Keeping the soil moist is one of the biggest problems in container gardening. When soil loses its moisture, your plant can wither or die easily. A trick in keeping the soil moist is with the use of a coir liner. This is a natural fiber made from the husk of ripe coconuts. It is a renewable resource with a pH that is ideal for planting. It is commonly found and used in hanging baskets or planters to provide moisture to soil. What is does is hold a great deal of water, which it later on releases when your plant’s roots become too dry.

Source : How to keeping the soil moist in the container gardening.