Hydrated Lime vs Quick Lime vs Slaked Lime: What Is the Difference for Soil?

Hydrated lime, quicklime and slaked lime are three stages of the same material, not three products. Quicklime (calcium oxide, CaO) is burned limestone; slaked lime and hydrated lime are the same thing (calcium hydroxide, Ca(OH)₂) made by adding water to quicklime; and agricultural lime is simply calcium carbonate (CaCO₃) that was never burned at all. Only one of the three belongs in most gardens and fields, and it is not quicklime. This guide covers what each one is, how to tell them apart on a bag label, how fast each moves soil pH, what hydrated lime actually costs per unit of neutralising value, and the one combination that must never be mixed.

Quicklime, hydrated lime and slaked lime at a glance

Name on the label	Chemical	Made by	Solubility / speed	Soil use
Quicklime, burnt lime, lump lime	CaO (calcium oxide)	Limestone burned at about 900 °C	Reacts violently with water	Not for gardens; industrial, road and some large-scale sodic-soil work only
Hydrated lime, slaked lime, pickling lime, calcium hydroxide	Ca(OH)₂ (calcium hydroxide)	Quicklime slaked with water	Slightly soluble, acts in days to weeks	Small, precise corrections: pots, greenhouse beds, remediation
Agricultural lime, ag lime, ground limestone, garden lime	CaCO₃ (calcium carbonate) or dolomitic CaMg(CO₃)₂	Limestone ground fine, never burned	Barely soluble, acts over months to years	The default for lawns, beds and fields

The wording is the trap. Slaked lime and hydrated lime are the same powder. “Quick” and “slaked” describe a reaction that has already happened: quicklime was slaked, and what you buy in the bag is the slaked product.

The chemistry: what slaking actually does

Slaking is the reaction of quicklime with water, and it releases a great deal of heat:

CaO + H₂O → Ca(OH)₂ + heat

Add water to quicklime and it crumbles from grey lumps into a fine white powder while the mass boils and steams, enough heat to boil the water it is absorbing. That is why quicklime is handled with protective equipment and never stored in a sealed container with any moisture present. Hydrated lime is the finished, cooled, stable powder that comes out of that reaction.

Both are alkaline, but they differ sharply in how much pH they can generate. A saturated solution of calcium hydroxide sits around pH 12.4; a suspension of calcium carbonate sits closer to pH 8.3 and stops there because it is barely soluble. That difference is the whole story of why one moves soil pH in a fortnight and the other takes a season.

Neutralising value: why “per bag” comparisons lie

Every lime product is compared against calcium carbonate as 100. The relative neutralising value (RNV) of the three is roughly:

Product	Relative neutralising value
Quicklime (CaO)	~178
Hydrated lime (Ca(OH)₂)	~136
Agricultural lime (CaCO₃)	100

So a kilogram of hydrated lime neutralises about a third more acidity than a kilogram of ag lime, before you account for fineness and purity. That sounds like a bargain until you compare prices properly. The only fair comparison is cost per unit of neutralising value (dollars per tonne ÷ RNV, or dollars per bag ÷ RNV ÷ bag weight), because the cheap bag with half the calcium carbonate content is not cheap. Purity matters too: dolomitic limestone (containing magnesium carbonate) should be preferred only when your soil is low in magnesium, since it also adds Mg.

The price of lime also moves with distance from a quarry. Freight dominates. Bagged hydrated lime is a manufactured chemical sold by weight in small packs, so it is priced like a chemical, while bulk ag lime is priced like a mineral. Compare on the RNV basis and the apparent bargain usually disappears.

Which lime should you use on soil?

Use agricultural lime by default. It is slow, forgiving, hard to overdose, and cheap per unit of neutralising value. The fineness specification is what governs how fast it works: particles under 0.25 mm react within a season, while anything over 1 mm may take years. Buy the finest grade you can get for the area you are treating, and work it into the top 10–15 cm.

Use hydrated lime when you need speed and precision on a small area. It dissolves enough to move pH within weeks, which suits greenhouse mixes, seed trays, container media and small correction jobs. It is also the material used in some sodic-soil work, where calcium is needed without waiting for limestone to dissolve.

Do not use quicklime on soil. The heat, the caustic dust and the violent reaction with soil moisture make it a hazard rather than an amendment; the small pH gain is not worth it, and where a genuine sodic correction is required, gypsum is the right material.

How fast does each one change soil pH?

• Hydrated lime: detectable within 1–3 weeks in a well-mixed, moist medium. Fast enough to overshoot: re-test at four weeks rather than at the end of the season.
• Agricultural lime: measurable in 4–8 weeks, with most of the response over 3–12 months. Fineness, soil moisture, temperature and organic matter all govern the rate.
• Quicklime: reacts on contact but is not an appropriate soil product.

Speed is not automatically an advantage. Lime reacts where it lands, and hydrated lime concentrated in one layer can create a high-pH band that roots avoid. Split applications, mix into the full rooting depth, and water in.

Never mix lime with ammonium fertiliser

Lime plus an ammonium source (ammonium sulfate, ammonium nitrate, urea) converts ammonium to ammonia gas, which leaves as odour and lost nitrogen. This is the classic failure behind “I fertilised and limed on the same day and the lawn looks worse”. Wait at least two weeks between a lime application and an ammonium fertiliser, or use a source of nitrogen that is not ammonium-based in the meantime. If you are wondering whether lime and fertiliser can be applied together at all, the dedicated guide on applying fertiliser and lime at the same time covers the timing rules, and the best time to apply lime covers the season.

Applying hydrated lime safely

• Wear eye protection, gloves and a dust mask, it is caustic and the dust is alkaline.
• Add lime to water, never water to quicklime.
• Do not store opened bags in damp air; hydrated lime absorbs carbon dioxide from the atmosphere and reverts to calcium carbonate over time, losing strength.
• Keep it away from ammonium fertiliser, cement and aluminium sulfate during storage and application.
• Keep children and pets off treated ground until the dust has settled and the surface has been watered in.

How much lime do you need?

The amount depends on your soil’s buffering capacity, not on its current pH alone, a pH 5.8 sand needs a fraction of the lime that a pH 5.8 clay loam needs. The lime requirement calculator works a tons-per-acre (or kg per 100 m²) rate from CEC, buffer pH or organic matter, corrected for the quality of the limestone you are actually buying. Feed it the RNV of your product rather than assuming 100, and the recommendation becomes comparable between ag lime and hydrated lime.

For the rest of the decision (timing, placement, pelletised versus bulk, calcite versus dolomite), see ag lime versus pelletised lime, calcitic versus dolomitic limestone and applying lime to the garden. For what the reaction is doing inside the soil, how lime works in soil explains the exchange complex side of it.

Bottom line

Quicklime is the raw material, hydrated lime and slaked lime are the same finished powder, and agricultural lime is the unburned original. Buy ag lime for anything larger than a container; use hydrated lime only where you need a fast, small, precise correction; compare products on cost per unit of neutralising value instead of per bag; never combine it with ammonium fertiliser; and always let a soil test (not a product label) set the rate.