Fertilizing rainwater - Orchid Board

malteseproverb · #1 07-12-2017, 03:19 PM

I've finally achieved my dream of having a rain barrel (yay) but now I'm wondering if I should be altering my fertilizing regimen in any way now that I'm using rainwater. I normally don't fuss over fertilizer: my approach has been to fertilize as little as possible and avoid it altogether if I'm lazy, but that was an attitude borne from dealing with somewhat hard tap water. Now that I'm going through the trouble of hauling gallons of rainwater from my yard to my tub on the second floor, I want to make sure I'm not missing anything here.

Here is my current fert schedule:
1/8 teaspoon Dyna Gro (7-8-6) + 1oz liquid kelp (0-0-1) once per week. I water twice per week for most of my plants now that it's summer so generally speaking I fertilize every other time I water (disregarding my mounts which are watered every day). According to Ray's TDS calculator I'm using about 12 ppm N and around 30 ppm TDS. I drop the kelp in August/Sept and I barely fertilize at all in winter, maybe once a month.

I'm worried about the amount of calcium I'm providing now that I've shirked my tap. Dyna Gro has 2% Ca and 0.5% Mg but I'm using it at 1/4 the strength recommended on the label. Should I use a Cal-Mag supplement? Should I purchase a fert that's formulated for rainwater? Is ph even something to worry about??

Maybe there is no right answer but let me know if anything leaps out at you as a bad practice. Thanks!

Ray · #2 07-12-2017, 04:50 PM

Personally, I'd use either MSU RO or K-Lite.

Dyna-Grow fertilizers can cause huge swings in the pH of a pure, unbuffered water supply. The fertilizers I mentioned will buffer it to a degree sufficient for application.

Orchid Whisperer · #3 07-12-2017, 05:48 PM

Concrete is chemically similar to limestone (calcium carbobate), and you can buffer the rainwater by storing it with pieces of concrete before using. I found an article a while back that mentioned a lack of calcium deficiency symptoms after using water stored in concrete. Fairly sure it was a Sue Bottom article on the St. Augustine Orchid Society website. I will post a link if I can find it.

Found the article later: here you go! http://staugorchidsociety.org/PDF/20...ps-Calcium.pdf

You can use other forms of calcium carbonate (limestone, eggshell, oyster or clam shells). Gypsum, or plaster of Paris, also provides calcium (as sulfate) without raising the pH as much.

malteseproverb · #4 07-13-2017, 12:42 PM

Hmm since I don't have the option of building a cement cistern, do you think adding a cal-mag supplement or epsom salts would create enough of a buffer that I could continue using dyna-gro safely? I literally just bought more this weekend. I was really excited to be able to use pure water on my plants but now it looks like I might be putting them in harm's way...

Orchid Whisperer · #5 07-13-2017, 06:00 PM

I looked up the chemicals in one brand of cal mag, Botanicare, it seems to be sulfates an nitrates of calcium and magnesium. So, a few pennys worth of Epsom salts and calcium nitrate maybe? Toss in a quart bottle, charge $18 for it. The nitrate and sulfate are not compatible in the same solution, so you are probably losing some calcium sulfate (gypsum) out of solution.

Having any solids in the rainwater probably does change/improve the buffering capacity somewhat but I am not sure how much.

Dollythehun · #6 07-13-2017, 07:16 PM

I crush eggshells and add them to the medium and occasionally use cal mag at 1/4 strength.

malteseproverb · #7 07-14-2017, 05:07 PM

Interesting! Thank you all for the info. I'm not sure if I want to risk playing around with the rainwater as I still might end up frying my plants with acid. I might just use tap water when I fertilize and flush with rainwater right after. As it is, I like to run a lot of water through the plants and get them thoroughly wet, so I usually apply fertilizer first and flush after with plain water. I could still use the cal-mag or epsom salts though. I would consider MSU but it seems like a pain to store and use. And there's a frightening thread on GardenWeb about it that I can't get out of my head.

Roberta · #8 07-14-2017, 07:10 PM

Quote:

Originally Posted by malteseproverb

Interesting! Thank you all for the info. I'm not sure if I want to risk playing around with the rainwater as I still might end up frying my plants with acid. I might just use tap water when I fertilize and flush with rainwater right after. As it is, I like to run a lot of water through the plants and get them thoroughly wet, so I usually apply fertilizer first and flush after with plain water. I could still use the cal-mag or epsom salts though. I would consider MSU but it seems like a pain to store and use. And there's a frightening thread on GardenWeb about it that I can't get out of my head.

You might want to invest in an inexpensive pH meter (online or your local hydroponics store) I just got an RO system, was concerned that I might have an issue with my existing fertilizer (since I won't have an easy chance to get the MSU pure-water stuff for a few weeks) I mixed up my regular brew (the RO water is about 20 PPM TDS as opposed to the 300 I start with... another meter worth having). I used plain old Miracid fertilizer, and behold, the pH was a very nice 6.8. (To get that pH with tap water I have to add about a tablespoon per gallon of vinegar, anything in the fertilizer to acidify the water is no match for all that calcium carbonate) So... depending on your water, it might actually be fine. (If you live in an area that gets acid rain, your results may be different...) You really can't know what you are working with unless you test. If you don't want to buy a meter, pH test paper is even cheaper (though not as accurate) Knowledge is power...

Orchid Whisperer · #9 07-14-2017, 07:43 PM

Quote:

Originally Posted by malteseproverb

Interesting! Thank you all for the info. I'm not sure if I want to risk playing around with the rainwater as I still might end up frying my plants with acid. I might just use tap water when I fertilize and flush with rainwater right after. As it is, I like to run a lot of water through the plants and get them thoroughly wet, so I usually apply fertilizer first and flush after with plain water. I could still use the cal-mag or epsom salts though. I would consider MSU but it seems like a pain to store and use. And there's a frightening thread on GardenWeb about it that I can't get out of my head.

I am not sure about Philly's water quality, but can't imagine it is sufficiently bad that you would need to flush it out of the medium right away. Actually, you can probably buffer your rainwater a lot if you just mix a certain amount of tap water with the rain water. Tap water is almost always buffered and pH adjusted to 7 or 8 before it gets into the water mains (the Flint, MI water quality problems were largely due to improper pH management before the water entered the pipes, resulting in lead dissolving from pipes into the tap water).

You can look up the water quality for just about all USA water treatment utilities online (water quality is part of my professional work). The water department should be able to tell you important things about your water, such as electrical conductivity (EC), total dissolved solids (TDS), pH, hardness, and alkalinity. The units of measurement are just as important to write down as the number. Anything that they don't automatically report, you can probably ask through a phone call or e-mail. Once you know the numbers, and then tell us what kind of orchids you are growing, we may be able to tell you if you tap water is a problem or OK for orchids.

Roberta · #10 07-14-2017, 07:58 PM

Actually, orchids are pretty amazing in their ability to adapt. Some of the Plwurothallids, Sophronitis, and highland Dendrobiums like cuthbertsonii are fussy about the water (which why I finally broke down and got RO) But a LOT of orchids just don't care - like Catts (except for Sophronitis), Cymbidiums, most Oncidiums, most Dendrobiums... they put up with my hard pH 8 water just fine. Best to be able to test your own water... the water company will give you ranges that can be next to useless (does it help to know that my TDS ranges from 150 ppm - pretty decent - to 500 ppm - fairly bad)? And may be different in a different part of town. Measure it... and know that especially the TDS changes from day to day. Mix up the fertilizer, test the pH, and there's no guesswork.