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Soil pH & Lime Calculator

Work out how much lime or sulphur it takes to move your soil pH, allowing for soil texture, the material’s neutralising value and the cap on a single pass.

How to use this calculator

  1. 1Get a real soil test. A pH figure from a lab, ideally with a buffer pH alongside it, turns this page from an estimate into an answer — and buffer pH is what a lab actually uses to write a lime recommendation.
  2. 2Sample properly: ten or so cores from across the area, mixed in a clean bucket, air-dried. One core from one spot is not the field.
  3. 3Identify the texture by feel. Moisten a handful and try to squeeze a ribbon between thumb and finger — no ribbon is sand, a short crumbly ribbon is loam, a long sticky ribbon is clay.
  4. 4Check the CCE on the bag before buying. It is the difference between the right quantity and a third too little.
  5. 5Split anything over 50 lb per 1,000 sq ft into two applications about six months apart, and re-test before the second.
  6. 6Apply in autumn for a spring crop, and incorporate into the top six inches where you can. Surface-applied lime on established turf takes far longer to register.

How the calculation works

Buffered pH units = ∫ (1 + 0.6 × max(0, 6.2 − pH)) dpH across the change Requirement as CaCO₃ = texture rate per point × buffered pH units Product = requirement ÷ (CCE ÷ 100) Passes = ceiling(product per 1,000 sq ft ÷ 50) To lower pH: elemental sulphur = texture sulphur rate × pH gap product = elemental sulphur × material factor
CCE
Calcium carbonate equivalent — neutralising strength as a percentage of pure CaCO₃. Printed on the bag in most states
Texture rate
Pounds of CaCO₃ per 1,000 sq ft to move pH one point: about 25 on sand, 60 on loam, 100 on clay
Buffer pH
A laboratory measurement of reserve acidity. The proper basis for a lime recommendation, and what this page approximates from texture
Exchangeable acidity
Hydrogen and aluminium held on soil particles. It is what lime actually neutralises, and it is far larger than the acidity in solution

The pH measured in water is only the acidity in solution. Most of a soil’s acidity is held on the clay and organic matter and only appears as the solution acidity is neutralised — which is why the requirement is so much larger than a simple pH calculation suggests.

The buffering model here rises as pH falls, which reproduces the documented behaviour. It is an approximation of a buffer pH test, not a substitute for one.

Gypsum has a calcium carbonate equivalent of zero. It is a neutral salt and does not change pH under any circumstances.

Worked example

Raising a loam lawn from pH 5.5 to 6.5 over 5,000 sq ft

  1. 1.Loam takes about 60 lb of pure calcium carbonate per 1,000 sq ft to move pH one point.
  2. 2.But the move starts at 5.5, where there is more reserve acidity than there would be at 6.0. Integrating the buffering factor across 5.5 to 6.5 gives 1.15 effective points rather than 1.0.
  3. 3.Requirement = 60 × 1.15 = 69 lb of calcium carbonate equivalent per 1,000 sq ft.
  4. 4.A flat "60 lb per point" rule would have said 60 lb — 13% short, and short in the direction that leaves the job unfinished.
  5. 5.Calcitic limestone is rated at 100% CCE, so the product quantity is the same 69 lb per 1,000 sq ft.
  6. 6.Over 5,000 sq ft that is 344 lb, or 8.6 bags of 40 lb, about $69.
  7. 7.At 69 lb per 1,000 sq ft the rate is above the 50 lb single-pass cap, so it goes down in two applications — 34 lb per 1,000 now, re-test in six months, the rest only if the test still calls for it.

Result: 344 lb of calcitic lime, in two passes six months apart

The same change on a heavy clay, using hydrated lime instead

  1. 1.Clay takes about 100 lb of calcium carbonate per 1,000 sq ft per point — four times what a sand takes.
  2. 2.The same 1.15 buffered points gives 115 lb of CaCO₃ equivalent per 1,000 sq ft.
  3. 3.Hydrated lime is rated at 135% CCE, so it takes only 115 ÷ 1.35 = 85 lb of product for the same effect.
  4. 4.Over 5,000 sq ft that is 425 lb rather than the 574 lb of calcitic limestone the same job would need.
  5. 5.Fewer pounds is not automatically better. Hydrated lime is caustic — it burns foliage and skin, it is easy to overshoot with because it acts in weeks rather than months, and on a soil this heavily buffered the slow release of ground limestone is generally the safer choice.
  6. 6.Even at the higher strength the rate is 85 lb per 1,000 sq ft, still above the 50 lb cap, so it splits into two passes.

