What is limestone?
When gardeners talk about "lime" or "limestone," they mean either calcium carbonate (calcite or calcitic limestone), or calcium-magnesium carbonate (dolomite or dolomitic limestone). Of the two forms, dolomite is preferred as it provides magnesium as well as calcium. Both are mined from open-pit quarries, then crushed and screened to the appropriate particle size. Our calculators refer to either of these materials.
Other forms include true lime (calcium oxide), and slaked, builder's, or hydrated lime (calcium hydroxide). True lime is highly caustic making handling for gardeners not worth the trouble. Hydrated lime is sometimes sold to home gardeners, but it has more neutralizing power than limestone meaning you should apply about two-thirds the recommended amount. It is less caustic than true lime, but is powdery so awkward to apply.
What is pH?
For you chemists, the symbol stands for the "negative logarithm of hydrogen ion concentration." For the rest of us, pH is a measure of the acidity ("sourness") or alkalinity ("sweetness") of soil. The scale ranges from 1 to 14, but few soils are more acidic that 5 or more alkaline than 9. A pH of 7 is perfectly neutral, but most plants grow best when the pH is a slightly acidic between 6.5.
Soils in areas with low rainfall tend to be alkaline. Where rainfall is high, soils are more often acidic. This happens because rainfall gradually washes calcium from soil. In practical terms, if you live east of the Mississippi River, you'll need to add calcium to your soil to raise the pH; in the arid West the reverse is true. There, you'll need to add amendments such as sulfur in order to gradually increase the hydrogen ion concentration.
Soil testing is the best way to learn about your soil, including the pH. Some home gardeners report good results using soil testing kits, and experienced farmers claim to taste the sweetness or sourness of soil by touching samples with their tongues. But here are two simple tests that will indicate if your soil is too acidic or alkaline:
Do you have sand, loam, or clay soil?
All soils are mostly collections of tiny mineral particles. The size of mineral most abundant in your soil determines whether you have sand, loam, or clay. This mineral composition of soil is called the soil's "texture."
Sand particles are small, but compared to other soil particles they are largest. The other two main categories of soil particles are silt, which is smaller than sand, and clay, which is still smaller.
Sand. Water and nutrients move through sand soils fast meaning both need more frequent replenishing. A handful of sandy soil won't clump or hold a shape when you press it.
Loam. These soils have a beneficial mix of all the mineral particle sizes.
Clay. Microscopic clay particles pack together tightly. Water and nutrients moves into and through clay soils slowly. Clay soils are heavy, and you can easily form a ball or ribbon with a clump of moist clay soil in your hand.
Most soils are mixtures of all three particle sizes, but in varying proportions. A predominance of sand particles makes a lighter, more open soil with lots of channels for air and water circulation. Minute clay particles pack together tightly making a clay soil heavier, more dense, and with less favorable air and water circulation. Because of the greater density of clay soils, more limestone or sulfur is required to change their pH.