The reasons in which plants benefit from rainwater are vast and at times complicated, to the point where one could write a dissertation on the subject. Seemingly, the basics are simple, but if you are the curious type this article may leave you with more questions than answers.
Hopefully, not too many questions.
Nonetheless, here are my top three reasons why plants prefer rainwater, and (some) of the science behind why this is.
Side note: Many people complain about the apparent differences they experience in tap water from city to city and state to state in taste and hardness level. Logically, it follows that if tap water is different from state to state the rainwater is too, even without considering human involvement. Not only does rain water change from place to place, it changes from shower to shower, and season to season in the SAME place (1). So, when you go through my list below just keep this in mind.
1. Rainwater deposits vital nutrients
Rainwater itself is a chemical cocktail that includes particles from local origins as well as particles that are transferred from elsewhere by the wind. To dive into some chemistry for just a moment, technically, “rainwater is a mixed electrolyte that contains varying amounts of major and minor ions” (1). The list of these ions is very long, but some of them are none other than potassium, calcium, nitrogen, magnesium, nitrate, nitrite, and iron (2).
This is partly where we get the term “water hardness” from. Water hardness is describing the amount of dissolved minerals (specifically magnesium and calcium) that are found in water.
Tap water is, generally, more hard than rainwater. In efforts to reduce tap water hardness salts are occasionally added. Salts can be found in many things, including fertilizers. However, with repeated watering from tap water that has been “softened” salt can accumulate in the soil and negatively affect the soil structure and decrease the plant’s ability to take in water through osmosis. On its own, hard water is ok for plants; however, the amount of magnesium and calcium in hard water can begin to exceed what is a useful amount.
Nitrogen, is a vital nutrient for plants and is taken up by plants through nitrogen fixation. Nitrates and Nitrites are limited in our tap water due to concerns with diseases. In addition, chlorine and fluoride are added to tap water, among other things, to give tap water a higher PH. A high PH can reduce the plants ability to take up nutrients in the soil, but more on that later.
2. Rainwater is more acidic.
Pure water or H2O has a PH level of 7.0 which is neutral. Anything higher than that will be more basic, anything lower more acidic. Rainwater, generally, has a PH of about 5.6, so it’s on the acidic side (3).
The reasons behind this can be found by looking up. The natural presence of Carbon Dioxide, Nitric Oxide, and Sulfur Dioxide found in the lowest layer of our atmosphere reacts with water in the air resulting in a more acidic compound. “Acid rain” is the result of industrial activity pumping out these gases at excessive rates and lowering the PH to harmful levels of 3.0 or below (3).
Austin soils are, generally, high on the PH scale due to the high level of limestone. An alkaline soil can be detrimental to plant growth because it doesn’t allow the nutrients to be in their most available state. “Availability” in this case is not referring to the quantity or amount of a specific nutrient found in the soil, but if that nutrient is in a chemical form that is accessible to the plant. The ideal PH range for a soil to receive the most nutrients is if it’s in a slightly acidic or neutral state. Thusly, rain’s more acidic nature helps lower PH, at least temporarily, and gives plants the ability to take in more nutrients than tap water.
3. Rainwater (usually) penetrates deeper and distributes more evenly.
Last but not least, rain is beneficial because it’s free! It falls from the sky over large areas, usually watering your entire yard for the same amount of time at the same rate, if we assume we are dealing with a flat plot of land with no elevation change or obstructions of course. Drip irrigation, while beneficial for saving water, can suffer from leaks, or human error may result in plants not being watered very deeply. When a plant is watered often, but not deeply, the roots are trained to remain at the surface and not search downward for water. If you forget to water, or we suffer from a drought that prevents you from watering, the plant is at risk of dying because the roots won’t be deep enough to collect water below the soil’s surface.
I hope that I’ve answered some of your burning questions on why rain water is preferred over tap water and if nothing else peaked your interest to go and explore the subject more. So, this month as we move into our flood season you can at least stay positive that your plants will enjoy the rain a little more than you will.
(1) Carroll, Dorothy, 1962, Rainwater as a Chemical Agent of Geologic Processes-A Review, United Staes Government Printing Office, Washington, G-2 p.
(2) Hutchinson, G. E., 1957, A treatise on limnology, v. 1, Geography, physics, and chemistry: New York, John Wiley and Sons, Inc., 1015 p.
(3) Casiday Rachel and Frey Regina, Acid Rain: Inorganic Reactions Experiment, Department of Chemistry, Washington University, Missouri, 1998. http://www.chemistry.wustl.edu/~edudev/LabTutorials/Water/FreshWater/acidrain.html