Since the rain water is usually collected from the roofs of houses, it picks up very little contamination when it falls. You’ll of course want to keep your roof clean of debris and potential contaminants to maximize purity. The material your roof is made of is also important in how much contamination the water will carry (see Safe Rainwater Harvesting Catchments). The chemicals and hard water from many of our municipal water systems can produce an imbalance in the soil of your garden. Chemical fertilizers, fungicides, pesticides, and drought can also disrupt the balance and harmony of the soil. This imbalance causes trees and plants to weaken and makes them more susceptible to disease.
Trees and plants have an efficient immune system that allows them to fend off diseases and other invaders as long as they have a healthy soil environment and aren’t stressed by other factors such as drought. Trees and plants rely on fungus, bacteria, and nematodes to help them absorb the minerals and nutrients they need. Trees and plants depend on a fungal root system called mycorrhizae. Mycorrhizae attaches itself to tree and plant root hairs and extends the root hair system.
Mycorrhizae uses some of the plant’s energy, but provides the plant with minerals it can’t otherwise absorb. In healthy soil, the mycorrhizae of one tree connects with mycorrhizae of other similar trees. When you look at your garden, visualize it as a vast interconnected community of trees, plants and tiny critters that live in the soil, all interacting and affecting each other. Thus, the type of water you use in your garden will affect the health of this intricate community.
And speaking of community, one of the best reasons to start harvesting rainwater with rain barrels is that if you teach and encourage others to do the same, you will help to spread the culture of rainwater collection and in turn help your larger community and the environment. It is always important to remember that every living thing on the planet needs water to survive so we as humans must expand our idea of community to the plants and animals that surround us.