Date
2025/01/15
Organisations
Department of Primary Industries and Regional Development
GRDC
Authors
Alice Butler
Key messages
- Soil acidification in cropping systems occurs as a result of the removal of harvested grain and other plant materials that are alkaline in nature, the addition of acidity through application of ammonium-based fertilisers, and leaching of nitrate beyond the root zone.
- Leaching of nitrate is influenced by soil texture and rainfall, with sandy soils most prone to leaching. In a 325-450 mm environment, the leaching percentage is estimated at 65% for sand and 25% for loam soils.
- High nitrogen inputs and significant rainfall events further increase the rate of acidification.
- Understanding soil type and soil pH to depth is essential to ensure lime is applied where it is most effective and acidification is properly managed.
- Lime requirement depends on soil type, rainfall, grain yield, and nitrogen application rates. For example, to maintain soil pH in a 325-450 mm rainfall environment producing a 4 t/ha wheat crop and receiving 120 kilograms of nitrogen annually, the 5-year lime requirement is 1.9 t/ha (90% neutralising value) on sandy soil and 0.8 t/ha on loam soil.
Context
In 2023/2024, the total amount of lime sourced from Lime WA registered suppliers was 632,000 tonnes (Lime WA pers. Comm. July 2024). This was a 16.5% decline on the 2022/2023 figure of 757,000 tonnes.
Various factors may have contributed to this decline:
- Growers may be using alternative lime sources, purchasing from other lime suppliers within WA, or utilising on-farm lime sources that significantly lower transport costs.
- The availability of transport, where in high production years, there is less availability of road transport to backload lime.
- Budget constraints associated with low production years, where spending may be redirected towards more immediate or prioritised inputs.
- Associated with the early adoption of liming practices in WA by growers, there may now be a shift from a recovery liming practice to a maintenance liming strategy to maintain topsoil pHCa above 5.5 and subsoil pHCa above 4.8 , with growers potentially purchasing smaller amounts of lime.
This article provides an overview of the soil acidification process and how that changes under different soil types and rainfall environments to assist with calculating maintenance lime rates.
The process of soil acidification
Soil acidification is caused both by the removal of alkalinity through harvesting grain and the addition of acidity through the application of ammonium-based fertiliser, which is exacerbated when nitrate leaches out of the soil.
Alkalinity removal
The amount of alkalinity removed depends on crop type and yield (Table 1). For example, the equivalent of 9 kg/ha of pure lime or calcium carbonate (CaCO3) is removed with each tonne of wheat grain removed, while significantly less is removed when growing canola and significantly more when growing a legume grain crop such as lupin (Table 1).
The amount of lime equivalent removed is a factor of the grain harvested. For example, a wheat crop averaging 2 t/ha would require the equivalent of 18 kg/ha of pure lime, where a higher-yielding 5 t/ha wheat crop would remove the equivalent of 45 kg/ha of lime.
Table 1. Alkalinity removal by crop type (grain only) determines the amount of lime required to neutralise potential acidification.
Addition of acidity due to ammonium-based fertilisers and leaching
The degree of acidification caused by nitrogen fertilisers will depend on the type of nitrogen applied and the level of nitrogen movement down the soil profile due to leaching.
Ammonium sulfate, monoammonium phosphate (MAP) and diammonium phosphate (DAP) fertilisers cause soil acidification even in the absence of nitrate leaching (Table 2). Ammonium nitrate and urea only result in acidification when nitrate is leached from the root zone (Table 2).
Table 2. Lime equivalent (CaCO3) required to neutralise acidity associated with the application of different nitrogen fertilisers and different levels of nitrate leaching.
Leaching refers to the process of water moving downwards through the soil profile beyond the rooting capacity of the plant, also taking with it water-soluble nutrients. The extent of leaching varies with soil texture and rainfall (Table 3). As an example, sandy soil has a low capacity to store water and is highly susceptible to leaching. Therefore, leaching of ammonium-based fertilisers is the major driver of soil acidification in this soil type.
Table 3. Percentage of nitrate leaching by soil texture and rainfall.
Source: iLime Technical Notes (DPIRD, 2019)
Estimating maintenance lime rates
To maintain soil pH at a target level, ongoing soil acidification must be balanced by the addition of alkalinity, such as a lime source. As acidification rates increase under higher leaching conditions, estimating lime requirements can become complex.
Using the values from Tables 1 and 2, estimates of the total amount of lime required to neutralise 120 kg of nitrogen applied to a 2, 4, or 6 t/ha wheat crop under 0%, 50%, and 100% leaching scenarios are provided (Table 4).
The following steps can be used to calculate a site-specific maintenance liming budget. Both rainfall and soil type need to be considered and are accounted for using the relevant leaching factors in Table 3 to adjust the amount of acidification from fertiliser applications.
Table 4. Estimating the total lime required to neutralise 120 kg of nitrogen applied to a 2, 4, and 6 t/ha wheat crop under 0%, 50%, and 100% leaching scenarios.
Table assumptions: MAP 0% leaching 3.6 kg CaCO3/kg nitrogen and 100% leaching 7.1 kg CaCO3/kg nitrogen. Urea 0% leaching 0 kg CaCO3/kg nitrogen and 100% leaching 3.6 kg CaCO3/kg nitrogen.
The influence of rainfall and nitrogen application rates is shown on the estimated amount of total CaCO3 (lime with 100% neutralising value) required to neutralise 60, 120 and 180 kg of applied nitrogen to a 2, 4 and 6 t/ha wheat crop, respectively, for six different soil texture groups within a 325-450 mm rainfall environment (Figure 1). This data can help inform decision-making on maintaining soil pH at target values.
Over a five-year period and using these estimates, a sandy soil in a 325-450 mm rainfall environment required an average 0.9 t/ha of lime (assuming a 100% neutralising value) to counteract the acidification associated with the application of 60 kg of nitrogen, compared to 1.7 t/ha of lime for 120 kg of nitrogen and 2.4 t/ha of lime for 180 kg of nitrogen. In contrast, a loam soil in the same environment requires an average of 0.4 t/ha of lime for 60 kg of nitrogen, 0.8 t/ha of lime for 120 kg of nitrogen and 1.1 t/ha of lime for 180 kg of nitrogen.
When sourcing lime, it is important to consider both the particle size and neutralising value, as these factors strongly influence the overall rate required, as well as the rate of change in soil pH. To demonstrate the impact of neutralising value, Figure 2 shows the total lime requirement using lime with a 90% neutralising value compared to lime with a 70% neutralising value (NV).
Understanding soil type and soil pH to depth is important to ensure lime is placed where it is most required and soil acidification is managed effectively. Higher applications of nitrogen combined with significant rainfall events that trigger leaching can increase the risk of acidification. Routine soil testing, together with the application of maintenance rates of lime, helps ensure the soil pH stays above target levels.
Page references and acknowledgements
Reference: Department of Primary Industries and Regional Development (2019). iLime Technical Notes [Access]
Reviewed by Chris Gazey and Gaus Azam, DPIRD.
Contact: [email protected]
Article produced with investment from DPIRD and GRDC in the Soil Water and Nutrition (SWAN) Strategic Collaboration, DAW2407-001SPX extending research undertaken by DPIRD and GRDC investments.
