Water quality and PPM

[Longroots]

I didn't plan to grow my collection into more sensative orchids but I have and I've been gathering a small amount of rainwater from a place it gathers in my garden and filtering it through coffee paper to remove debris.

I have a water butt but I so rarely use it I worry that the water in there now is not good and there will be a lot of dissolved salts and other things from the rod of my house. The roof has a fair amount of moss on it so water has travelled past and through parts I'm worried will increase the PPM of the water I gather. I cannot afford the space or the cost of a RO system, so please do not suggest this.

Primarily I want to know if simply emptying the water butt and letting it refill will be fine for the plants I have or will have (Masdevallia Measuresiana and Dracula Lotax). And the suggested PPM to have for them when using rainmix fertalizer. I'd also like some recommendations for decently priced TDS measurers for my water in the UK.

[Ray]

I think you’d be fine just collecting and using the rainwater, as-is. Moss on the roof is unlikely to add dissolved mineral to the water.

[estación seca]

Let a cup of collected water evaporate in a vessel with a narrow base. If there's no visible residue it's probably OK.

[Longroots]

I use rainmix for fertalizer, I don't have a PPM meter and have read that only decent ones can read accurately without callibration. What ones do people use, or in the knowledge of me using good rainwater, what strength should I use per litre.

Rainmix recommend 0.5g per 1 litre, so I was thinking a quarter of this per litre for my Dracula/Masdevallia. Thoughts?

[Shadeflower]

Give the old rain butt a scrub out if you are worried. There will be a lot of leaves in there but eventually they will just turn to mud and the water won't be much different to pond water. I know emptying out 250 liters, then climbing in there and cleaning it is not something I'd particularly want to do

Maybe you can drain all the water, tilt it so all drains out and use a pressure sprayer to wash any remaining gunk out of the tap.

When it comes to measuring my conclusion is that the cheap ph meters are completely unreliable and you need to spend over 40 pounds on a ph meter. No question about it but when it comes to the tds... well it doesn't need to be that accurate.
I have used an £80 tds meter and budget ones. Yes the budget ones might not be calibrated as well but they give an indication of what the water contains. Then when you add fertilizer they are accurate enough to not need an expensive one.

For arguments sake my rainwater is 30ppm on an accurate meter and 60ppm on the budget one. Ok woa, completely off, throw it in the bin?

Well, no, add your fertilizer and the accurate one will read 210 ppm. The budget one for arguments sake will read 250ppm - it might not be 100% accurate but for the price difference that is completely good enough.

When it comes to the ph that is a completely different matter, the budget ones can be over 1 ph off and that is not acceptable at all.

This is the one I use, they last a good 3 years and either need the battery replacing or for the price I usually just get a new one. Probably been using this model for a good 10 years, I'm happy with the accuracy or innacuracy of it.

Bestgle Digital TDS Meter Water Quality TDS ppm Tester for Testing Salt Water Pool Water Purity Monitor: Amazon.co.uk: Garden & Outdoors

[Roberta]

Even a lab-grade pH meter needs fairly frequent calibration. You likely will be better off with pH paper or test strips. They won't necessarily give you the last decimal point, but they're not so subject to gross errors.

[Longroots]

I have a hot tub so I have a variety of pH testing tools so thats not too bad, including strips. Perhaps I'll use them for pH rather than a digital pen. Thank you for recommendations

[Ray]

TDS meters are merely (and usually cheap) EC meters to which an arbitrary “conversion factor” is built into the circuitry. More often than not it’s standardized to NaCl content in distilled water, although sometimes they come with a “500” or “700” setting, whatever that means.

Nonetheless, if you think about the physics of conductivity - the travel of charged ions in the solution - it is pretty obvious that, due to their sizes and charges, Na+ and Cl- ions move at different rates than do natural- and fertilizer-derived ions, so it’s pretty clear that they will not give you an accurate “TDS” value.

I had two such meters, and at 100 ppm N MSU RO, the true fertilizer-contributed TDS is 740 ppm, but my two meters told me the solution was 395 or 480 ppm.

To answer Longroots’ question about how much “Rainmix” to use, divide 2.6 by the %N in the fertilizer formula, and the result is ml/L for 25 ppm N.

In S/H culture, that would be my recommendation for every watering. For traditional cultural methods, it depends upon the frequency of feeding, but I think about 100 ppm N per-week is reasonable - that is, use 100 ppm N if you feed weekly, 200 if you feed every two week, 50 if you feed 2x/week, etc.

[claypot]

Don't worry about water here in the UK. I use tap water regularly with no ill effects. As for rainmix, they give you a small plastic spoon to measure the product. Add one of those to 10 ltrs of water and it works well. Too many people get hung up on TDS meters - I am sure the Good Lord does not measure TDS when the wild plants get watered.

[Orchidtinkerer]

claypot, that is actually what I do too, one spoon per bucket which for me is I think 8 liters but the instructions tell you to add 5 spoons per bucket so it isn't as straightforward as following the instructions.

Once one does add a spoon per bucket the ppm should always come out roughly the same and the ppm meter should only confirm this so once one knows how much to feed the ppm meter is not really needed but for how little they cost I like confirming my 1 spoon comes out roughly right.