Certain plants prefer a particular pH range, so knowing this can help you choose shrubs and perennials and/or amend the soil to alter its pH. A pH test of your garden's soil is the best way to determine whether it is acidic, alkaline or neutral. However, the performance of certain annuals can give you a clue as to the soil's pH.
Annuals that prefer a higher pH (over 7, alkaline) include geraniums (Pelargonium), celosia and marigolds. Annuals that like acid soil (pH lower than 7) are petunias (shown), pansies and bacopa. So if you have a bed with both geraniums and petunias and the geraniums are doing better, one cause may be that the pH is shifting to alkaline (or vice versa).
Keep annuals' pH preference in mind when planting and maintaining your containers, too. According to an article in the University of Minnesota's Yard and Garden News (June 2009), "Often the pH of city water is elevated to a pH of over 7.5 with sodium hydroxide or other bases to protect metal pipes from slowly deteriorating. High water pH can elevate potting medium pH over time. The rate of pH increase is dependant on some properties of the water (dissolved minerals) and the particular planting medium and its pH buffering ability." If your municipal water is alkaline, you might do best planting alkaline-preferring annuals in your pots.
Learn all about growing annuals with Horticulture's Annuals CD