Moisture

• Petunias require moist, well-drained soil. Iowa State University Extension suggests adding compost or other organic matter to the soil to improve drainage. Other options include planting petunias in flower pots or raised beds.
  
  

Composition

• Petunias do not like extremes when it comes to soil composition. Very sandy and heavy clay soils result in poor growth in petunias. Organic matter corrects this problem as well, according to the National Gardening Association.
  
  

Nutrients

• Petunias need nutrient-rich soil for optimal growth. Monthly fertilization with a balanced fertilizer provides the needed components. Trailing petunias perform best and develop the deepest color when given an additional nitrogen boost with iron chelate spray.