pH range and TDS limits

[smweaver]

For all you chemistry-inclined people, I'd like to ask for your views regarding what pH range and total dissolved solids (TDS) values you attempt to maintain for the water that you apply to your orchids. I just got a pH meter and a TDS meter and have been playing around with them this afternoon. I've discovered that the pH and TDS values of my water supply (after adding fertilizer) are, respectively, 6.5 and 128 PPM. Do these seem like decent values for the requirements of most orchids? I have a mixed species collection, but the majority of plants are in the cattleya, cymbidium, lycaste and stanhopea genuses. Any feedback you care to provide will be much appreciated. Thank you!

Steve

[Ray]

A pH of 6.5 is on the upper end of the range I shoot for (5.5-6.5) and should be fine.

128 ppm TDS is probably low, but there are all sorts of reading that could make it inaccurate.

What is the water supply, and how much of what formula fertilizer do you use?

[smweaver]

Ray, the water supply's from an RO unit. I've been putting approximately half a teaspoon of fertilizer (usually whatever brand happens to be on hand out in the garage, but nothing with urea in it) in a five-gallon bucket of the water. But up until this point in time I didn't have either the pH or TDS meter to take any readings. I'll try to increase the TDS reading a bit. Thank you for your response!

Steve

[Ray]

You didn't answer my question completely (no fertilizer formula), but we can "scrape by" on what you did share:

While not entirely correct, for practical purposes we can assume the RO water has no dissolved solids, therefore a TDS of zero. That means that all of the solids measured were from the fertilizer.

I recommend feeding at 125 ppm N (not the TDS, as you measured), so I think you're starving your plants. An easy way to achieve that strength is to dived 10 by the %N on the fertilizer label (hence the reason I inquired about the formula) to get the teaspoons per gallon needed.

As a point of reference, I use the MSU RO formula, which is 13% N. For 125 ppm N, 10/13=0.76 - or about 3/4 teaspoon per gallon.

Using the label, I need 3.55 grams of fertilizer in a gallon of solution for that concentration. That's - on a gross level - 3.55g / 3785g of water, or 938 ppm TDS. In reality, however, not all of the mass of a water-soluble fertilizer contributes to the "dissolved solids", so the true reading will be lower.

Then there's the issue that TDS meters are just cheap electrical conductivity meters with a built-in conversion, so their output, while good as a reference point to see if you're being consistent, should never be used as an absolute measurement. TDS and EC

[smweaver]

It's a 20-20-20 formula fertilizer, not specifically designed for RO water (at least it doesn't say so on the package). The only source of nitrogen (according to the label) is from ammonia. Using the information you provided (and thank you for the link to your site concerning TDS meters), I'll increase the amount of fertilizer to get the correct 125 PPM of nitrogren. Thanks, Ray!

Steve

[Ray]

Steve,

I use the 125 ppm target at every watering, as my plants are mostly in s/h culture. In "regular" culture with bark- or CHC-based media, or moss, I recommend that you use the fertilizer solution about every-other watering or three out of four during active growth.

[mojomick]

Oops. I have been using MSU for well water at 125ppm at each watering, but not all of my plants are sh. I have about 25 plants in bark media. Should I be using a different ratio for them? Betty

[Royal]

Just to jump in here...

Betty, the concentration is fine. I use the same - MSU for well water (from First Ray's) at about 125 ppm at every watering. I only have a few in s/h, everything else is mounted or potted in a conventional orchid medium. I think what Ray suggested here is that every fourth watering it is good to use plain water to flush out or rinse away any salt build up. So, once a month or so I really soak em' with plain water.

Ray, Did I get that right? Do you ever flush your plants in s/h?

[mojomick]

Thanks Royal, I do flush all my plants with plain water about once per month. I was just concerned with the concentration for the bark based vs. the sh. I will continue to do as I have been unless otherwise informed. Betty

[gixrj18]

I don't use MSN, but I keep my TDS around 1400ppm. Some people will think this is high, but as long as you flush them real good...they love it!