The pH scale runs from 0 – 14, with 7 considered neutral. Numbers below 7 are acidic; numbers above alkaline. In much of the country, most plants grow in a range of 6.0 to 7.5. In addition, plants will generally tolerate some fluctuation in pH over time, with the “neutral” being a goal rather than an absolute.
pH levels can be adjusted somewhat on a temporary and localized basis, but it’s not a one-time fix, and it’s harder to do for large areas. For acid soils, ground limestone is a good form of soil amendment to use because it is available to the plants over a long period of time, slowly dissolving over a period of years. Limestone does take a while to work—most often, we recommend that people apply lime in the fall so it’ll have achieved its work by the following spring. If you spread ground limestone in the fall on freshly worked soil, it will work its way through the soil over the winter in time for spring. Be sure to spread it evenly, as it does not spread but rather sinks into the soil.
To make our alkaline soils more acidic, we apply soil sulfur. Pine needles are acidic and if we can find them, they are added to the soil as mulch or in compost. For acid-loving plants, you should also use a special plant food.