where there is a large amount of harvested product per unit area and/or high ammonium-based fertiliser inputs.
- Delaying management responses to the acidification inherent in an agricultural system results in the development of subsoil acidity, which is difficult and costly to correct.
- Awareness of this issue and timely intervention are the tools that land managers can use to prevent this insidious land degradation process.
Indicators and summary of status
Indicator |
Status of indicator |
Soil acidification hazard |
In the absence of a liming program, the soil acidification hazard predicts that the pH values of more than 20% of the soils cropped to sugarcane, banana, irrigated maize, vegetables, irrigated macadamia, papaya and passionfruit will decline by one unit in less than 20 years. |
A Soil Acidification Index based on land use and lime statistics was developed in State of the Environment Queensland 2003 ; this was based on 2002 ABS data at the Statistical Local Area level. Because of the lack of updated data since then, no recalculation of the index can be undertaken to assess trends and status. Until statistics of lime use in identified agricultural systems are collected periodically, it will not be possible to assess trends in agricultural soil acidification.
A new indicator for assessing the hazard of agricultural soil acidification based on the time for soil pH to decline by one unit under the current land use, in the absence of a liming program, is presented in 'Pressure and condition'.
Importance
Soil acidification reduces crop production through a range of toxicities (such as aluminium, manganese) and/or deficiencies (such as calcium, molybdenum). If allowed to continue without amendment, soil acidification has increasing impacts on subsurface layers. Subsoil acidity is difficult and costly to correct. An indirect effect of acidification is reduced surface cover, which leads to the off-site impacts of increased runoff and erosion and nitrate pollution of groundwater. Thus this degradation has both production and environmental consequences.
Soil acidification is caused by acid inputs mainly arising from the use of high rates of ammonium-based fertilisers in excess of crop requirements, leaching of nitrate below the root zone, and high rates of product removal.
Pressure and condition
The major pressures causing soil acidification are productivity-related: the application of high rates of ammonium-based fertilisers to maximise crop yields, and high rates of product removal. Community and government concerns about water quality in environmentally sensitive areas such as the Great Barrier Reef, together with field research that has demonstrated that nitrogen fertiliser inputs can be reduced without yield penalty in the sugar and banana industries, have led to a reduction in the average amounts of nitrogen fertilisers being applied in these industries. For example, the average nitrogen application for bananas in north Queensland has been reduced from 525 kg N/ha/year in 1995 to 310 kg N/ha/year in 2005. Application of lime is the necessary management response to high rates of product removal.
As indicated in State of the Environment Queensland 2003 , the pressure indicator is the size of the discrepancy between the rate of acid addition inherent in the current agricultural system and the amount of amendment being applied to ameliorate this acid addition. However, no more recent lime application figures are available than the ABS 2002 figures used in that report, so the Soil Acidification Index developed in that report cannot be updated to indicate condition trend.
In the likely continuing absence of lime statistics, a new indicator, Soil Acidification Hazard, has been developed (Bloesch et al. 2006). The assessment of hazard can be updated as land use data are updated. The framework for assessing the hazard of agricultural soil acidification is based on the time for soil pH to decline by one unit under the current land use in the absence of a liming program. The input data are soil properties which allow soil pH buffer capacity to be calculated and the Net Acid Addition Rate (NAAR) of the current land use. The Soil and Land Information (SALI) database provides key soil properties and the Queensland Land Use Mapping Program (QLUMP) database provides the current land use according to the Australian Land Use and Management (ALUM) classification scheme. Soil pH buffer capacity is calculated from clay content and organic carbon content of the surface soil (Aitken et al. 1990). It is very likely that the calculated pH buffer capacity is an overestimate because the soil organic carbon contents derived from SALI for use in the pedotransfer function are for undeveloped soils; following development, soil organic carbon levels decline and so would pH buffer capacity. The NAAR values assigned to the various land uses are based on published data and expert opinion.
In the absence of a liming program, the pH values of more than 20% of the soils cropped to sugarcane, banana, irrigated maize, vegetables, irrigated macadamia, papaya and passionfruit are predicted to decline by one unit in less than 20 years. It is essential that appropriate liming programs be implemented in these industries to prevent subsoil acidification.
Response
A concerted government and non-government response to the issue of agricultural soil acidification is needed. Subsoil acidity is often an unknown crop growth restraint because soil tests are often conducted only on surface soils (0-15 cm), thus ignoring the important part of the root zone below this depth. Because subsoil acidification results from lack of attention to a surface soil acidification problem, it is in the interests of both industry and government to ensure that all land managers are aware of soil acidification and its importance to their agricultural systems.
Technical knowledge gaps are the method and frequency of application of liming materials to the reduced tillage systems that are rapidly gaining acceptance in many agricultural industries, and the NAAR associated with leguminous crops that are grown as a component of agricultural rotations.
References
Aitken, R.L., Moody, P.W. and McKinley, P.G. 1990, 'Lime requirement of acidic Queensland soils, 1. Relationships between soil properties and pH buffer capacity', Australian Journal of Soil Research 28: 695-701.
Bloesch, P., Le Grand, J., Moody, P., Moss, J. and Searle, R. 2006, Soil erosion and soil condition hazard maps for Great Barrier Reef catchments of Queensland , QNRM06214, Department of Natural Resources, Mines and Water, Brisbane.
<<Return to State of the Environment Queensland 2007 content page
Last reviewed 16 May 2011
Last updated 4 September 2007