- Aluminium (Al) is major constituent of most soils but is not required for plant growth
- Aluminium can only affect plants when it moves into a soluble or exchangeable form
- Soluble or exchangeable aluminium levels will increase as a consequence of increasing acidity (pHCa below 4.5), usually as a result of land management practices
- Plant species differ widely in their tolerance of exchangeable aluminium in soil
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Understanding the problem
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| Why is it important to me as a farmer?
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- In a situation of increasing soil acidification with time, it is important for farmers to be conversant with the signs of acidity related infertility pro
- Plant growth, particularly growth of seedlings, can be greatly harmed by high levels of exchangeable soil aluminium
- Plant species differ widely in their tolerance of excess aluminium in soil
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- The acid soil conditions in parts of south west Victoria are known to limit plant production
in agricultural industries. For example, crops and pastures such as barley, phalaris and
lucerne have been limited by aluminium toxicity in the Central Highlands. This is because
aluminium phytotoxicities (along with manganese) are more soluble in acid soils
- Soil acidification is a process that is more common in sandier soils, where nitrogen/nitrate leaching occurs more readily
- Soil acidification is found throughout the Corangamite region, but appears to be most prevalent in areas of higher agricultural production. These areas have high levels of agriculture-related inputs and outputs, which also increase the likelihood of soil acidification
- The cumulative impact of loss of nitrate nitrogen below the root zone, the use of acidifying fertilisers and removal of alkaline products from the paddock all contribute to increasing soil acidity and increasing aluminium levels
Figure 1 - Areas of moderate, high and very high soil acidification susceptibility in the
Corangamite region. - Source: CCMA
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- Aluminium has not been shown to be essential for plant growth. However aluminium
becomes increasingly soluble as the soil pHCa decreases below 5.0. The soil solution
aluminium reacts with root cell wall materials and cell membranes, restricting cell wall
expansion and hence root growth
- High aluminium levels can be toxic to plants, but aluminium generally falls to harmless
levels once the pHCaCl2 exceeds 5.0 (see below)
Figure 2 - Effect of pHCa on the availability of plant elements. - Source: DEPI
Victoria
[View larger image]
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| How to recognise it in the paddock
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- Signs of aluminium toxicity may resemble those of phosphorus deficiency, including:
- overall stunting
- small, abnormally dark green leaves
- purpling of stems, leaves and leaf veins; and yellowing and
- death of leaf tips
- Aluminium-injured roots are typically stubby and lacking fine branching, while the root tips are thickened
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Soil Testing
- Plant analysis is of limited use in detecting aluminium toxicity in the field. Small amounts
of dust contamination on the plant material can easily dominate the measured aluminium
levels, even where aluminium is at toxic concentrations in the plant
- Soil pH levels and soil aluminium analyses are more reliable than plant analysis in
detecting aluminium toxicity
- There are three common tests for the presence of aluminium:
- Potassium chloride (KCl) extractable Al method. It was developed to assist in the
establishment of lucerne
- Extraction by calcium chloride (CaCl2)
- Exchangeable Al as a percentage of Cation Exchange Capacity (CEC)
- These are offered by commercial laboratories. Interpretation is specific to method, soil,
plant species and varieties
- Exchangeable aluminium is used to determine the requirement for lime where
aluminium-sensitive species, such as lucerne, white clovers and, to a lesser extent, sub
clovers, are concerned. High aluminium levels can be toxic to plants, but aluminium
generally falls to harmless levels once the pH (CaCl2) exceeds 5.0
- Exchangeable aluminium should be the lowest amount of the cations. The desirable
amount is less than 1%
- CEC is a measure of the soil�s capacity to adsorb and hold cations (positively charged ions). This measurement
provides an indication of the amount of nutrients available in the soil, their ratios, and the
soil�s ability to hold nutrients (the higher the CEC, the more nutrients a soil can hold)
Use of Lime
- The only way to reverse soil acidification which may be increasing the levels of
aluminium in the soil is to raise the pH through the application of alkaline minerals such
as agricultural lime to the soil
- The effectiveness of an application of lime will depend on
the relationship between pH, soil type, buffering capacity and enterprise. Lime can be
incorporated into the soil or simply top-dressed and left to leach into the soil with
subsequent rainfall
Use of suitable pasture species in response to aluminium in the soil
- The following table ranks the suitability of certain pasture species to aluminium levels, expressed as a percentage of the soil�s cation exchange capacity. Also listed are some crop species, indicating the exchangeable aluminium levels above which yields are reduced
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Table 1 - Aluminium ranges and their suitability to most pasture species and specific crop
species. - Source: DEPI Victoria
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Exchangeable Aluminium (% of Soil CEC) |
Comment |
Pasture and crop species whose yields reduced if exchangeable Al is above the base of the range indicated |
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1-3 |
Most sensitive species not
affected |
Lucerne |
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3-5 |
Generally accepted |
Annual medics, barley |
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5-10 |
Marginal |
Phalaris seedlings, red
clover, some sub clovers,
wheat |
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10-20 |
Moderate |
Maize, white clover, some
oats, sub clover, ryegrass,
fodder rape, some
triticales |
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20-30 |
High |
Lupins, cocksfoot, some
oats, cereal rye |
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>30 |
Very High |
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- Lucerne establishment and persistence are particularly susceptible to exchangeable
aluminium in both the topsoil and the subsoil. The desirable aluminium levels in the
topsoil for lucerne establishment, as measured by the three methods mentioned earlier,
are:
- Less than 1%, if measured as part of the cation exchange capacity
- Less than 2 mg/kg (or ppm), if measured by the CaCl2 method
- Less than 50 mg/kg (or ppm), if measured by the KCl method
- Subsoils should also be soil tested if lucerne is to be grown. Lucerne is a deep-rooted
plant, and it should not be sown if the level of aluminium in the subsoil, as measured by
the KCl method, is above 50 mg/kg
Other related questions in the Brown Book
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Brown Book content has been based on published information listed in the Resources and References sections below
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- Surface Soil pH. Victorian Resources Online - Department of Primary Industries,
Victoria.
- Tools and systems for assessing soil health. Victorian Resources Online - Department
of Primary Industries, Victoria.
- Interpreting Soil and Tissue Tests. Section 8.1.8 - Department of Primary Industries,
Victoria.
- Acid Soils. Victorian Resources Online - Department of Primary Industries, Victoria.
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- Clarkson T, Department of Primary Industries on behalf of the Corangamite Catchment
Management Authority (2007).Corangamite Soil Health Strategy 2007. Corangamite
Catchment Management Authority, Colac, Victoria.
- Helyar K.R. The symptoms and effects on plants of nutrient disorders in acid soils. (1981) - Agricultural Research Centre, Wollongbar, NSW 2480.
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