Research proves rainwater is better for your garden than tap water

To provide safe, clean drinking water to the public, water treatment plants in Australia use physical and chemical processes to removing solid particles and potentially harmful pathogens from the water supply. After cleansing, fluoride is added to strengthen tooth enamel and prevent tooth decay and, depending on the source, the water might also be treated to reduce the hardness. 

By the time mains water is delivered to your tap, it is clean and safe to drink. It does however contain fluoride and residual chlorine, minerals, and coagulants, and while these are not considered to be harmful to health, what about the impact on your plants if you use tap water in the garden? 

Nitrogen

Nitrogen is essential for plant growth and is a major component of chlorophyll, the green pigment in plants that absorbs sunlight and uses its energy to produce sugars from carbon dioxide and water. If plants are deficient in nitrogen, they might look yellowish, but prolonged nitrogen deficiency will cause plants to wither and die. 

Australian soils are notoriously poor in nutrients and nitrogen is no exception.

While nitrogen makes up about 78% of the earth’s atmosphere, plants cannot access it directly as it requires too much energy to convert it into a form they can use. Instead, plants must rely on a process called ‘nitrogen fixation’ by which nitrogen in the air is converted into ammonia or related nitrogenous compounds by microorganisms in the soil.  

Plants can also get their nitrogen from high-energy processes in the atmosphere, like solar radiation and lightning. The enormous energy that lightning generates is enough to break down and convert atmospheric nitrogen into nitrogen compounds. When these mix with oxygen and water in the atmosphere, the resulting rainfall will contain greater levels of nitrates and ammonium. 

A summer thunderstorm will not only give your plants a good watering, but it will also give them a good top-up of nitrogen. 

pH

Gardeners will know that most plants prefer a slightly acidic soil, and that pH levels between 5.5 and 6.5 are perfect for nutrient absorption. Depending on where it is sourced, tap water is generally more alkaline, and the pH is too high for optimum plant health.  

Rainwater is your friend. As it falls through the atmosphere, each raindrop absorbs carbon dioxide. When carbon dioxide in rain combines with other atmospheric minerals, the drops become slightly acidic. As soon as these slightly acidic raindrops reach the soil, they facilitate the release of micro-nutrients like copper, manganese, iron, and zinc (which are important in plant growth) from the soil.  

Naturally soft

Tap water is often ‘hard’ and contains various minerals and salts that could build up in the soil and create unfavourable conditions for the plant uptake of nutrients and water. Rainwater is free from minerals and salts and is naturally soft.