Exchangeable Acidity & Lime to Neutralise It
Neutralises aluminium
Acid soils carry exchangeable acidity — mostly toxic aluminium — on their exchange sites. Enter the acidity, bulk density and depth to get the lime requirement as CaCO₃ in kg/ha and t/ha.
Compute lime requirement
Next: apply roughly 2.9 t/ha of ag-lime and incorporate to 15 cm; re-test in a season before topping up.
A 1.5× over-lime factor accounts for incomplete reaction. Choose dolomitic lime if magnesium is also low.
Lime requirement — key facts
- Driver
- exchangeable acidity (cmol⁺/kg)
- Soil mass
- bulk density × depth × area
- Lime as
- CaCO₃ equivalent (CCE)
- Output
- kg/ha and t/ha
- Neutralising factor
- ≈ 1.5–2× theoretical
- Real target
- toxic aluminium (Al³⁺)
- Sample depth
- plough layer 0–20 cm
- Privacy
- Runs in your browser; nothing uploaded
Lime the acidity, not just the pH number
In an acid soil the damage is done by exchangeable aluminium and hydrogen sitting on the clay's exchange sites — aluminium in particular stunts roots. A KCl-extractable acidity test reports this in cmol⁺/kg, and that is what lime has to neutralise. Scale the per-kilogram acidity up by the soil mass of the field (bulk density × depth × area), convert to calcium carbonate, and apply a factor for the slow, partial way lime reacts in the ground. The result is a defensible field rate.
This tool gives the lime requirement in kg/ha and t/ha, the soil mass per hectare and your input acidity from the exchangeable acidity, bulk density and sampling depth. Use it to order lime, budget the field and target the aluminium that limits yield. Pair it with the Sulphur Nutrient, Fertiliser Spreader Uniformity and Fertigation tools for a full soil and nutrient plan.
Target the aluminium
Neutralise exchangeable acidity, not just a pH reading.
Get a real field rate
Scale cmol⁺/kg up by soil mass to kg/ha and t/ha.
Allow for field reaction
Apply a neutralising factor for incomplete liming.
Order with confidence
Convert to CaCO₃ equivalent to compare any product.
Frequently Asked Questions
How is the lime requirement calculated?+
From a charge balance. Exchangeable acidity in cmol⁺/kg tells you how much acidity sits on the exchange sites per kilogram of soil. Multiply by the soil mass per hectare (bulk density × sampling depth × area) to get the total acidity for the field, then convert to calcium carbonate using its equivalent weight and apply a neutralising factor for incomplete reaction. The result is lime as CaCO₃ in kg/ha and t/ha.
What is exchangeable acidity?+
It is the acidity held on the soil's cation exchange sites — mainly exchangeable aluminium (Al³⁺) and hydrogen (H⁺) — measured in cmol⁺/kg by extracting with an unbuffered salt such as 1 M KCl and titrating. In acid soils it is dominated by aluminium, which is toxic to roots, so neutralising it is the real goal of liming, not just nudging the pH number.
How is soil mass per hectare worked out?+
Soil mass = bulk density × sampling depth × area. For one hectare (10,000 m²) at a 0–20 cm depth with a bulk density of 1.3 g/cm³, the mass is about 2.6 million kg (2,600 t). The calculator computes this for your bulk density and depth so the cmol⁺/kg acidity scales correctly to a field rate.
Why multiply by a neutralising factor?+
Laboratory neutralisation is complete, but in the field lime reacts slowly and imperfectly — particle size, mixing, soil contact and moisture all reduce efficiency. A neutralising factor (often 1.5–2× the theoretical amount) lifts the applied rate so the field actually reaches the target. Set it to match your lime quality and local recommendations.
Does liming raise the pH directly?+
Indirectly. Lime supplies carbonate that consumes H⁺ and precipitates aluminium as harmless hydroxide, which removes the source of acidity. As exchangeable acidity falls, base saturation and pH rise. Targeting the exchangeable acidity is more reliable than aiming at a pH number because it addresses the aluminium that actually harms roots.
What units should I enter?+
Exchangeable acidity in cmol⁺/kg (the same as meq/100 g), bulk density in g/cm³ (typically 1.1–1.5 for mineral topsoils), and sampling depth in centimetres (commonly 15–20 cm for the plough layer). The tool returns lime as kg/ha and t/ha plus the soil mass in tonnes per hectare.
Why convert to calcium carbonate (CaCO₃)?+
CaCO₃ is the reference liming material, so rates are quoted as its calcium carbonate equivalent (CCE). One cmol⁺ of acidity is neutralised by half a millimole of CaCO₃ (it carries two charges), which fixes the conversion. If you use dolomite, burnt lime or a product with a different CCE, divide the CaCO₃ rate by that product's CCE fraction.
How deep should I sample?+
Match the depth to the layer you will incorporate lime into — usually the plough layer, 0–15 or 0–20 cm. Deeper sampling spreads the same acidity over more soil mass and raises the calculated lime, so be consistent: sample, calculate and incorporate to the same depth for the rate to make sense.
Are the figures precise?+
They are sound agronomic working figures. Real fields vary in bulk density, acidity and lime quality across the block, and lime acts over months to years. Use the result to plan a rate and order material, then re-test the soil after a season to confirm the acidity has dropped and fine-tune the next application.