Petunias make attractive colourful garden additions, and they bloom annually. The University of Rhode Island points out the versatility of petunias, because they can grow in baskets, outdoor gardens and many types of planters. Like many types of plants, petunias grow best when soil meets specific pH requirements, which gardeners can test for and adjust with relative ease.
The soil pH refers to how acidic or basic a soil is. Pure water has a neutral pH of 7.0; a soil with a pH lower than 7 is acidic and one with a pH higher than 7 is basic. Different types of plants grow better with varying soil pH levels, but most grow best with a pH relatively close to neutral.
Petunias grow best with a soil pH between 5.5 and 5.8, according to the University of Massachusetts. This pH is considered relatively low because it is more acidic than many soils.
With a higher soil pH, petunias begin to suffer from an iron deficiency, according to the University of Minnesota. Petunias have a difficult time taking up iron compared with other plants, but more acidic soil helps them intake the nutrient. Therefore, petunias may not grow well among flowers that take up iron very easily and need less acidic soil, like marigolds.
The National Gardening Association recommends testing a garden's soil to make sure that nutrient levels and pH levels will provide ideal growing conditions for intended garden plants. Local gardening stores often sell at-home soil testing kits, and gardeners can also send soil samples to testing centres at local universities or nurseries.
If a garden's soil pH falls outside of the ideal range for petunias of 5.5 to 5.8, gardeners can still grow the flowers. The National Gardening Association recommends adding lime to increase soil pH or sulphur to decrease it. Gardening stores sell these additives to help gardeners achieve the right soil conditions for different plants. A naturally acidic compost, like one made out of pine needles, can also help reduce the soil pH for petunias.