All plants need water. When rainfall does not provide water in the necessary amounts, the gardener must. When to apply water and how much are decisions anyone growing ornamental plants must make.
All soils have air spaces, and it is in the air spaces that soil water is stored.
When water falls on the soil, it moves into the spaces in the soil. The rate at which the water moves into the soil is called the infiltration rate. After rain or watering, gravity pulls water out of the largest air spaces deeper into the soil. The more quickly this happens, the better the soil drainage. Sandy soils have many large air spaces, so they are well drained, but the small spaces in clay make it poorly drained.