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What is the best water for our garden soil and plants?

“Water is perhaps the most amazing compound on Earth. Able to dissolve most substances if given sufficient time, water is a carrier of geological information, a medium of communication between biological systems, and a vital component of every living cell.”

— Nigel Palmer, “The Regenerative Grower’s Guide to Garden Amendments,” 2020

Today it was 90 degrees outside and I procrastinated on writing in favor of watering plants on my patio with the garden hose.

I enjoyed gazing into the rainbow I made by spraying with my back to the sun, contemplating just how amazing, and precious, water is for healthy plants and healthy people.

According to Palmer, however, not all water is equal for growing optimal food crops. He says, “Rainwater is the best water to use for watering plants and for making (soil) amendments.” There are several reasons why rainwater is so beneficial.

Rainwater doesn’t contain chlorine, which is used as a disinfectant in municipal tap water. Chlorine can be toxic to plants, as evidenced by leaf edges that looked burnt. Some tap water also contains high levels of sodium bicarbonates that can inhibit plant growth and damage soil structure by preventing soil particles from clumping together.

Palmer says high amounts of dissolved minerals, mostly calcium and magnesium, in tap water increase the water’s “hardness.” Rain water is naturally “soft,” with a hardness value of only 18-20 parts per million (ppm), whereas acceptable hardness values for municipal drinking water can be as high as 170 ppm. Palmer cautions gardeners against using municipal water straight from the tap for watering plants if hardness values exceed 70 ppm.

I did a bit of investigation to find out the hardness value for the tap water in Medford. Our city water comes from Big Butte Springs about 30 miles north of Medford. In summertime when water needs are highest, the supply is supplemented with water from the Rogue River. Both water supplies are considered moderately soft with hardness values that range from 32 to 45 ppm. Sodium content in our tap water varies from 4-6 ppm.

The high quality of our tap water means we don’t have to worry about burning our plants with excess minerals; however, some gardeners store tap water for at least 24 hours to allow the chlorine to dissipate before using it for plants and soil.

Still, rainwater offers benefits that tap water can’t match. For instance, rainwater contains nitrogen from the atmosphere in the form of nitrates that plants can immediately absorb through their leaves and roots. That’s why our garden looks so fresh and green after a rain.

Rainwater also has more oxygen than tap water, and this helps to prevent plants from becoming waterlogged and developing root rot. Carbon dioxide in rainwater lowers the soil pH to slightly acidic — the ideal pH for most of our garden plants. Rainfall triggers the release of micronutrients in the soil — zinc, copper, manganese and iron — so the minerals become available to plants.

Rainwater also does a better job of flushing accumulated salts down through the soil beyond the root zone of our plants where higher levels of sodium have less effect on plant growth.

I know I’m being somewhat of a tease by discussing the benefits of rainwater for our garden during a period of drought. We’ve had only 5.48 inches of rain so far in 2021, which is about 6 inches below the average amount in our area by this time of year. The outlook for rain in the near future looks grim as there is no rain forecast for the next 10 days, and we are quickly moving toward our typical drought period from June through September (recently stretching into October and beyond).

We still may get a bit of rain, though, and Palmer suggests collecting rainwater when we can to make the soil amendments described in his book. (In case you’re wondering, it is legal for homeowners to capture rain from their rooftops in barrels and buckets to water their plants.) I bought a pretty rain chain to direct rainwater from the house gutters into a bucket. Unfortunately, rain hasn’t touched the rain chain yet, but it’s there waiting for its moments of glory.

I’ve been able to capture more rain in Bandon, where we have a rain barrel connected to the gutters affixed to the shelter over our camper. During a downpour, we can fill up the 50-gallon barrel, and then we pump the water to a storage tank by the hoophouse. We’ve been able to water our raised beds this way so far, but we’ll have to switch to delivered municipal water during the dry summer months until we install a pump for our well.

It will be interesting to observe any differences in plant health and plant growth after we transition from using rainwater to municipal or well water. Note to self: Check out hardness values for well water and municipal water in Bandon.

Whatever source we use for watering, practicing water-wise gardening is an important part of sustainable land stewardship. To learn more about water-wise gardening, visit the Medford Water Commission’s website at www.medfordwater.org or register for the OSU Extension Service’s free, online Water-wise Gardening class, presented live from 3-4:30 p.m. Tuesday, May 11, and available as a Zoom recording afterward. Sign up or view the class at https://extension.oregonstate.edu/mg/growing-oregon-gardeners-level-series.

In the meantime, here are a few tips for water-wise gardeners:

• Check irrigation system and garden hoses for leaks.

• Water plants in the early morning to avoid evaporation; water twice a week for longer periods of time, rather than daily applications for short periods of time.

• Place plants close together in raised beds and apply mulch around the root zones.

• Improve garden soil so water and nutrients are more available to plants.

The last tip brings us back to Nigel Palmer’s book about soil amendments and the close connection between what he calls “good water” and healthy soil. I’m still holding out hope that at least a little bit of that “good water” for my garden might still come directly from the rain.

Rhonda Nowak is a Rogue Valley gardener, teacher and writer. Email her at Rnowak39@gmail.com. For more about gardening, check out her podcasts at https://mailtribune.com/podcasts/the-literary-gardener and her blogs at www.literarygardener.com.