Effects of soil acidity

In WA, the major problem when soils acidify is aluminium toxicity in the subsurface soil. Low pH in topsoils primarily affects nutrient availability and decreases nodulation of legumes and nitrogen fixation in pastures. These problems are minimised if the topsoil pH_Ca is maintained above 5.5.

Aluminium toxicity

Aluminium is present in soils in a variety of forms and bound to the soil constituents, particularly clay particles and organic matter. When soil pH drops, aluminium becomes soluble and the amount of aluminium in the soil solution increases. As a rule of thumb, soil aluminium concentration of 2-5 parts per million (ppm) is toxic to the roots of sensitive plant species and above 5ppm is toxic to tolerant species.

In most Wheatbelt soils, aluminium will reach toxic levels when subsurface pH_Ca falls below 4.8. Generally, there is sufficient organic matter in topsoil so that aluminium can remain bound and does not become toxic to plant roots even though it is extractable in a laboratory analysis.

Toxic levels of aluminium in the soil solution affect root cell division and the ability of the root to elongate. The root tips are deformed and brittle (Figure 1) and root growth and branching is reduced (Figure 2). Poor crop and pasture growth, crop yield reduction and smaller grain size occur as a result of inadequate water and nutrition. The effects of aluminium toxicity are most noticeable in seasons with a dry finish. Roots are unable to effectively grow through acidic subsurface soil, which forms a barrier and restricts access to stored subsoil water for grain filling.

Figure 1 Healthy root tip (left) compared to a deformed root tip affected by aluminium toxicity (right). Photos: CSIRO

Figure 2 Wheat seedlings grown in soil with a range of aluminium concentrations demonstrate restricted root growth at high aluminium concentrations. Photo: S Carr

Liming soil to increase the soil pH is effective in reducing the availability of aluminium to non-toxic levels. Figure 3 shows 11 day old barley seedlings grown in acidic subsurface soil. The seedlings on the left were grown in soil that was limed to increase pH_Ca to 5.1; the available aluminium concentration was less than 2ppm and the seedlings show no symptoms of aluminium toxicity. The seedlings on the right were grown in the same soil without lime, at a pH_Ca of 4; the available aluminium concentration was 15ppm and root growth was severely restricted by toxic levels of aluminium.

Figure 3 Barley seedlings grown in limed (left) and unlimed (right) acidic subsurface soil; there are no symptoms of aluminium toxicity in the limed treatment

Measurement of soil aluminium

In most cases, the subsurface soil pH will be a good indicator of aluminium levels.

The measurement of aluminium in the soil solution is complicated and is affected by many factors. Depending on the methods used it is not always possible to distinguish between toxic and nontoxic forms of aluminium. A rough guide to the levels of aluminium can be achieved by measuring aluminium concentration in the same 0.01M CaCl₂ solution used to measure the soil pH.

The measurement of aluminium in topsoil is further complicated by the presence of higher levels of organic matter because aluminium can be bound to the organic matter (and therefore in a nontoxic form) but is released when extracted with the 0.01M CaCl₂ solution.

Soil analyses that present aluminium concentrations for topsoil are not meaningful. Even if a high level of aluminium is extracted from the topsoil during analysis, it is unlikely to be available in toxic concentrations in the soil.

At a subsurface pH_Ca above 4.5 aluminum concentration is usually less than 2ppm. A pH_Ca of 4.8 or above in the subsurface will avoid aluminium toxicity for most crop species. Below pH_Ca 4.5 aluminium concentrations increase rapidly and quickly become toxic to most crop and pasture species (Figure 4).

Figure 4 The relationship between pHCa and aluminium concentration in subsurface soils from a farm near Beacon