Why use this calculator
Soil acidity is the slow, invisible constraint on a great deal of cropping and grazing country, and it costs yield long before anything looks wrong in the paddock. Below about 4.8 on the calcium chloride scale aluminium becomes soluble and prunes roots, phosphorus availability falls, rhizobia struggle to nodulate legumes and the whole nitrogen cycle slows down.
Lime fixes it, but the rate is not a single number: a sand needs a fraction of what a clay needs for the same pH movement, a surface spread on no till country acts on far less soil than an incorporated application, and a quarry product at 85 neutralising value needs noticeably more tonnes than one at 98. This tool puts all four drivers into one rate so you buy the right tonnage once.
How to use it
- First select the scale your laboratory used, because Australian tests almost always report pH in calcium chloride, which reads roughly 0.7 units below a water test.
- Enter the current topsoil pH from your most recent test, sampled from the 0 to 10 centimetre layer.
- Enter the target pH, which for most Australian growers means 5.5 in calcium chloride terms.
- Choose the soil texture, since heavier soils and high organic matter resist pH change strongly.
- Enter the depth the lime will be mixed into, remembering that surface spreading on no till acts on roughly the top five centimetres.
- Put in the neutralising value from the supplier docket.
- Then enter the paddock area, the delivered and spread price per tonne, and the payload of the truck so the tool can tell you how many loads to book.
What each result means
- Lime rate
- Tonnes of the product you specified per hectare, already adjusted for texture, depth and neutralising value.
- Applications to split it into
- How many separate spreads the lime rate needs so that no single pass moves pH by more than one unit or puts on more than 5 tonnes a hectare.
- Total lime required
- Whole paddock tonnage at that rate, which is the figure to put on the quarry order.
- pH gap to close
- How far the topsoil has to move. Anything beyond one unit should be split across two or more applications.
- Current pH on the calcium chloride scale
- Your reading expressed on the scale Australian laboratories and agronomists use, so thresholds line up correctly.
- Neutralising value adjustment
- The multiplier applied because your product is weaker or stronger than pure calcium carbonate at one hundred.
- Cost per hectare
- Delivered and spread cost of the lime on one hectare, which is what to compare against the yield response you expect.
- Total lime cost
- The whole paddock bill at your delivered and spread price. Cartage usually dominates it beyond an hour from the quarry.
- Truck loads to book
- Deliveries needed at the payload you entered, rounded up, so spreading and traffic can be scheduled.
The formula
Lime rate in tonnes per hectare equals the soil texture factor multiplied by the pH gap, multiplied by depth in centimetres divided by ten, multiplied by one hundred divided by the neutralising value of the product. Texture factors are sand 1.2, sandy loam 1.9, loam 2.7, clay loam 3.5, clay 4.4 and peat or high organic matter 6.0 tonnes per hectare per pH unit at ten centimetres. The pH gap is target pH minus current pH, floored at zero. Total tonnes equals rate multiplied by paddock area. Neutralising value expresses a product's acid neutralising power against pure calcium carbonate at one hundred.
How this number is used in practice
Lime is a capital improvement dressed up as an input. The response usually takes twelve months to appear and then persists for five to ten years, which means it should be judged against the whole rotation rather than the next crop. The most common mistake is treating it as an annual cost, deciding it is too expensive in a tight year, and then repeating that decision until the topsoil drops below the point where legumes will nodulate.
At that stage the paddock is not just yielding less, it has stopped producing its own nitrogen and the fertiliser bill rises to compensate.
Where the lime ends up matters as much as how much you buy. Lime is very immobile, so a surface application on no till country raises pH in the top few centimetres and takes years to influence the layer below. If the constraint is subsurface acidity at 10 to 20 centimetres, which is very common in long cropped country, surface lime alone will not reach it inside a decade and incorporation or a strategic deep tillage pass becomes part of the plan.
Always sample in layers rather than taking one bulked 0 to 10 sample, or you will not see the problem at all.
Buy on neutralising value and particle size, not on price per tonne at the quarry gate. A product at 85 neutralising value needs about fifteen per cent more tonnes than one at 98 for the same effect, and coarse material can take two to three years to react while fine material works within months.
Once cartage is included, the cheap coarse lime from the closer quarry is often genuinely the better buy, but it has to be worked out rather than assumed, and the tonnage has to be adjusted accordingly.
Worked examples
80 ha of loam moving from pH 4.6 to 5.5, incorporated to 10 cm, 95 NV lime at $78 a tonne
About 2.56 t/ha and roughly 205 tonnes for the paddock.
The same paddock surface spread on no till, acting on the top 5 cm only
The rate halves to about 1.28 t/ha.
40 ha of clay moving a full unit from pH 4.5 to 5.5 with a weaker 85 NV product
About 5.18 t/ha, or roughly 207 tonnes.