Result: 425 lb of hydrated lime — fewer pounds, and the harder material to use safely

Why the number is so much bigger than pH suggests

Soil pH measured in water tells you about the acidity in solution. That is a tiny fraction of the acidity present.

The rest — the reserve, or exchangeable acidity — is hydrogen and aluminium ions held on the surfaces of clay particles and organic matter. As lime neutralises the acid in solution, more comes off the exchange sites to replace it. The soil pushes back, and it keeps pushing back until the reserve is spent.

That is why a soil with a pH of 5.5 might need seventy pounds of lime per thousand square feet rather than the trivial amount the solution chemistry alone would suggest. And it is why texture matters so much: clay and organic matter have enormous surface areas and hold correspondingly large reserves, while sand holds almost none.

It also explains the non-linearity. The further below neutral a soil sits, the more of its exchange sites are occupied by acid ions, so each successive tenth of a pH point costs more lime than the one above it. Going from 5.0 to 6.0 takes appreciably more lime than going from 6.0 to 7.0 on the same ground — a fact that flat-rate calculators miss entirely.

Buffer pH, and why a lab does it differently

The honest position is that texture is a proxy for the thing that actually matters, and laboratories measure the thing itself.

A soil testing lab adds a buffer solution — a liquid of known pH and known resistance to change — to a soil sample and measures how far the soil pulls it down. A soil with a large reserve acidity moves the buffer a long way; one with a small reserve barely moves it. That displacement is a direct measurement of the reserve, and it converts to a lime requirement through a published table.

This is why two soils that both read pH 5.5 in water can have lime requirements differing by a factor of three, and why a lab recommendation is worth more than any calculation from texture. Buffer pH is normally included in a standard soil test at no extra cost, and it is often reported as "SMP buffer", "Adams-Evans" or "Mehlich buffer" depending on the region.

Where the texture estimate on this page is most useful is planning and budgeting — working out roughly what a job will cost and whether it needs one pass or three — and as a sanity check on a recommendation you have been given. Where it is least reliable is on soils high in organic matter, which buffer far more strongly than their mineral texture suggests.

Reading the bag: CCE, fineness, and what you are actually buying

Liming materials are not interchangeable pound for pound, and the two properties that matter are both printed on a good bag.

  • Calcium carbonate equivalent (CCE) — neutralising strength as a percentage of pure calcium carbonate. Calcitic limestone is about 100%, dolomitic slightly above because magnesium carbonate is lighter per unit of neutralising power, hydrated lime about 135%, wood ash roughly 50% and wildly variable. Gypsum is zero.
  • Fineness — lime only reacts where it dissolves, and dissolution depends on surface area. Coarse material sits in the soil for years; material passing a 100-mesh sieve reacts almost entirely within a season. Two bags at the same CCE can differ by a year in how quickly they work.
  • Effective neutralising value — some states require a single figure combining CCE and fineness. Where it is available it is the number to compare, because it answers "how much of this will actually do something this year".
  • Dolomitic versus calcitic — dolomitic supplies magnesium as well as calcium. That is useful where a soil test shows magnesium low, and unhelpful where it is already high — excess magnesium can worsen soil structure on clays. Test before defaulting to it.
  • Pelletised — the same ground limestone bound with a binder so it flows through a spreader without dust. Convenient and worth the premium on a lawn; no faster and no stronger once it breaks down.

Gypsum is not lime

Gypsum appears in this calculator’s material list for one reason: it is the most persistently misunderstood product in soil amendment, and it is sold in the same aisle as lime.

Gypsum is calcium sulphate. It supplies calcium, which is genuinely useful, and sulphur, which is often useful. What it does not do is neutralise acidity, because sulphate is the anion of a strong acid and contributes nothing to raising pH. Its calcium carbonate equivalent is zero, and that is not an approximation.

Where gypsum earns its place is sodic soil — ground where sodium has displaced calcium on the exchange sites and destroyed the structure, leaving a dense, dispersed, unworkable soil. Gypsum supplies calcium to push the sodium off, and the sodium leaches away as sodium sulphate. That is a real and important use, and it is confined to arid regions and coastal soils that have taken salt water.

It is also widely sold as a clay-breaking amendment for ordinary garden soil, where it does very little. Compacted clay that is not sodic has a structure problem that organic matter and reduced traffic fix, and gypsum does not.

Lowering pH, which is the harder direction

Raising pH is a matter of adding a base and waiting. Lowering it is slower, less reliable, and sometimes impossible.

