One way to check for adequate drainage is to dig a hole 10 inches deep and fill it with water. After it drains, refill it with water. If the water drains in 8 to 10 hours, the site is adequately drained for most flowers. The amount of air in the soil depends on the type of soil, soil compaction, and how quickly water drains from the soil. Clay soils normally have poor drainage and aeration but excellent water retention.
Sandy soils have good drainage and aeration but retain little water.
The addition of organic matter such as pine bark or composted yard waste will enhance soil aeration, water movement into the soil, and drainage of clay soils. It will also improve water retention in sandy soils. Be careful when selecting leaf mold, making certain that the material is fully composted and not merely aged. Peat moss, sand, hardwood bark, wood chips, and pine straw are not recommended. Organic matter content must be increased to 25 percent, by volume, to be effective as a soil conditioner.