Soil Test & Lime / Sulfur Calculator
How much lime or sulfur a pH change really needs — and why soil texture decides the amount far more than the pH reading does.
The same job in each material
Texture decides the amount, not pH
A clay loam moving 0.5 pH needs more lime than a sand moving 1.5
pH tells you how acidic the soil is right now. It tells you almost nothing about how much lime it will take to change that, because the acidity in solution is a vanishing fraction of the acidity held on the clay and organic matter. Texture decides the amount. A clay loam going from 6.0 to 6.5 needs about 55 lb per 1,000 sq ft; a sand going from 5.0 to 6.5 — three times the pH change — needs about 45. The soil with the smaller job needs more material. A buffer-pH test measures that reserve directly; texture is the free approximation to it, and it's the one your eyes can do.
Sulfur is 3.12× as potent as lime, and that's chemistry rather than a table. Calcium carbonate has an equivalent weight of 50.04 g; elemental sulfur — which oxidises to sulfuric acid, yielding two hydrogen ions per atom — has 16.03. The ratio is 3.12, and published field rates carry exactly that ratio at every soil texture: lime 30 / sulfur 10 on sand, lime 110 / sulfur 35 on clay loam. The same reasoning gives every liming material its strength — dolomite 108.6%, hydrated lime 135.1%, burnt lime 178.5%, against published figures of 109, 136 and 179. Nothing on this page is looked up.
Fix pH before buying fertiliser. Below about pH 6, phosphorus locks up in aluminium and iron compounds that roots can't reach, so part of what you apply to an acidic lawn is money spent on a nutrient that won't be taken up. Limestone costs a fraction of what fertiliser does per bag, and it makes the fertiliser work.
- "High" means stop, not keep it up. It's the reading people most often misread — a high number isn't a score to defend, it's a signal that further application is money spent on a surplus. Excess phosphorus doesn't wash down through soil, it runs off the surface.
- Centipedegrass wants acid soil — pH 5.0–6.0. Liming it to the usual 6.5 causes iron chlorosis, so the standard advice is actively wrong for that one grass.
- Never more than about 50 lb per 1,000 sq ft at once. Beyond that it sits on the surface rather than reacting, and on turf it can scorch. Big corrections are split over seasons.
- Pelletised lime isn't faster lime. Same ground limestone with a binder that dissolves on contact — pleasanter to spread, no quicker to act. Fineness of grind is what changes the speed.
- Don't apply lime and nitrogen together. Raising pH around a urea granule drives nitrogen off as ammonia gas. A few weeks between is enough.
- Sample properly: a dozen cores from around the lawn mixed in a clean bucket, not one scoop from the worst patch.
How to use
- Enter the pH from the test and the target for your grass.
- Pick the soil texture, which stands in for the buffer capacity.
- Compare materials by strength rather than by price per bag.
- Split anything over about 50 lb per 1,000 sq ft across seasons.
Frequently asked questions
How much lime do I need to raise soil pH?
It depends far more on the soil than on the pH. Raising a sand by one unit takes about 30 lb of ground limestone per thousand square feet; the same change on a clay loam takes about 110. The pH reading tells you the direction and roughly the multiplier, but texture sets the magnitude, and the spread across textures is nearly four to one.
Why does soil texture matter more than pH for liming?
Because the acidity in solution is a vanishing fraction of the acidity held on the clay and organic matter, and it is that reserve you have to neutralise. A clay loam moving from 6.0 to 6.5 needs about 55 lb per thousand square feet, where a sand moving from 5.0 to 6.5 — three times the pH change — needs about 45. The soil with the smaller job needs the larger amount.
What is buffer pH on a soil test?
A second measurement that estimates the reserve acidity directly, by adding a buffered solution and seeing how far it shifts. It is the number a lab uses to calculate a lime recommendation, because ordinary pH cannot. If your test reports one, use its recommendation; if it does not, soil texture is the free approximation and it is what this tool uses.
How much sulfur lowers soil pH by one point?
Roughly a third of what the equivalent lime application would be — about 10 lb per thousand square feet on sand up to 35 on clay loam. The ratio is 3.12 and it comes from equivalent weights rather than a table: calcium carbonate is 50.04 grams per equivalent and elemental sulfur, which oxidises to sulfuric acid, is 16.03.
Why is sulfur stronger than lime per pound?
Because each sulfur atom yields two hydrogen ions when soil bacteria oxidise it to sulfuric acid, and a sulfur atom weighs 32 where a calcium carbonate unit weighs 100. That gives equivalent weights of 16.03 against 50.04, a ratio of 3.12 — and the published field rates carry exactly that ratio at every soil texture, which is a good sign the tables are not arbitrary.
What is calcium carbonate equivalent?
A material strength rating: how much neutralising power it has compared with pure ground limestone. It is simply the ratio of equivalent weights, so it can be derived rather than looked up — dolomite comes out at 108.6 per cent, hydrated lime at 135.1 and burnt lime at 178.5, against published figures of 109, 136 and 179. A material at 136 per cent means you need about three quarters as many pounds.
Should I use dolomitic or calcitic lime?
Calcitic unless the soil test shows magnesium low. Dolomitic lime supplies magnesium along with calcium and is very slightly stronger per pound, which makes it a reasonable default in many places — but using it every time can push magnesium high enough to interfere with calcium and potassium uptake. Let the test decide rather than habit.
Is pelletized lime better than powdered?
It is pleasanter to spread and no faster to act. Pelletised lime is the same ground limestone held together with a binder that dissolves the moment it gets wet, so within an hour of watering you have powdered lime either way. What genuinely changes the speed is fineness of grind, which appears on the label as a mesh or fineness rating rather than on the front of the bag.
How much lime can I apply at once?
About 50 lb per thousand square feet. Beyond that the surplus sits on the surface rather than reacting, and on established turf it can scorch. Large corrections are split across seasons, roughly six months apart — which costs nothing in practice, because limestone takes three to six months to move a pH reading at all.
Does a soil test showing high phosphorus mean I am doing well?
It means stop applying it. This is the reading people most often misread: a high number is not a score to defend but a signal that further application is money spent on a surplus. Phosphorus does not wash down through soil, it runs off the surface into waterways, which is why many places now restrict its sale for established lawns.
Should I fix pH or fertilize first?
pH first, and it is not close. Below about pH 6 phosphorus locks up in aluminium and iron compounds that roots cannot reach, so a proportion of anything you apply to an acidic lawn will not be taken up. Limestone costs a fraction of what fertiliser does per bag and it makes the fertiliser work — on a genuinely acidic lawn it is the highest-return purchase available.
What pH does centipedegrass want?
Five to six, which makes it the exception to nearly every piece of standard lawn advice. Liming centipede to the usual 6.5 target induces iron chlorosis and thins the stand, so the ordinary recommendation is not merely unnecessary for that grass but actively wrong. It is the same grass that wants barely any nitrogen, and the pattern is consistent: centipede is the low-input outlier.
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