When people use it
- Sizing a lime order and booking the right number of truck deliveries
- Comparing two quarry products once neutralising value and cartage are taken into account
- Budgeting a multi year acidity program across a whole cropping rotation
- Deciding whether to incorporate lime or accept a slower surface response on no till country
- Working out zone specific rates where a paddock has both sandy rises and heavy flats
- Checking a contractor's recommended rate against an independent calculation
Tips
- Sample in layers, 0 to 10 and 10 to 20 centimetres, because subsurface acidity is common and a single bulked sample hides it.
- Do not try to lift pH more than one unit in one pass. Split the rate across two or more years instead.
- Buy on neutralising value and particle size rather than price per tonne at the gate, then add cartage before comparing.
- Spread lime six to twelve months before the crop that needs it, since the reaction is slow and moisture driven.
- Lime is immobile. If the constraint sits below ten centimetres, surface spreading alone will not reach it for years.
- Retest after two years rather than every season. Soil pH moves too slowly for annual testing to tell you anything useful.
- Use dolomite only where magnesium is genuinely low. It is dearer and unnecessary on soils with adequate magnesium.
What it assumes, and where it stops being accurate
- Texture factors are indicative for temperate agricultural soils and are no substitute for a laboratory buffer or lime requirement test.
- The model assumes even spreading and reasonable soil moisture. Dry soil slows the reaction dramatically regardless of the rate applied.
- The 0.7 unit offset between water and calcium chloride readings is an average. The true offset varies with salt content and soil type.
- Only the topsoil layer you nominate is treated. Subsurface acidity below the incorporation depth is not addressed by this rate.
- Costs cover lime delivered and spread. Tillage passes, extra traffic and soil testing are not included.
- Organic matter, cation exchange capacity and existing exchangeable aluminium all affect the true requirement and are only approximated by the texture factor.
Terms used on this page
- pHCa
- Soil pH measured in dilute calcium chloride, the standard Australian laboratory method.
- Neutralising value
- A liming product's acid neutralising power relative to pure calcium carbonate, which is set at 100.
- Buffering capacity
- A soil's resistance to pH change, driven mainly by clay content and organic matter.
- Subsurface acidity
- Low pH in the 10 to 20 centimetre layer, common under long term cropping and hard to reach with surface lime.
- Exchangeable aluminium
- Soluble aluminium released as soil acidifies, which damages root tips and limits yield.
Questions people ask
- How much lime do I need to raise soil pH by one unit?
- It depends almost entirely on texture and depth. Moving a loam one full unit through the top ten centimetres takes roughly 2.7 to 3 tonnes a hectare of good quality lime, while a sand may need only 1.2 and a heavy clay 4.4 or more. High organic matter soils resist change strongly and can need double the loam rate.
- What is the difference between pH in water and pH in calcium chloride?
- They are two laboratory methods on the same sample. Calcium chloride uses a weak salt solution that mimics soil solution more closely and reads about 0.6 to 0.8 units lower than a water test. Australian laboratories report calcium chloride by default, so a target of 5.5 in calcium chloride is roughly 6.2 in water.
- What is neutralising value and why does it matter?
- Neutralising value measures a liming product's acid neutralising power against pure calcium carbonate, which is set at one hundred. A product at 85 needs about eighteen per cent more tonnes than one at 100 to do the same job. It is printed on the supplier docket and should be the first thing compared when quotes differ.
- How long does lime take to work?
- Fine lime incorporated into moist soil begins changing pH within weeks and reaches most of its effect over six to twelve months. Coarse material can take two to three years. Surface applied lime on no till moves down very slowly, often less than a centimetre a year, so incorporation matters where subsurface acidity is the problem.
- Can I apply too much lime?
- Yes. Pushing pH above about 7 reduces the availability of iron, manganese, zinc and boron and can induce deficiencies that are harder to fix than the original acidity. Move no more than one pH unit per application, base the rate on a current soil test, and retest before applying again.
- Should I use lime or dolomite?
- Use agricultural lime, which is calcium carbonate, unless a soil test shows magnesium is genuinely low. Dolomite supplies both calcium and magnesium but usually costs more per unit of neutralising value. Applying dolomite to soil with adequate magnesium wastes money and can unbalance the calcium to magnesium ratio.
Use this with
Soil pH and Lime Rate Calculator answers one part of the question. These pick up where it stops: Fertiliser Application Calculator, Crop Rotation Nitrogen Credit Calculator, Seed Rate Calculator, Cost Per Hectare Calculator and Harvest Yield Estimator.
Where these figures come from
Figures and rules on this page follow Australian law and ATO guidance.
- Soil Quality Australia. Soil pH, liming rate and soil health fact sheets
- Primary Industries and Regional Development, Western Australia. Soil acidity, liming and dryland cropping guidance for low rainfall zones
- Agriculture Victoria. Extension notes on pastures, soils, liming and livestock management
- Grains Research and Development Corporation. Australian grains agronomy, nitrogen budgeting and yield estimation research
Soil pH and Lime Rate Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.