Is soil aluminium the cause of lost pasture production?

A recent soil survey of beef-producing farms in the South West’s high rainfall zone has highlighted the potential for aluminium toxicity in our soils.

Aluminium toxicity can have a significant effect on pasture productivity by inhibiting subsurface root growth and restricting plant access to water and nutrients. However, the potential constraint is rarely investigated in high rainfall permanent pastures where few producers soil test below 10 centimetres. That changed recently when South West Catchments Council (SWCC) and the Western Beef Association Inc conducted tests to a depth of 30 cms on 24 farms (44 sites) through funding from the Australian Government’s National Landcare Program.

Soil pH is a good indicator of aluminium solubility and its likely concentration in soil solution. When soil pHCa drops below 4.8, aluminium becomes more soluble and available to plants. This increases dramatically below 4.5 and can become toxic to tolerant species such as subclover and ryegrass if it drops below 4.3.

Aluminium toxicity is more likely to occur in the subsurface below 10 cm where organic matter is lower compared to the topsoil. This is because organic matter or its associated acids can detoxify aluminium. Given high levels of soil organic carbon in high rainfall soils (600-1,000 mm), aluminium may also be detoxified in the subsurface.

Soil samples taken from a depth of 10-20 cm revealed that 39 per cent of sites had a soil pH below 4.5, with an average value of 4.4 on sandy soils and 4.7 on clayey soils. Seven of the 24 farms (29 per cent) had at least one site with a soil pH below 4.3 at this depth. Average soil organic carbon content at this depth was 1.8% in sandy soils and 2.7% in clayey soils.

In the 20-30 cm depth, 25 percent of samples were below 4.5 with median values of 4.6 in sandy soils and 4.9 in clayey soils. Average soil organic carbon content at this depth was 1% in sandy soils and 1.5% in clayey soils. (Note: average soil organic carbon at 0-10 cm was approximately 4%).

These trends are supported by data held by the Soil Quality website (soilquality.org.au), which shows 66 per cent of 169 permanent pasture sites in WA had a soil pH below 4.5 in the 10-20 cm zone, a much higher proportion compared to cropping and mixed farm sites.

While an acidic subsurface can be ameliorated with alkaline products such as lime, alkalinity will only move through the soil profile where the topsoil pH is above 5.5. The SWCC survey recorded an average topsoil pH value of 4.8 on sandy soils and 4.7 on clayey soils, and no site with a low subsurface pH had a topsoil result above 5.5. This suggests that recovery is likely to be slow and only manifest itself long-term.

Low pH in the topsoil can reduce productivity in other ways, particularly where legumes such as sub clover are grown for nitrogen fixation. Nitrogen is fixed by rhizobia, which form nodules on legume roots. But rhizobia are sensitive to low pH, with subclover strains preferring a range of 5.0 to 8.0. Another factor affecting nitrogen fixation is the availability of molybdenum, which decreases with decreasing soil pH. Nutrient availability for macro-nutrients such as phosphorus is also reduced by low soil pH. However, it should be noted that micro-nutrients such as copper and zinc become less available as pH increases.

SWCC is supporting trials at four focus farms, including the effect of hydrated lime with gypsum on subsoil acidity and aluminium, and the effect of limesand incorporation, which is often recommended to increase the liming effect. Management of other common soil constraints are also being trialled.

Project results will be presented at an event planned for early 2020. For more information, contact SWCC on 9724 2400.

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