This is the calculator on SoilSums with the widest real-world error bars, and it is worth saying so before the numbers rather than after. It gives a starting figure from your soil's texture. What actually decides how much lime a soil needs is its buffering capacity, and no online form can see that.
How to use it
- Enter the area you are treating.
- Choose the soil texture. Squeeze a handful of damp soil: gritty and falling apart is sandy, holding together but crumbling when poked is loam, sticky and ribboning between finger and thumb is clay. The description under the dropdown describes each one.
- Enter your current pH, from a soil test or a meter, and the target you want. Most vegetables are happy between 6.0 and 6.8, so there is rarely a reason to aim higher.
The calculator will tell you when it is being asked to extrapolate too far, and will point you at elemental sulfur instead if the target is below the current reading — lime raises pH, it cannot lower it. For the material choice, the timing and why the whole job takes
four to six months, how to raise soil pH covers what the quantity alone does not.
How it is calculated
lb of limestone = rate for texture x pH change x (area / 1000)
The rate is pounds of ground limestone per 1,000 square feet per one pH unit, taken from the University of Kentucky's lawn liming table: 25 for sandy soil, 60 for loam, 95 for clay. Those figures assume ground agricultural limestone, calcitic or dolomitic, worked into the top six inches of a mineral soil low in organic matter.
Each texture also carries a published spread, and the calculator shows it next to the single figure. A range is the honest shape of this answer: the midpoint is not more accurate than the range, it is just easier to put on a bag.
The calculator treats the requirement as linear in the pH change, which is an approximation. It holds up reasonably over about one unit and gets progressively less reliable beyond that, so the tool warns above 1.5 units rather than quietly extrapolating.
A worked example
A 400 square foot bed of loam soil, currently pH 5.5, targeting 6.5:
- pH change: 6.5 − 5.5 = 1.0 unit
- Rate for loam: 60 lb per 1,000 sq ft per unit, published range 45-75
- Per 1,000 sq ft: 60 x 1.0 = 60 lb
- Scaled to the bed: 60 x (400 / 1,000) = 24 pounds of ground limestone
- The same arithmetic on the range: 18 to 30 pounds
The same bed in sandy soil needs 10 pounds. In clay, 38.
Lime rates by texture
| Soil texture | Per 1.0 pH unit | Published range | 0.5 unit | 1.5 units |
|---|---|---|---|---|
| Sandy | 25 lb | 20-30 lb | 12.5 lb | 37.5 lb |
| Loam | 60 lb | 45-75 lb | 30 lb | 90 lb |
| Clay | 95 lb | 90-100 lb | 47.5 lb | 142.5 lb |
All figures are pounds of ground agricultural limestone per 1,000 square feet, from AGR-214, Liming Kentucky Lawns (University of Kentucky Cooperative Extension, 2014), Table 1.
Note the bottom right cell. Correcting clay by a full 1.5 units comes to 142.5 lb per 1,000 square feet, well above the 50 lb this calculator treats as a single-application ceiling, so it splits the job for you.
Why published lime rates differ so much by region
If you look up lime rates from two different extension services you will often find numbers that disagree by a factor of two or more for what sounds like the same soil. That is not one of them being wrong. It is three separate things at once.
The texture name is a proxy, not the mechanism. What sets lime requirement is cation exchange capacity and organic matter: how many acidic sites there are to neutralise. Texture correlates with CEC but does not determine it, because clay mineralogy varies. A smectite clay and a kaolinite clay of identical texture want very different amounts of lime.
The services are describing their own soils. A state table is tuned to the soils that state actually has. Kentucky's is built on limestone-derived soils; western Oregon's on leached, high-rainfall soils that are acid to start with.
They are not even answering the same question. Some publish a rate per pH unit, some a rate to reach a target pH, some a cap on what is safe in one application. Those are different quantities and cannot be compared cell for cell.
| Kentucky (AGR-214) | Colorado (GardenNotes #222) | |
|---|---|---|
| How it is expressed | lb per 1,000 sq ft per 1.0 pH unit, by texture | a cap on a single application, plus adjustments |
| Sandy | 25 lb | not broken out by texture |
| Loam | 60 lb | not broken out by texture |
| Clay | 95 lb | not broken out by texture |
| Single-application limit | not stated | 50 lb per 1,000 sq ft on established turf |
| Organic matter | assumes low | add about 20% at 4-5% organic matter |
| Hydrated or burned lime | not covered | halve the rate, and never above 10 lb per 1,000 sq ft |
| Context it was written for | cool-season lawns on Kentucky soils | Colorado soils, most of which are alkaline already |
Two states, and the rows barely line up: one gives a rate per pH unit, the other a ceiling on what is safe at once. Neither is wrong, and no arithmetic turns one into the other.
