Agricultural lime and how it works

Lime sources

Agricultural lime is any inorganic product that is used to increase the pH of soil. In Western Australia, the main sources are limesand, limestone and dolomitic lime. Application of one of these products is usually the most economical method of ameliorating soil acidity.

Less common lime sources include Cretaceous chalk, lake bed marls, on-farm calcareous soil, and from time to time, industrial by-products with liming properties may be available.

Calcium oxide (burnt lime or quicklime) and calcium hydroxide (slaked or hydrated lime) are not recommended for use as agricultural lime. Storage and handling of these products is difficult. They are caustic, so skin contact and inhalation need to be avoided. Also, burnt lime reverts to calcium carbonate unless stored in air-tight conditions.

How lime works

Excess hydrogen ions in the soil solution cause soil acidity. When agricultural lime is applied, carbonate from calcium carbonate (or from magnesium carbonate) neutralises acid in the soil.

The soil chemistry can be simplified into a few steps:

  • In wet acidic soil, calcium carbonate ionises (separates) to calcium and carbonate ions.
  • The carbonate ions react with hydrogen ions in the soil solution, forming bicarbonate ions.
  • The bicarbonate ions react with hydrogen ions in the soil solution and form carbon dioxide and water.

The end result is that the soil has more calcium ions on the exchange surfaces of the soil, carbon dioxide is released into the soil air and hydrogen ions that were contributing to acidity are bound in soil water.

How lime reacts with hydrogen ions in soil. From Soil Quality: 4 Soil Acidity (Gazey et al. 2019). Diagram: Science with Style.

Liming quality and liming rates

Liming rates need to be adjusted to account for lime quality so that the desired acid neutralising effect is achieved. The key indicators of agricultural lime quality are neutralising value and particle size, regardless of the source.

Neutralising value (NV)

The carbonate content of lime determines the capacity of the lime to neutralise acidity. Neutralising value is expressed as a percentage relative to pure calcium carbonate, which is given a value of 100%. With higher neutralising value, a lower rate can be used to achieve the same pH change.

Particle size

The size of the lime particles determines how quickly the lime can neutralise acid. Lime with a higher proportion of finer particles has a larger surface area to react with the acid in soil. Research shows that finer limes (a high proportion of particles less than 0.5 millimetres) increase pH faster, which is necessary for rapid amelioration of acidic soil.

A way to visualise the impact of particle size on surface area is to imagine the particles as roughly cube shaped; then the surface area can be imagined as an opened-out box.

A way to visualise the impact of particle size on surface area is to imagine the particles as roughly cube shaped; then the surface area can be imagined as an opened-out box. From Soil Quality: 4 Soil Acidity (Gazey et al. 2019). Diagram: Science with Style.

Calculating effective neutralising value

Lime comparison calculators use a calculated effective neutralising value of agricultural lime based on particle size distribution and the neutralising value of particles in the size categories to estimate the relative value of different lime sources. The neutralising value of larger particle sizes is discounted to account for the reduced capacity to change soil pH in the short term. Different calculators use different discount factors.

Lime comparison calculator examples developed for WA

App iLime

  • iLime is a decision tool to assist liming decisions developed by the Department of Primary Industries and Regional Development.
  • Users are able to specify crop, fertiliser and soil characteristics specific to their farm. Different lime sources, rates of application and timing may be compared to estimate the optimum economic return for your paddock.
  • Yield, soil pH, soil aluminium and cash flow responses are displayed for lime strategies compared to not applying lime, allowing a long-term strategic view to liming decisions made today. It is available for Android and iOS.
Link

Web calculator Lime comparison calculator

This soilquality.org.au calculator discounts the neutralising value to account for the reduced effect of larger particle sizes which reduce the short term capacity of the lime to change the soil pH.

Link

The effective neutralising value of each particle size (particle size ENV) is calculated as:

Particle size ENV = proportion of lime in the category ​ × neutralising value (NV) of lime in the category ​ × particle size discount factor

The overall effective neutralising value of the lime (lime ENV) is calculated as:

Lime ENV = the sum of all the particle size ENVs

Page references and acknowledgements

Material on this page adapted from:

  • Gazey C, Azam G, Clausen J and Rengel Z (2019). Soil Quality: 4 Soil Acidity. SoilsWest, Perth, Western Australia. [Access]

Last updated July 2024.

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