Most gardeners notice that their gardens look especially good after a rain, even if they have been routinely irrigated with tap water. Indeed, many gardeners regularly gather rainwater and save it to water their gardens. Rainwater is especially good for plants in a variety of ways, including a relatively low pH level.
Alkalinity and pH
Although gardeners often talk about alkalinity and pH as though they were the same thing, this is not accurate. The pH scale measures the concentration of hydrogen ions in liquids such as rainwater or tap water. Liquids below 7.0 on the scale are called acidic; liquids above 7.0 are called basic or alkaline. This is not the same as alkalinity, which is determined by the carbonates, hydroxides, and bicarbonates in the water as measured in parts per million. The alkalinity level indicates the ability of the water to neutralize acidity. High alkalinity slows plant growth.
Pure water has a pH of 7.0, or neutral. Rainwater is usually mildly acidic, with a pH of 5.5 to 6.0. It also has very low alkalinity. This low pH makes it easier for plants to absorb nutrients from the soil and encourages the activity of beneficial microorganisms in the soil.
Other Benefits to Plants
Rainwater is free of the additives present in tap water, such as chlorine and fluoride. It will not contribute to the buildup of salts in the soil, which is bad for plant roots, and it often forces salts already present to a deeper level, well below the root systems. Rainwater is also generally free of contaminants that accumulate in lakes and rivers, such as pesticide and fertilizer residue.
Rainwater and Conservation
In some areas, roughly half of the water consumption in each household is used to water the garden. Saving rainwater for the garden reduces pressure on municipal water supplies. Using rainwater instead of tap water can save the home gardener some money, too. In many urban and suburban areas, storm water runoff is a real problem. Collecting rainwater helps reduce storm water runoff.
Acid rain occurs when sulfur dioxide and/or nitrogen oxides combine with water and oxygen in the air. It reaches the ground in the form of rain, snow, or other precipitation. Although the very high acidic levels in this precipitation is harmful to plants, even greater damage is caused by soil contamination, especially when heavy metals are carried along. Gardeners who live in areas with frequent acid rain should have their rainwater tested before saving it for irrigation.