Sub-surface acidity in Mid North SA

A significant contributor to soil acidification is product removal, and cutting hay can contribute significantly due to the amount of biomass removed.

The primary objective of this project was to increase the capacity of producers and advisors in the Mid-North high rainfall zone (MNHRZ) region of South Australia to cost-effectively measure and manage subsurface acidity and acid throttles.

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Producer: Craig Jaeschke
Location: Clare, SA
Annual Average Rainfall: 540 mm
Soil Type: Red Sodosol/Black Vertosol
Enterprise: Dryland Cropping: Oaten hay – Wheat – Canola. Sheep grazed paddock in summer months.

Soil acidity affects approximately 50 million hectares or about 50 percent of Australia’s agricultural land (NLWRA 2001)1. In South Australia, it’s estimated that more than two million hectares of agricultural land is susceptible to soil acidification, and under current farming systems, the area prone to acidification is expected to double over the next 40 years. Soil acidity issues are becoming more widespread in the medium and high rainfall cropping areas such as the Mid North and the Eyre Peninsula (GRDC Acid Soils SA)2. Subsurface acidity is increasingly recognised as a widespread and significant constraint to crop production. While the constraints imposed by sub-surface acidity and thin acidic layers (often known as acid throttles) are known, there has been little research on how to measure or spatially define these constraints.

The primary objective of this SAGIT-funded project was to increase the capacity of producers and advisors in the Mid North high rainfall zone (MNHRZ) region of South Australia to cost-effectively measure and manage subsurface acidity and acid throttles. Specifically, the aim was to investigate the variability in sub-surface soil pH and whether topsoil pH maps and alternative soil data layers such as electrical conductivity, elevation, and radiometrics can be used to target strategic sub-surface sampling more accurately.

The variability in surface soil pH was determined using 2 ha grid sampling (0-10cm). Grid samples were also analysed for exchangeable cations, Cation Exchange Capacity (CEC) and Colwell P. Sub-surface acidity was measured at five locations in Craig’s ‘E4’ paddock to a depth of 0-20 cm, segmented into four equal parts, i.e. 0-5, 5-10, 10-15, and 15-20 cm depths. Locations were selected according to data from the pH and CEC maps to capture the variation across the paddock. The separate soil segments were analysed for pH (CaCl2), exchangeable cations, and CEC.

Figure 2. Variability in surface pH (CaCl2) and CEC from 0-10cm grid soil sampling. The pH map also shows the location for the 5cm incremented strategic soil sampling.

RESULTS
Soil pH in the 0-10 cm layer ranged from 4.4 to 5.7, from quite acidic to optimum for most crops (Figure 2). While lime recommendations can be made based on the 0-10cm pH maps, understanding whether sub-surface acidity is present is important to ensure that appropriate management strategies are targeted.

Across the paddock sub-surface acidity was observed at all five of the strategic sampling locations, whether the 0-10cm pH was 4.4 or 5.2. At all locations, the soil pH dropped by more than 1 unit in the 5-10 and 10-15cm segments compared to the 0-5cm depth. Despite previous lime application on the paddock in 2020 at 2 T/ha, the 5-10cm soil layer was the most acidic layer and at all 5 locations could potentially limit root growth and therefore crop production, ranging from pH 4.2-4.7. A significant contributor to soil acidification is product removal, and cutting hay can contribute significantly due to the amount of biomass removed.

For 80% of the strategic samples, the average 0-5, 5-10 cm pH was within 0.2 pH units of the 0-10 cm pH measured along a transect on the 2ha grid. The exception was Point C (Figure 2) where there was greater variation (0.6 pH units) between the grid and strategic sampling results. This is consistent with the expectation that if you measure the average of a transect of a paddock, or multiple smaller transects (grid sampling) there is always some variability that is averaged out.

Figure 3. Stratified soil pH in 5 cm increments to 20 cm depth, at five locations in the paddock, and nearby soil pH (0-10 cm depth) from previous grid sampling (top line) decline in pH from surface 5cm to depth.

Cation exchange capacity (CEC) was also mapped on the paddock, providing insights into the variation in soil pH buffering capacity across the paddock. As the CEC of the soil increases, the ability of the soil to buffer a change in pH (either acidifying or increasing after lime addition) also increases. This effect is clearly seen in the results where the strategic samples with the lower CEC (<10 cmol(+)/kg) and therefore pH buffer capacity had a greater decline in pH from surface 5 cm to depth. While CEC was a good
predictor of sub-surface acidity, EM38 which measures apparent electrical conductivity to depth (1-1.5 m), and radiometric data were poorly correlated to sub-surface acidity and therefore a poor predictor of the occurrence of sub-surface acidity.

CONCLUSIONS
The segmented sampling at 5 cm intervals highlighted the presence of significant pH stratification and acidity (pH <5.0) down to at least 15cm. Using standard 0-10cm sampling across this paddock masked the acidity at 5-10cm depths as well as further down the soil profile. With a typical sowing depth of 2.5-5 cm, seeds may inadvertently be placed directly above an undetected hostile layer, which is likely to impact on root development and hence grain yields, especially if more acid-sensitive crops such as pulses or canola are part of the rotation.

Research has shown that surface pH needs to be maintained above pH 5.5 for downward movement of the neutralising effect of lime. Even when these higher pH levels are maintained, amelioration past the surface soil layer will take time. More rapid amelioration of pH to depth of the acidic sub-surface soils requires incorporation to at least 10-15cm depth. However, this needs to be considered in the context of other possible soil constraints present at depth such as sodicity, salinity or high boron, and susceptibility of the soil to erosion. Lime application rates also need to be increased where sub-surface acidity is identified due to the greater volume of soil requiring lime.

ACKNOWLEDGEMENTS
This project was supported by the Mid North High Rainfall Zone grower group, through funding from the South Australian Grain Industry Trust. We would like to thank all of the landholders involved in the project for their cooperation and support.

1 NLWRA 2001. Australian Agriculture Assessment 2001. National Land and Water Resources Audit, Canberra.
2GRDC Acid Soils SA. https://acidsoilssa.com.au/

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