Key messages
- Poorly performing crops, poor nodulation in pulses, and weeds that thrive in low pH soils, could indicate sub-surface acidity
- Segmented pH soil testing in problem paddocks can help farmers get a clearer picture of what’s happening within each soil layer
- Once a soil constraint such as pH is identified, it’s important to understand other soil characteristics before committing to deep incorporation of an ameliorant such as lime; some machines may go too deep and cause further issues, for example by bringing up toxic sub-layers
- Where surface lime was applied in our project trials, pH increased in the 0-5cm range only, while machinery incorporation resulted in a pH increase at depth, with a higher pH achieved as liming rate increased
We know that soil issues in the Riverine Plains region are often complex and can be variable across both vertical and horizontal profiles. For example, acidity may not be present at the surface (< 10 cm) but can be quite profound at depth (e.g 15-20 cm).
We also know that different constraints, such as acidity and sodicity can exist within different layers of the same soil; these can cause serious yield losses and reduce profitability in susceptible crop types when left unmanaged.
So how can you work out if your subsurface is holding back your production potential?
A highly acidic soil is usually considered to have a pH (CaCl2) less than 4.8, below which aluminum availability often increases dramatically. A soil with pH (CaCl2) less than 5.2 will cause yield loss in sensitive species; typically, the most pH sensitive crop types are lentils, faba beans, chickpeas, and vetch.
It’s recommended that farmers target pH (CaCl2) above 5.5 in the root zone to avoid crop losses and maximise nitrogen fixation from pulse crops.
The vertical stratification of pH means soil testing needs to be comprehensive to understand where problems lie, so that they can then be effectively addressed.
Traditional bulked soil testing at 0-10cm depth won’t identify acidity issues below 10 cm, which means many farmers may be unaware of the sub-surface constraints in their paddocks. While comprehensive soil mapping, ground truthing of soils and amelioration is expensive, this should be balanced against the gains in potential yield that can be realised when the constraint is identified and treated.
It’s recommended farmers 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, to better understand if subsurface acidity is limiting production.
As part of our project work looking at identifying and treating subsurface acidity, we soil sampled two sites with known acidity issues at Rand and Hopefield, NSW. We also established trials which focused on different machinery options to incorporate various lime rates at both these sites.
The Hopefield demonstration site pre-treatment soil tests showed pH (CaCl2) to be below 4.8 at all depth increments. Soil pH (CaCl2) below 4.8 results in aluminium becoming more soluble, which is a problem because solubilised aluminium is highly toxic to plant roots. Aluminium was particularly problematic between 5 and 30 cm in this soil.
A demonstration trial then looked at five incorporation methods (lime mixing methods) to treat the acidity. This included a control (no incorporation), speed tiller, deep offset discs, deep offset discs plus speedtill, and a Horsch Tiger, combined with four liming rates of 0, 1, 2.5 and 5 t/ha. Lime was applied and incorporated in March 2023 and the site was sown to barley in early May 2023.
Post-incorporation soil tests show that where lime was surface applied, pH increased in the 0-5 cm range only, with higher pH achieved as the liming rate was increased (Figure 1). In this surface-only application, the soil pH increased 0.39 pH units for every tonne of lime applied.
Figure 1 The effect of surface applied lime (no incorporation) at nil, 1, 2.5 and 5t/ha application rates, sampled during August 2023
Figure 2 shows the response to lime spread at 5 t/ha. The control, speed tiller and the deep offsets plus speed tiller treatments showed an increase in soil pH in the 0-5 cm range, however this drops back to starting levels in the 5-10 cm range, indicating the lime had not yet moved to this depth. Both the Horsch Tiger and deep offset disc treatments were able to incorporate lime to a greater depth, which is reflected in the increased pH at the 5-10 and 10-15 cm depths, respectively.
As well as showing the benefits of deeper incorporation, these results demonstrate the importance of segmented testing to depth. In this instance, a traditional 0-10 cm soil test would have been misleading, because it would have only captured the effects of the lime sitting on the surface, and not provided a true picture of the acidity remaining at depth.
Figure 2 The effect of 5t/ha lime incorporated using different techniques when sampled in August 2023
A second demonstration site at Rand was sown to faba beans in May 2024. This site was a heavier clay and had soil pH of <4.8 in the 0-15 cm depth, beyond which pH increased to above 5. Aluminium toxicity was present in the 5-10 cm layer. An incorporation trial was established at this site in late February 2024, with treatments including a control (no incorporation), Kelly chain plus offset discs, and compact disc harrows (Rubin 12” and 10”). Lime was applied at rates of 0, 1, 2.5 and 5 tonnes/ha, and the site also received 0.8 t/ha gypsum post incorporation. Post harvest soil testing is planned to establish the effect of incorporation and the rate applied.
Pre-incorporation soil tests also showed that soil sodicity (ESP) starts to increase above 15 cm in this soil, while soil pH is not in the problematic range below this depth. Consequently, at the Rand site, applying lime and gypsum to 15 cm is likely the best option to correct the low pH stress; this will avoid bringing sodic dispersive soils to the surface and ensure gypsum is fast-tracked to greater depths, where sodicity is more prevalent.
It’s recommended farmers complete segmented pH soil testing in problem paddocks first, to get a clear picture of what is happening at each layer. In addition to soil pH, the ECEC test is necessary to help estimate the lime amount required for the target depth.
Once the pH is identified, including severity and depth, it is important to understand other soil characteristics before incorporating lime, as some soils may have other non-pH related problem, such as sodicity.
For further information about developing a soil testing and amelioration program, talk to your agronomist or soils advisor.
This article is an excerpt from the article "Evaluating different lime rates and incorporation techniques when ameliorating acid soils", published in Research for the Riverine Plains, 2024. Read the full article.
This project was supported through funding from the Department of Agriculture, Fisheries and Forestry through the Smart Farms Small Grants program and is a co-investment of the Grains Research and Development Corporation.