Summary

With acidity an increasingly significant issue in SA soils, this project studied levels of aluminium (Al) and manganese (mn) that would cause toxicity, and molybdenum (Mo) levels that would cause deficiency, at a range of acid-soil sites across SA in comparison with data generated from interstate.

Critical values to prevent yield loss were identified in wheat and lentils for soil Al and plant Al, Mn and Mo.

The use of a range of measures to identify acidity allows better determination of its impacts. Plant test levels can be used where soils data is not available.

Background

In the past, growers and advisors have used pH to indicate the potential presence of toxic soil aluminium (Al) levels.

However, the relationship between soil pH and toxic Al levels varies by soil type. In addition, none of the current Al tests have been calibrated with SA soil types and conditions.

While manganese (Mn) toxicity has not been considered an issue in SA, recent data from Spalding and Sandilands lime trials has showed that Mn levels are high, suggesting further investigation is warranted.

Research Aims

The core objectives of the project were to:

• Determine accuracy and critical levels of different available soil Al tests across a range of soil types and various crop species.
• Assess if plant sampling in season can provide guidelines to Al and Mn toxicity levels.
• Provide guidelines for growers on the critical soil and plant Al and Mn levels in South Australian conditions.

In The Field

Extensive sampling of soils and plants was conducted at Wirrabara, Spalding, Koonunga, Mallala and Sandilands lime trials that had showed responses to lime.

These sites covered a range of soil types, crops, rainfall and soil pH, and samples were taken from plots with and without lime or sulphur addition to adjust pH up or down respectively. Soil samples were taken at 0-10cm, and tested for soil pH (CaCl2), exchangeable aluminium and extractable aluminium. Plant tissue testing including Al, Mn and Mo was conducted.

Whole tops were used for lentils and youngest emerged blades for cereal crops.

Results

The project has helped to establish critical values for soil and plant tests for Al and Mn to avoid toxicity in lentils and wheat in acid soils.

The sampling demonstrated the limitations of using pH alone as an indicator of toxicity due to variation between soils. As an example of the variation, at a pH of 4.2 (CaCl2) extractable Al levels were approx. 3mg/kg at Mallala compared to approx. 10mg/kg at Spalding.

Levels of Mo in plants were examined and found to be the best indicators of crop stress due to acidity. Unlike Al and Mn which become increasingly toxic in acid soils, Mo becomes less available as soils become more acidic, therefore a minimum, rather than a maximum, value was identified.

The soil test critical values (CV) of the studied elements to avoid a yield loss of 5 to 10 per cent were:

• Soil extractable Al 0.02-1.81mg/kg in lentils and 5.4-12.8mg/kg in wheat
• The plant test critical levels before a yield loss occurs were
  • Plant Al: Less than 34mg/kg for wheat youngest emerged blades (YEBs)
  • Plant Mn: Less than 250mg/kg in lentils and less than 260mg/kg in wheat
  • Plant Mo: More than 0.1mg/kg in lentils