Elemental sulphur is the standard material, and it is not itself an acid. Soil bacteria — mostly Thiobacillus — oxidise it to sulphuric acid, and they only work when the soil is warm, moist and aerated. Applied in autumn in a cool climate, nothing measurable happens until the following spring. Applied to a cold or waterlogged soil, nothing happens at all.

Aluminium sulphate and iron sulphate act immediately because they hydrolyse rather than needing biology, but both take six to eight times the weight of elemental sulphur for the same effect. Aluminium sulphate has the further problem that aluminium is itself toxic to plant roots at the concentrations involved, which makes it a poor choice for anything you intend to grow in.

And there is a case where none of it works. Soils in arid regions, and any ground near limestone bedrock, old mortar or a concrete foundation, frequently contain free calcium carbonate. That carbonate buffers against acidification indefinitely: any acid you add is consumed neutralising it, and the pH returns. Test by putting a few drops of vinegar on a dry sample — if it fizzes, there is free lime, and growing acid-loving plants in raised beds of imported soil is a far better plan than fighting the ground.

What this assumes, and where it stops

Assumptions

  • Texture-based rates are pounds of pure calcium carbonate equivalent per 1,000 sq ft per pH unit, for the top 6 to 7 inches of soil.
  • The buffering model raises the per-unit requirement as pH falls below 6.2, reproducing the documented non-linearity.
  • Product quantities are derived from the material’s calcium carbonate equivalent.
  • A single application is capped at 50 lb of product per 1,000 sq ft, with the remainder deferred about six months.
  • Sulphur rates for lowering pH are treated as linear in the pH gap.

Limitations

  • A texture estimate is a proxy for reserve acidity. A laboratory buffer pH measures it directly, and the two can differ by a factor of three on soils high in organic matter.
  • Incorporation depth changes the requirement roughly in proportion. These rates assume the standard 6 to 7 inch tillage layer; a deeper bed needs proportionally more.
  • Fineness is not modelled. Two materials at the same CCE can differ by a year in how fast they react.
  • Soils containing free carbonate cannot be acidified by any practical quantity of sulphur, and this page cannot detect that from the inputs.
  • The sulphur model is linear in pH, which is a rougher approximation than the liming model — acidification depends heavily on soil biology, temperature and moisture.

Common questions

How much lime do I need to raise soil pH?

It depends far more on texture than on the pH gap. Roughly 25 lb of pure calcium carbonate per 1,000 sq ft moves a sand one point, 60 lb a loam and 100 lb a clay — and starting from a lower pH takes more than the flat rate suggests, because there is more reserve acidity to neutralise. Then divide by the material’s CCE to get pounds of actual product.

What is calcium carbonate equivalent?

The neutralising strength of a liming material as a percentage of pure calcium carbonate. Calcitic limestone is about 100%, dolomitic slightly above, hydrated lime about 135% and wood ash roughly 50%. It converts a calculated requirement into a quantity of the material you actually bought, and skipping it is how people end up a third short.

Does gypsum raise soil pH?

No. Gypsum is calcium sulphate, a neutral salt with a calcium carbonate equivalent of zero. It supplies calcium and sulphur and genuinely helps sodic soils by displacing sodium, but it will not move pH by any amount under any circumstances. To raise pH you need a carbonate — calcitic or dolomitic limestone.

How long does lime take to work?

Six to twelve months for ground limestone, and longer if it is coarse or surface-applied to established turf. Lime only reacts where it dissolves, so fineness and incorporation both matter — working it into the top six inches roughly halves the time. Apply in autumn for a spring crop, and re-test after six months rather than re-applying on a hunch.

How do I lower soil pH?

Elemental sulphur, at roughly 10 lb per 1,000 sq ft per point on sand up to 25 lb on clay. It is slow, because soil bacteria have to oxidise it and they need warmth and moisture. Aluminium and iron sulphate act faster but take six to eight times the weight, and aluminium is toxic to roots. If the soil contains free carbonate — drop vinegar on it and see if it fizzes — acidification will not hold at all.

Can I apply all the lime at once?

Not above about 50 lb of product per 1,000 sq ft. Beyond that the material sits on the surface rather than reacting, and a heavy dose disrupts soil biology. Split it into two applications about six months apart and re-test before the second — overshooting pH is harder to correct than undershooting, because you then have to move it back in the more difficult direction.

Sources

Formula and content last reviewed on .

Results are estimates for information only, not professional advice.

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