A national study has highlighted the advantages of using agricultural lime to combat the production problems related to acid soils. The study predicted that the need for lime and accurate knowledge about its application will only increase.
The GRDC-commissioned study by the W A Department of Agriculture set out to answer questions such as: How much lime is being used on Australian farms? How much lime should Australian farmers be using now and in the future? What stops farmers using lime where they should? Are there alternatives to lime for managing acidity?
Researchers say there are two main causes of acid soils: acidification caused by the nitrogen cycle as nitrogen is lost from the soil by leaching and removal of lime in agricultural products.
Lime by the truckload
ln the five main cropping states of Australia, about 550,000 tonnes of lime were applied to all agricultural soils in 1989-90. a year of high lime use. Estimates for treating the nation 's 1.5 million hectares of extremely acid cropping soils with an ameliorative dressing of lime (1.5 t/ha) are the application of 2,295,000 tonnes of lime.
Just to maintain the current pH of the combined area (7.4 million ha) of very acid and extremely acid cropland soils would require preventative dressings of about 740,000 tonnes per year, according to report author, William Porter. The first step is soil testing for acidity Dr Porter said current estimates indicate that over the next few decade, a further 7.7 million ha of mildly acid soils will probably become very acid. Based on the same limited estimates of rates of acidification, stabilising the pH on this larger area will require annual preventative lime dressings of about 1,530,000 tonnes per year. "The estimates of lime requirements are by no means accurate predictions. but better numbers are not available." said Dr Porter. But the trend indicates that lime needs will only increase.
Lack of information
Dr Porter said farmers are often unfamiliar with the proper treatment of acidity which leads to either too much or too little lime being applied. The first step is soil testing for acidity - both topsoil and subsurface. Dr Porter said the results will vary from state to state and across a single farm. "Farmers should pick out areas that need lime and consult local guidelines for liming as appropriate." The study said the following often led to misuse of lime:
- • The need for more basic information. To manage soil acidity. farmers need to have quantitative information about the effect of soil acidity on production for their farming system and the rate of acidification of their soils. This information is available for few farming systems.
- Uncertainty about lime material. There is a need to define the quality of lime material for both treatment and prevention. There is also a need for clear, consistent description of lime quality throughout Australia.
- Concern about the high cost of lime. Farmers often perceive liming as prohibitively expensive without taking into account the fact that the time between re-applications of lime is likely to be between 5 and 20 years, and that not every paddock has to be limed in the same year. Agricultural liming is not out of line with other farm input costs. But, the study found, farmers need to be assured of the short-term or guaranteed productivity benefit of lime use to regard it as a necessary rather than optional input.
- Lack of awareness of subsurface acidification. and lack of knowledge of its management. The study found that current knowledge is inadequate to evaluate likely alternative methods for managing subsurface acidification. This holds particularly for the likely success of using lime to manage subsurface acidity. and alternalive methods of delivering lime (or other material) to the sub-soil.
Are there good alternatives to lime
Dr Porter said the perception that liming is very expensive has tempted farmers to look for alternatives. Lime is in fact the cheapest and most effective method of neutralising soil acidity and raising soil pH. Other options are to reduce the rate of acidification, or to reduce the sensitivity of the system to low pH. The study reached two conclusions about these opltons:
- Management options to reduce soil acidification exist and include improved nitrogen management. However, some of these options have not yet been properly tested.
- Certain materials (such as phosphate, gypsum. fluoride) and acidtolerant plants may be used in the short term. Long-term acidification is not corrected by these measures.
In general, the study cautions farmers about relying on alternatives to lime. Alternatives do not have the potential to delay the need for a regular lime program, and to reduce the amount of lime required to maintain the system. But without lime, signilicant long-term problems could develop, such as continued subsurface acidification, which may go unrecognised. This could ultimately lead to an extremely acid subsurface profile requiring very expensive liming to correct.
Other potential problems include: loss of clay minerals, leading to a permanent reduction in the soil's ability to maintain a supply of nutrients and water; accelerated loss of nutrients, leading to higher fertiliser requirements; reduced diversity in the the farming system leading to farmers becoming dependent on a few, very acid-tolerant plant species.