Prepare the Soil With Organic Matter
Petunias are heavy bloomers, and that means they need rich, well-draining soil to perform at their best. If you’re planting in a field, this often means improving what’s already there. Whether your soil is clay-heavy or sandy, adding plenty of compost or well-rotted manure will boost fertility and texture.
Loamy, slightly acidic soil (around pH 6.0 to 6.5) is ideal. Organic matter also invites beneficial soil organisms that improve nutrient uptake and keep your plants healthy from the ground up. I like to till compost into the top 6 inches of soil and let it rest for a week before transplanting—this gives earthworms and microbes time to work their magic and sets the stage for strong root growth.