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 through funding from the Australian Government’s National Landcare Program.
The survey conducted comprehensive testing of the subsurface, including soil pHCa (pH measured in calcium chloride) and exchangeable aluminium.
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.
But not all soils have large proportions of aluminium, so another important indicator is exchangeable aluminium. If the proportion of aluminium in a soil’s cation exchange capacity (the sum of exchangeable cations including calcium, magnesium, potassium, sodium) is above 30 per cent, there is an increased likelihood that the concentration of aluminium is toxic to even tolerant plant species. However, it should be recognised that soil organic matter can bind this aluminium and limit toxicity. This is why aluminium toxicity is more likely to occur in the subsurface below 10 cm where organic matter is lower compared to the topsoil.
Given the difficulty in confirming toxicity, the SWCC project paired sampling on twenty farms, comparing a poor performing and better performing site identified by farmers on similar soils, resulting in a total of 44 sites on 24 farms.
Results from the survey suggest that aluminium may be having a significant impact on high rainfall pastures. Samples taken from a depth of 10-20 cm revealed that 39 per cent of sites had a soil pH below 4.5, with a median 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.
In the 20-30 cm depth, 25 percent of samples were below 4.5 with median values of 4.3 in sandy soils and 4.9 in clayey soils.
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.
SWCC’s survey also found that nine of the 44 sites (20 per cent) had an exchangeable aluminium proportion greater than 30 per cent of cation exchange capacity in the 10-20 cm zone. An additional four sites exceeded 30 per cent in the 20-30 cm zone. These occurred on both sandy and clayey soil types.
Preliminary comparisons between poorer and better sites suggest that aluminium toxicity may explain some difference in production on five of the 20 farms (25 per cent). However, investigations on a range of soil constraints will continue through winter and spring.
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 a median topsoil pH value of 4.7 on both sandy and 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.Tags: aluminium toxicity soil acidity soil pH