Penn State adds a third answer: a soil-test-driven recommendation with a flat ceiling of 100 lb per 1,000 square feet on any single turf application, splitting larger corrections into semiannual ones. Maryland puts that ceiling at 50, which is a genuine disagreement rather than an error — see the split question above for which one this calculator uses and why.
Oregon State has no row here at all, and that is worth saying rather than passing over. Its western-Oregon guidance works from a soil test rather than a texture table, so there is no per-texture figure of theirs to compare — see Applying Lime to Raise Soil pH for Crop Production (Western Oregon). The more carefully a service works, the less willing it tends to be to answer from texture alone, which is the same conclusion this page keeps arriving at.
This calculator uses the Kentucky table because it is the one that states a rate per pH unit per texture, which is the shape this tool needs. The practical consequence is worth being blunt about: if you garden outside the mid-South, treat the figure as the middle of a wide range and let a soil test settle it.
Tips and common mistakes
Get a soil test. For the price of a couple of bags of lime, a test tells you your actual pH, usually a direct lime recommendation, and what else the soil is short of. In the United States and Canada your county or provincial extension service will do one cheaply. It is the single best value purchase in gardening.
Apply less than you think and retest. Lime is slow and hard to reverse. Splitting a large correction across two seasons, with a test in between, is how to avoid overshooting. Above 50 lb per 1,000 square feet you should be splitting it anyway, on Maryland's reading, and Colorado State and Ohio State put the ceiling at 50 as well; Penn State is the one publication of the four that would let you go to 100.
Count your organic matter. A bed that has had compost worked into it for years is buffered by that organic matter as well as by its clay, and needs more lime than the texture alone suggests — Colorado puts the uplift at about 20% where organic matter runs 4 to 5%.
These figures are for ground, dolomitic or pelletized limestone. Ohio State University Extension caps all three at 50 lb per 1,000 square feet in a single application, and caps hydrated lime at 20 and burned lime at 10 — Colorado is stricter again at 10 for either. So hydrated and burned lime are not this product at a different price: they are caustic materials with a ceiling a fifth of the one above, and they will burn skin and foliage. Ground or pelletised limestone is the right product for a garden, and the only one these rates describe.
Work it in rather than leaving it on top. Limestone barely moves through soil on its own. Raking or digging it into the top six inches gets it where the roots are; left on the surface it can take years.
Do not lime potato beds to neutral. Potatoes prefer acid soil and scab gets worse as pH rises. If a bed is in rotation, lime it in the year it grows brassicas rather than the year it grows potatoes.
Skip the lime entirely if your pH is already above 6.5. Most vegetables need nothing at that point, and pushing higher causes the micronutrient problems described in the questions below.
Check what your acid-loving plants want first. Blueberries, rhododendrons and azaleas want soil far more acid than a vegetable bed. Liming near them undoes what they need.
Sources
- AGR-214, Liming Kentucky Lawns — University of Kentucky Cooperative Extension, 2014. Table 1 is the rate table this calculator uses.
- Liming Turfgrass Areas — Penn State Extension. Source of the 100 lb per 1,000 sq ft single-application ceiling, the semiannual split, the target pH ranges by grass, and the instruction not to lime without a test.
- Lime and Lawns — University of Maryland Extension. Source of the 50 lb ceiling this calculator uses, the six-month split, and the three-to-four-year soil test interval.
- Changing Soil pH, CMG GardenNotes #222 — Colorado State University Extension. Source of the 50 lb established-turf limit, the 20% organic matter uplift, and the hydrated and burned lime figures. (The extension site has since redesigned its soil-pH page, so the document is cited by title rather than URL.)
- Lime and the Home Lawn — Ohio State University Extension, Ohioline HYG-4026. Source of the maximum single-application rates by material — 50 lb per 1,000 sq ft for ground, dolomitic and pelletized limestone, 20 for hydrated lime, 10 for burned — the ideal lawn pH of 6.0 to 7.0, and the statement that it may be several months before the soil pH changes.
- Timing of Lime and Fertilizer Applications — UMass Amherst Soil and Plant Nutrient Testing Laboratory. Source of the four-to-six-month range for limestone to raise soil pH, and of the advice that established plantings may be limed twice a year, spring and autumn, with the amount limited to avoid damage.
- EM 9057, Applying Lime to Raise Soil pH for Crop Production (Western Oregon) — Oregon State University Extension. Cited only for its soil-test-led approach. No figure from it appears on this page: its own Table 2 states that it must not be used for lime rate recommendations.