Lime incorporation to ameliorate subsurface acidity

Key messages

• Farmers in the Riverine Plains should assume some degree of subsurface acidification is occurring in their soils, unless soil testing proves otherwise
• Poorer performing areas of a paddock could indicate a potential subsurface acidity issue; soil testing will confirm its presence or absence
• Lime incorporation is essential in cropping soils to optimise benefits - when lime is applied without incorporation to the targeted depth, it only impacts pH levels at the surface and does not flow down through the soil profile
• Farmers should only incorporate lime to the target depth that is suitable for that soil, as other soil constraints (eg sodicity, slaking), seedbed preparation, emergence and trafficability can come into play

Subsurface acidity and the re-acidification of previously limed soils is a major issue for grain farmers in the Riverine Plains. With the issue being so widespread, farmers can assume their farm has some degree of subsurface acidification, unless soil testing indicates otherwise.

To support farmers in managing this issue, and prevent associated yield loss, Riverine Plains is leading the GRDC project investment Best practice liming to address sub-soil acidity in north east Victoria. This project is specifically looking at how the rate and quality of lime applied, and incorporation, can better ameliorate subsurface acidity. 

With liming traditionally kicking off in the Riverine Plains from late summer onwards, we thought it timely to revisit some of the key outcomes from our 2022 and 2023 project trials, to support farmers in managing their upcoming programs.

What we looked at

Riverine Plains established replicated trials at Lilliput near Rutherglen during mid February 2022, looking at various rates of lime and incorporation methods. The treatments included;

1. Control (nil lime, nil incorporation)
2. Nil lime, with incorporation by shallow discs 
3. Lime to target pH 5.2 (1.2 tonnes/ha applied lime), incorporated by sowing 
4. High rate of lime to target pH 5.8 in 0-10cm depth (5.0 tonnes/ha applied lime), incorporated by sowing
5. High rate of lime to target pH 5.8 in 0-10cm depth (5.0 tonnes/ha applied lime), incorporated by shallow discs to a depth of 15cm
6. High rate of lime to target pH 5.8 (5.0 tonnes/ha applied lime), deep incorporation with Horsch Tiger to 20cm, followed up with speed-tiller
7. High rate of lime to target pH 5.8 at depth (8.5 tonnes/ha applied lime), deep incorporation with Horsch Tiger to 20cm, followed up with speed-tiller (Deluxe option)

A Horsch Tiger was used for the deep incorporation, with calibration to ensure that the depth of the lime incorporation was kept above 20cm. The speed tiller was run over both incorporated treatments to ensure a smooth surface for ease of sowing. Once the treatments were completed, the host farmer sowed and managed the trial site in line with the management practices used for the remainder of the paddock.

Incremented soil testing to a depth of 50 cm allowed Riverine Plains to monitor treatment effects from 2022–2024. 

What we found 

Results from the trial indicated that applying no lime, or a small amount of lime (1.2 t/ha) incorporated by sowing or shallow discs, resulted in an increase in year 1, but the benefits of the lime did not persist into the second year of production. This suggests that the rate of acidification from crop production on this soil is greater than the benefit from rate of lime applied. 

Lime is slowly soluble and soil testing results from the trial to date (2022-2024) show that when lime is applied without incorporation, it only impacts pH levels at the surface and does not change the pH down through the soil profile (Figures 2a and 2b). Figures 2c and 2d show that incorporating lime by sowing impacts pH in the top 5cm, with the rate of change depending on the quantity of lime applied. Incorporation of lime using shallow discs (Figure 2e), or deeper incorporation with the Horsch Tiger (Figure 2f, g), enables the lime to move further down the profile, to the depth of incorporation.  Shallow discs resulted in lime movement to 10cm, while the Horsch Tiger moved lime to 20cm. 

 

Summary

While deep incorporation has shown positive results, it’s important that farmers only incorporate lime to the depth that is suitable for that soil, as other soil constraints (eg sodicity, slaking), seedbed preparation, emergence and trafficability can come into play. For example, if you can only cultivate to a depth of 10cm, load up that zone with adequate lime for full amelioration, so that it can move to depth over time.

To better understand if subsurface acidity is limiting your production, it’s best to undertake incremented (5 cm) soil sampling to a depth of at least 20 cm and further incremented (10 cm) sampling to a depth of 60 cm.

For further information about using incorporation techniques, speak to your agronomist or contact Jane McInnes at jane@riverineplains.org.au.

Acknowledgements

This project article is an investment by the Grains Research and Development Corporation. (GRDC).