Aluminium in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 58

plants rapidly increases as pH drops below 5.5. This may be compounded by
increasing iron toxicity in acid soils. (In strongly alkaline soils, aluminium is not a
problem, but calcium will make phosphorus unavailable.)

Treating aluminium toxicity
Aluminium toxicity in the soil can be overcome by increasing the soil pH by
adding lime. The amount of lime needed can be determined by soil tests of topsoil
to a depth of 20 cm. In soils that are acid at depth, it may be necessary to test down
to 100 cm to fully understand the management implications.
Soil test results are expressed as exchangeable aluminium in milliequivalents
per 100 grams of soil (meq/100 g) and calculated as a percentage of the cation
exchange capacity of the soil (% CEC). The aluminium that affects plant growth is
the aluminium in the soil solution.

Aluminium tolerance of plants
Some plants are more tolerant than others to toxic aluminium in the soil. In an
acidic soil with high aluminium it is possible to carefully select species that are
tolerant to high levels of aluminium. However, it is far better to reduce aluminium
toxicity in the soil by liming as this extends the range of plants that can be grown
economically. Table 16 shows a list of plants and their tolerance to aluminium.

4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0

Aluminium toxicity
increasing

aluminium calcium

phosphates

iron

Insoluble complexes of aluminium, iron and phosphorus are found in this pH range.

Figure 19 The effects of soil pH on aluminium. ( Soil Sense C-07)