Effects of Rainwater and Water pH on Plant Growth

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Rainwater is generally an excellent source of irrigation for plants. Plants are not only watered when it actually rains, but rainwater can be collected in buckets and barrels for later use. In fact, commercial rain collection containers have a spigot and hose attached for easy watering. Collecting rainwater and watering your plants with it is a good idea all year long, but stored rain water is especially invaluable during times of drought and water restrictions.

rainwater for plants

Clean Water

Most rainwater is clean and gives plants water free from chemical additives, such as chlorine or salt, found in tap water. And because it’s free to collect rainwater, plants are more likely to be watered more often than in homes where city water can be costly or restricted.

Reduces Salt

Clean rainwater seeps into the soil and washes salt–which is not healthy for plants–deep into the ground away from the roots. As an added bonus, plants are then able to absorb more water, which reduces their overall need for irrigation.

Raises Soil Acidity

Acid rain has a pH level below 5.6 often caused by air pollutants but also by natural sources such as volcanoes. More acidic soil can slow down plant growth and damage foliage. To find out if acid rain is an issue in your geographic region, conduct a search on the Environmental Protection Agency’s website. Then talk to your local county extension office to remedy the acidic soil with fertilizer.

Source: gardenguides.com

Water pH

Pure water has a pH of 7, which is neutral. Higher numbers indicate alkaline water, the greater, the more alkaline. Lower numbers indicate acidity, the lower the pH, the more acid. Plants, each being adapted to some of the wide variety of conditions found on earth, have certain preferences in pH for rainfall and soil conditions. When you measure the pH of your soil, you are actually measuring the pH of the moisture it holds, which is affected by the soil particles and microorganisms. Of course, the pH of the water you apply will affect this also. The pH does have an effect on plant growth.

Nutrient Availability

Many of the elements most used by plants in their growth are less available when the pH is in the acid range, below 6, and many of the micronutrients, those used in small quantities, are less available when the water, or soil, is alkaline. Some plants prefer, even require, acid soils and are not bothered by the lack of nutrients. Other plants will tolerate, but do not require, pH levels above 7, and are adapted to the lack of micronutrients, such as iron and zinc.


One common effect of the application of water with a high pH, or of a high soil pH, is chlorosis, or the yellowing of the leaves in such a way that the leaf veins remain green. Common in gardenias and citrus, this indicates a lack of iron, usually caused by a pH that is too high for the plant. A lack of nitrogen is indicated by leaves that turn yellow evenly, without the green veins.

Soil Microorganisms

The tiny bacteria and other organisms that inhabit the soil, huge numbers of them in a teaspoon of soil, are most active at a pH of 6.3 to 6.8, and so the processes that break down plant waste, such as leaves, are most active at that level. Peat bogs, for example, having a very acid environment, act as preservatives for organic matter.


One of the more unusual effects of water pH is the change of color that can be seen in the blossoms of hydrangeas. Watering with an acid solution (as long as the soil isn’t strongly alkaline) will give blue flowers. Applying alkaline water will cause the bush to bloom pink. Of course, varieties differ also, so if you want pink flowers, for example, it’s best to get a pink variety and then keep the pH high also.

Acid-Loving Plants

One group of plants that needs acid soil (below a pH of 6.0) is the heather family, which includes rhododendrons and azaleas as well as heathers. Some, such as cranberries and blueberries, need soil even more strongly acid, around 5.0. Trying to grow these in an area where water from the faucet tests at a pH of 7.0 or higher will be difficult without amending the soil with a large amount of peat moss or watering with rainwater rather than tap water. If, however, your area has a problem with acid rain, you can try these in your garden.

Neutral-Soil Plants

Many common vegetables and ornamentals prefer soil with a pH of around 6.5 These include spinach, parsnips, dahlias, chrysanthemums, sweet peas and tulips. If your faucet water or soil is acid, or your area has acid rain, you’ll likely notice that plants are stunted, without obvious reasons for their lack of growth. This is because the major nutrients, nitrogen, phosphorous and potassium, are simply unavailable to these plants. Apply lime to the soil–which increases pH–and plant growth should pick up.

Source: gardenguides.com

Ask any successful gardener or farmer and they’ll most likely tell you that rainwater is the best water for plants.  Assuming that your area is free from acid rain, rainwater harvesting for household use and for garden irrigation makes sense.  Rain water is without doubt much better for your plants than highly chlorinated municipal water.  Groundwater can be contaminated by man-made and natural conditions.  Irrigating lawns and plants with ‘hard water’ can result in a build-up of minerals and salts in the topsoil which stunts or in some cases kills the plants. Drier areas of the Lowveld such as Hoedspruit and Phalaborwa have hard water problems.  We have been irrigating the lawn at our camp in Klaserie Private Nature Reserve with hard borehole water so long that patches of lawn have died.  There are some grass species that can tolerate these conditions but the ultimate solution is to switch to rainwater irrigation.  Large quantities of soft rainwater can be collected during the rainy season and stored in water tanks for winter irrigation.

The pH of water has very important implications for all life, including humans.  It is interesting that there are close similarities between how water pH affects plants and how it affects human health.  We should be drinking water that has a pH close to neutral.  Our body fluids should have a slightly alkaline pH (around 7.5) for optimum health.  We also cannot assimilate nutrients when our pH is too acid or too alkaline.  In fact, there is a natural health supplement company that bases it’s philosophy around pH (see Sevenpointfive).  If you get your body’s pH to the optimum level of 7.5, you should be able to heal yourself  of wide range of illnesses.

Contact Water Rhapsody for all your rainwater harvesting, water tank and grey water recycling requirements.  Our products are eco-friendly and carry a guarantee.  Contact us for a free quote and assessment!

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