Moisture

• Petunias are somewhat drought-tolerant plants. They do not require a lot of moisture every day to thrive. The ideal soil is one that is well-drained and does not retain moisture like clay soils do. The ideal amount of water for a petunia is a rainfall or hand watering about once a week. The soil should be allowed to dry out on the top 2 inches before the plants are watered again. Some spreading varieties of petunias prefer water twice a week. Every watering should soak the soil to a depth of 6 to 8 inches for best results.
  
  

Composition

• Petunias can grow in a variety of soils. However, they prefer a rich, fertile soil. Soils that contain a mixture of organic matter, such as compost, manure, leaf compost or peat moss, provide the best growing medium for petunia plants. The best color for soil is a rich brown or black. Potting soils provide an ideal soil composition for growing petunias.
  
  

Nutrients

• The soil petunia plants grow in should contain a fair amount of nutrients. The more nutrients in the soil, the larger the plants and the more numerous the flowers. Fertilizer, such as a 10-10-10 or 8-8-8 fertilizer, should be applied to the soil every three weeks throughout the summer and fall to provide the petunia plant with the necessary nutrients. Potted petunias should be fertilized about every two weeks. Soluble fertilizers provide the best nutrients for the soil.
  
  

Location

• Petunias require a lot of light each day. Soils that are not located in sunny areas will not be able to provide the necessary light that petunias demand. Petunias must have at least five to eight hours of sun each day to survive. Soils in sunny locations will provide the best place for petunias to thrive and grow.
  
  

Considerations

• Poorer soils can also grow petunias, but it is best to amend the soil before planting the flowers. Adding 2 inches of organic matter, whether it is compost, manure, peat moss or even potting soil, over the existing soil will go a long way toward producing healthy plants. The soil should be mixed in to a depth of 10 inches with a tiller.