Lime: why, how much, and how
Lime raises soil pH, but a pH number alone does not say how much lime it takes. This guide covers where soil acidity comes from, why texture changes the rate, what the strength numbers on a bag mean, and how to get lime into the soil. The sliders and quick checks along the way use the same arithmetic as the Lime Calculator.
Why soil turns acidic
Soil pH falls over time in most places, for three reasons:
- Rain. Water moving down through the soil carries calcium, magnesium and potassium with it, and hydrogen and aluminum take their places on the soil particles. High-rainfall regions trend acidic for this reason alone.
- Nitrogen fertilizer. When soil bacteria convert ammonium to nitrate, they release acid. Urea and ammonium nitrate leave behind acidity that takes about 1.8 lb of limestone to neutralize per pound of nitrogen applied. Ammonium sulfate takes about 5.4 lb.
- Plant growth and decay. Roots release acid as they take up nutrients, and breaking down organic matter produces more.
Below about pH 5.5, aluminum and manganese dissolve into the soil water at levels that injure roots, phosphorus binds to iron and aluminum and stops being available, and the bacteria that release nitrogen from organic matter slow down. Raising the pH back to the 6 to 7 range undoes all of that. Lime also supplies calcium, and dolomitic lime supplies magnesium.
Same pH, different rate
A pH reading measures the acidity dissolved in the soil water, called active acidity. Most of a soil's acidity is held on clay and organic matter instead, called reserve acidity, and it moves back into the water as lime neutralizes what is there. Lime has to neutralize both.
Sand holds very little reserve acidity. Clay and high-organic soils hold a great deal. That is why a sandy soil and a clay soil both reading pH 5.5 can need lime rates three times apart, and why a lab runs a second test, the buffer pH (SMP, Sikora, Mehlich or Adams-Evans, depending on the lab), to measure the reserve. Without a buffer pH, soil texture is the best stand-in.
Try it
Lime to move pH, by soil texture
Quick check
Two lawns both test pH 5.5. One is on sand, one on heavy clay. Which needs more lime to reach 6.5?
Answer: The clay lawn. Clay holds far more reserve acidity, so the same pH change takes about 90 lb of lime per 1,000 sq ft on heavy clay and about 30 lb on sand.
Reading the bag: CCE, fineness, ECCE and ENM
Every lime recommendation assumes a reference material: pure, finely ground calcium carbonate. Real products differ from it in two ways, and the label carries a number for each.
Try it
What a coarse ag lime is worth
Pelletized lime is ground limestone, mostly finer than 100 mesh, bound into granules so it spreads without dust. The pellets fall apart at the first rain, and from then on it behaves like the powder it was made from. Products sold as "fast-acting" are also finely ground. They react sooner, but the CCE still caps how much acid each pound can neutralize, so a smaller bag cannot stand in for the full rate.
Quick check
A 40 lb bag of pelletized lime and a 40 lb bag of pulverized lime both list a CCE of 95%. Which raises pH more in the end?
Answer: About the same. Both are fine ground limestone with the same CCE. The pellets cost more for easier spreading, not for extra strength.
Spreading it
- Apply it ahead of when the pH needs to be right. Fine lime takes two to six months to react fully, so lime a garden in fall for the next season. On a lawn, fall and early winter are good times: freeze and thaw work it into the surface.
- Mix it in where possible. On a garden bed or before seeding a lawn, till or dig lime into the top 6 inches. Lime left on the surface moves down about an inch a year, so an established lawn takes several seasons to change below the top couple of inches.
- Split large rates. On established turf, put down no more than 50 lb per 1,000 sq ft at a time. More than that can crust on the surface and spike the pH in the top half inch while the root zone barely changes.
- Keep it a week or two apart from urea and ammonium fertilizer. On the surface together, lime raises the pH around the fertilizer granules, and some of the nitrogen escapes as ammonia gas.
- Pick calcitic or dolomitic by the magnesium on the soil test. Dolomitic adds magnesium, which helps where magnesium tests low and adds nothing where it is already high.
Gypsum (calcium sulfate) supplies calcium but does not raise pH, so it does not replace lime. Wood ash does raise pH, at anywhere from a quarter to most of limestone's strength depending on the wood and the burn. It is also high in potassium and quick to overdo.
Why pH drifts back
Lime is used up by the same processes that made the soil acidic. Rain keeps leaching, and every pound of nitrogen fertilizer keeps adding acid. A lawn fed 3 lb of nitrogen a year as urea needs roughly 5 to 6 lb of lime per 1,000 sq ft each year just to stay where it is. A soil limed to 6.5 will usually be back near 6.0 within three to five years.
Retest every three years, or every year where pH is being moved, and lime again when it drops below the target. Irrigation water with high alkalinity works the other way, raising pH slowly every season. Where that is the case, the irrigation water test explains it.
Quick check
A lawn was limed from pH 5.6 to 6.4. Three years later, fed urea every spring and fall, it tests 5.9. What happened?
Answer: Normal drift. Liming resets the pH, but the acidifying never stops. That is why lime is a repeat job every few years rather than a one-time fix.
Taking a soil past pH 7.0 locks up iron, manganese and zinc, and bringing it back down takes elemental sulfur and a year or more. Lime from a soil test, not on a schedule.
Summary
- Rain, nitrogen fertilizer and plant growth all acidify soil. Lime neutralizes the acid and supplies calcium.
- pH says whether lime is needed. Buffer pH, or soil texture without one, says how much.
- Compare products by ECCE: CCE discounted for coarse particles. Pellets and powder with the same CCE do the same job.
- Mix it in where possible, split lawn rates into passes of 50 lb or less, and allow months for it to react.
- pH drifts back down. Retest every three years.
Keep reading
EC, ppm, and the 500 and 700 scales
The same bucket reads 1,000, 1,280 or 1,400 ppm depending on the meter. Which scale a meter uses, how to convert, and what temperature does to a reading.
5 min read LawnFertilizing a lawn
Pounds of nitrogen per 1,000 sq ft is the number that matters. What the bag says, how much to spread, when, and how to check the spreader.
5 min read IndexAll Bizon guides
Everything published so far, and what is being written next.
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