Answer: The pH scale runs from 0 - 14, with 7 being 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. I happen to live in the Southwest, which has alkaline soil (8-8.5), so our native plants are geared to thrive in those conditions. 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 acheived its work by the following spring. You can hurry things along if you use a liquid lime product (sold by Gardener's Supply Co., http://www.gardeners.com; ph# 800/863-1700) which works more quickly.
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. As a quick fix in mid season, you might consider using a dusting of wood ashes, but you will find that the wood ashes leach out very quickly and need to be repeated in much smaller and more frequent doses.
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. We tell people to never use limestone or wood ashes here! I hope this information is useful. Thanks for writing!
Q&A Library Searching Tips