BEST PRACTICE SOIL TESTING & LIMING RECOMMENDATIONS
Recent research based in the Riverina and Central West regions is changing the way that growers and advisors address soil acidity. This article considers the changes to soil testing, pH targets and lime incorporation strategies.
Soil Testing Practices & Liming
Soil testing for lime requirement has traditionally relied on 0-10cm deep comprehensive tests conducted along a transect from a paddock or block. While this has been relatively effective, issues have arisen with the introduction of minimum tillage, causing the soil pH profile to become stratified. It is not uncommon to observe a distinct decrease in pH below a point anywhere from 2-15cm, commonly referred to as an ‘acid throttle.’ This is problematic as an acid throttle may be hidden in a traditional 0-10cm soil test. In a soil profile for example, the 0-5cm band could have a pH of 6, while the 5-10cm layer may be a pH 4.6, but a 0-10cm test could read pH 5.4. Traditionally, a low or nil rate of lime may have been recommended based on the result of the 0-10cm test, despite the high lime requirement below 5cm deep.
To identify pH stratification, subsampling down the soil profile has now become the preferred soil testing process. Sub-sampling soil at 5cm intervals, ideally to a depth of 20cm, is considered best practice for identifying soil acidity, as it provides a detailed profile of pH variation through the soil layers, allowing for more targeted and effective management. The additional benefits of this approach are:
1. Enhanced Lime Application Efficiency: With detailed pH data, farmers can adjust lime applications. This reduces costs and prevents over- or under-application, which can lead to poor results and a waste of resources.
2. Better Crop and Pasture Growth: Despite the topsoil appearing neutral or mildly acidic, acidic subsoil layers can limit root growth and restrict access to nutrients and water. Identifying and treating subsoil acidity can help crops establish deeper root systems, supporting better resilience to drought and improving nutrient uptake. This is of particular importance regarding sensitive crops such as Faba Beans.
3. Tracking Soil Management Effects Over Time: By sub-sampling at 5cm intervals, farmers can monitor the progress of liming and other soil amendments, in neutralising acidity over time and in specific layers. This is important, as soil pH can change gradually in response to practices like liming, fertiliser use and nutrient removal by crops.
An alternative practice to subsampling, is engaging dedicated soil testing companies or retail stores, who now have the technology to intensively test paddocks on a 2ha grid to identify spatial variation in pH, allowing the application of lime on a variable rate basis.
Target Soil pH
The optimum pH growers should be targeting has steadily increased over time, as research outcomes have improved the understanding of soil acidity. Historically a pH target of >5 (CaCl2) was considered optimum, as this is when negative yield impacts are minimised for most crops, as shown by the graph below. Current research is now indicating that a pH target between 5.8-6.5 is optimal, particularly when subsurface acidity is also an issue.
As pH increases above 5.5, soluble alkali (molecules that neutralise acidity) moves down the profile and starts neutralising the acidic soil below the lime treated band. This is useful, as it is difficult and costly to address subsurface acidity directly with lime, due to limitations in depth of cultivation and incorporation. Not only is it important to fix an acidity issue already present, but it is also important to prevent the development of subsurface acidity through normal agronomic practices in future.
Source: Lisa Miller Southern Farming Systems
Deep Incorporation
In some cases, deep incorporation of lime may be important to effectively neutralise sub-soil acidity. Moreover, solely relying on the movement of lime (i.e. alkali) down the profile is a slow process. However, deep incorporation of lime does come with certain disadvantages:
Soil Structure Disturbance: Cultivation, can disturb soil structure, reducing organic matter and increase the risk of erosion. This is especially problematic in undulating or dispersive soils prone to compaction or erosion.
Labour and Equipment Costs: Cultivation can be very expensive due to significant labour, fuel and R & M requirements. Deep tillage such as using a Horsch Tiger, may be required to target subsoil acidity below 10cm, adding further expense and requiring specific equipment.
Variable Results with Different Soil Types: The effectiveness of lime incorporation can vary based on soil type, soil conditions and the cultivator used. For example, different cultivators will lead to different levels of lime mixing throughout the cultivated profile, which can also change depending on the soil conditions at the time of cultivation.
Loss of Moisture: Cultivation during a summer fallow can lead to significant losses in soil moisture, impacting subsequent yields. Alternatively, in a very wet season post cultivation, particularly deep cultivation, trafficability can be a greater issue.
Return on Investment
The average return on investment (ROI) from liming to address soil acidity in southern New South Wales varies, but on-farm trials and research suggest that each dollar invested in lime can yield between $2 and $5 in increased productivity over the long term, with variations depending on soil type, acidity depth, crop type, and other environmental factors.
Studies indicate that the ROI is more favourable when lime is proactively applied and adequately incorporated. Regular liming can prevent declines in productivity and avoid more costly interventions to correct severe acidity, particularly in the subsoil. Lime application typically takes several years to achieve maximum benefits, but the cost of not applying lime can result in substantial yield losses and increased management costs over time.
Situations where the ROI is questionable, include where the subsoil is not acidic, which minimises the need to target high pH levels in the soil surface. In addition to this, growers need to determine how often acidity is limiting yield in their system. Identifying other major constraints to yield is important, such as sub soil sodicity, weeds, presence of aluminium, frost and drought. While ameliorating acidity in these situations may improve crop resilience, the yield and pasture benefit may not be significant in the longer-term.
Overall, for most clients, subsampling when conducting soil tests will provide greater insights into pH changes down the profile. This will provide greater confidence in lime application decisions and maximise the return on investment from an expensive input, applied intermittently.