When Watering The Garden Rainwater Has Many Benefits Over Tap Water. One Of The Most Surprising Is The Level Of Major Plant Nutrients To Be Found in Rainwater. Nitrogen, Phosphorus, Potassium, Calcium, Magnesium and Sodium Have All Been Detected In Rainwater. But that’s not all, there are many other benefits.
Tap water is quite variable, depending on the source from which is it collected. For example, water collected from chalky areas will be quite hard and contain a lot of dissolved calcium. This can be problematic for acid loving plants such as azaleas and rhododendrons. Rainwater as it comes from the sky it’s extremely soft and it’s largely free of calcium (though a very small amount may be dissolved in it).
Rainwater is also free of the chemicals used in treatment plants and of the pharmaceuticals that are frequently found in tap water.
2. Pharmaceutical and Chemical Risks
In the a report entitled, Risk assessment of chemical contaminant in drinking water, 2014 Public Health England said traces of six pharmaceutical compounds had been found in drinking water: benzoylecgonine, caffeine, the painkillers ibuprofen and naproxen; carbamazapine, used in treating epilepsy, and its metabolised form carbamazapine epoxide.
Tap water also goes through a process called “Chloramination”. This is where chemicals are added to make the water safe to drink. Chlorine and sometimes other chemicals are added to make it safe to drink. Obviously rainwater isn’t treated in this way!
3. pH Levels: Acidity v Alkalinity
Because it contains small amount of nitrates rainwater is naturally very slightly acidic. The pH of tapwater is artificially controlled. As Scottish Water says on its website, “The pH level in your water supply must be controlled so it doesn’t corrode the metal distribution pipes by being too acidic, or leave deposits on the pipes if it’s too alkaline.”
Grey water from kitchen and laundry equipment was reflect the underlying pH of it’s mains source PLUS any additives such as detergents. Thus the pH is often as high as pH 10, which can be injurious to many plants if used in volume.
4. Organic Matter
When rainwater is collected from our roof and subsequently stored in our garden water tanks it contains some organic matter. This is because our roof tiles have moss on them and some of this moss is dislodged by rain and collected with the rainwater. Large pieces of moss are easily filtered out through a coarse strainer and cause no problems in the garden.
Other debris including bird droppings, leaf litter, dust and pollen are also present on the roof and get collected in the rain water. I’ve never studied the microbiology of collected rainwater but it must be teaming this life. When we use our stored rainwater to water our plants we are adding a light application of fertiliser and abundant life.
5. Nitrate Levels in Rainwater
The most bio-available form of nitrogen are nitrates, the form of nitrogen most readily available to plants. Nitrogen is vital for plant growth, especially leaves and stems. Plants are able to absorb the nitrates in rainwater through their roots.
6. Major Plant Nutrients in Rainwater
Research by Allen, Carlisle, White and Evans, at the UK’s Nature Conservancy, measured the total quantity of major plant nutrients in collected rainwater over a three-year period at five sites in Britain. The following figures are reported
N (nitrogen) 8.7-19.0 kg/ha/yr,
P (phosphorus) 0.2-1.0, kg/ha/yr,
K (potassium) 2.8-5.4 kg/ha/yr,
Ca (calcium) 6.5-24.0 kg/ha/yr,
Mg (magnesium) 2.9-6.1 kg/ha/yr
Na (sodium) 14.0-51.0.