Answer: First you need to have a soil test done to determine what the pH of your soil is. 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. 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. If you determine that your soil is acidic, ground limestone will help raise the pH somewhat. It 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. You shouldn't need to apply it more than once a year. 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.
There are many different types of iris some with different soil preferences. However, most iris will grow in any fertile garden soil. If you want, send another email to the Scotts Q&A with the specific iris you are interested in. http://www.scottscompany.com/html/b08_qa.cfm
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