Plants, H2O Connoisseurs: Why Rainwater is the Gourmet Option

The previous two A Nourishing Harvest articles explored practical ways to reduce the synthetic load of rain barrel water, making that resource safer for application on plants we will eat. Though I mentioned the water-saving benefits of rain barrels, this article takes a deeper look into the benefits of rainwater for the plants themselves.

 When it comes to healthy-looking plants, many gardeners have observed that all the tap water in the world can't compare to a couple of good inches of rainfall. According to Joshua Siskin, garden writer for the Los Angeles Times, "Rainwater collection is not just about saving water but about storing up a reserve of high-quality water for irrigation" (2017). 

So what's so different about rainwater, compared to what comes out of the tap?

Beautiful native Solomon's Seal (Polygonatum biflorum) loves rich, moist woodland soils.

Benefits of Rain Water for Plants

Folks who have shopped for an organic or conventional garden fertilizer have seen three big numbers on the bag, which represent the ratio of nitrogen to phosphorus to potassium (N:P:K) in the product. When it comes to supporting sturdy plants and healthy, lush foliage, nitrogen is key.  Nitrogen makes up part of chlorophyll molecules, which create food for the plant through photosynthesis and give plants their green color  (Mas). Soil is not the only source of nitrogen for plants. Air is 78 percent nitrogen, and some of it-- in its ammonium and nitrate forms-- falls with the rain. Plants absorb this nitrogen through their leaves and roots (Siskin).

Carbon dioxide is also dissolved by rainwater, forming carbonic acid. This is partly responsible for making rainwater somewhat acidic, as discussed in this article (USEPA). The naturally acidic pH of rainwater helps to increase the bioavailability of zinc, iron, copper, and manganese--nutrients essential to plant growth that are less bioavailable in naturally alkaline to neutral soils (Siskin). Of course, the pH of acid rain caused by industrial pollution falls low enough to be harmful to plants (USEPA).

What about when plants get too much rain, and flooding occurs?  Gardeners know that waterlogging can suffocate a plant's roots and encourage undesirable anaerobic soil conditions and bacteria. Compared to tap water, rainwater offers an interesting safety buffer in this situation: a higher oxygen content. Anyone remember that scene in Hook where Robin Williams (playing Peter Pan) is drowning, and a rainbow of mermaids comes along to breathe air into his lungs? I think of all that dissolved oxygen like little rescue mermaids, helping to prevent root rot and keep soil bacteria a bit happier until conditions change.

Bloodroot (Sanguinaria canadensis), another lover of moist woodland soil, offers one of the first springtime blooms.

The Downside of Tap Water for Plants

Tap water has some characteristics that plants simply aren't adapted to. Some tap water here and there is fine for many species, but watering primarily with tap water risks the following elements accumulating in the soil. This is particularly a concern for plants grown in containers.

Chlorine is used in many US water supplies as a disinfectant, and fluorine is often added to prevent tooth decay. According to Joshua Siskin,

Nearly all plants...are susceptible to chlorine toxicity, usually expressed in burnt leaf margins. Indoor plants such as Dracaenas and spider plants (Chlorophytum comosum), pines, yuccas, and fruit trees, in particular, are subject to fluoride toxicity as well, with symptoms ranging from burnt, discolored, or spotted leaves to stressed fruit that may become diseased (2017).

"Hard" tap water, which is higher in magnesium and calcium, can damage pipes, and sodium is added to some water supplies as a softener. Softeners may reduce the amount of lead from pipes dissolved into the water, which has its benefits for human health, too (World Health Organization). However, like chlorine and fluoride, sodium is detrimental to many plants. (This is why armies have salted enemy farm fields at various points in history). Plant species are categorized into two broad groups based on their ability to tolerate saline. Most cultivated species are glycophytes, which are salt-sensitive plants that are intolerant of long exposure--even to mild salinity (Pardo).

When assessing the quality of soil, organic farmers and gardeners look for soil aggregation. Rather than being fine and dusty--easily washed or blown away--soil rich in organic matter clumps into tiny balls, or aggregates. This structure aerates the soil, helps to prevent drought by holding moisture, and slowly releases nutrients. Interestingly, sodium is damaging to this soil structure, dissolving these aggregates. Tap water is more likely to result in soil that appears cracked and dry, rather than rich and moist (Siskin).

A benefit of rain for in-ground plants is that it leaches salts downward, past the root zone.  It washes grime from air pollution and mineral deposits from tap water off of the leaves, improving a plant's ability to photosynthesize--just like wiping our glasses clean improves our ability to see.

The simple act of collecting rain for your garden also serves your community as a whole. Check out this EPA article to learn how your rain barrel helps to prevent water pollution, improve climate resiliency, and more.

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Sources:

Emmerich, Robert. "Breeding Salt-Tolerant Plants." Oct 10, 2017. https://phys.org/news/2017-10-salt-tolerant.html 

Mas, Joseph. " How Does Nitrogen Help Plants Grow?" PhosLab Blog. May 7, 2013.  www.phoslab.com/how-does-nitrogen-help-plants-grow/ 

Pardo, Jose M. and Francisco J. Quintero.  "Plants and sodium ions: keeping company with the enemy." Genome Biology. 2002; 3(6). www.ncbi.nlm.nih.gov/pmc/articles/PMC139373/ 

Siskin, Joshua. "For Plants, Rain Has Benefits That Tap Water Simply Can’t Deliver."  The Mercury News. February 13, 2017. www.mercurynews.com/2017/02/13/for-plants-rain-has-benefits-that-tap-water-simply-cant-deliver-9/ 

 United States Environmental Protection Agency (USEPA). "What is Acid Rain?" Updated Dec 20, 2019.  https://www.epa.gov/acidrain/what-acid-rain  

World Health Organization.  " Calcium and Magnesium in Drinking-water: Public health Significance." ://apps.who.int/iris/bitstream/handle/10665/43836/9789241563550_eng.pdf;jsessionid=93D7815DB71B42157F577561F5C02BEC?sequence=1