Fertilizer and Mag/Ca Questions

[Christine1227]

Hi! A bit of a newbie here, but have lurked for a very long time. I'm unsure where to post this question, so my apologies if this is section is incorrect!

I have 3 phals, a vanda, an oncidium, a mini cattleya, a den phal, and a cymbidium in my current collection. My question is regarding fertilizer and mag/ca. I water with tap and understand that tap water generally contains enough mag/ca for plants to survive on. However, do people general supplement additional mag/ca, even if they water with tap?

To piggyback off of that, if I was to purchase the MSU pure water fertilizer (13-3-15 8Ca 2Mg), AND use tap, would it still be suggested to get a mag/ca supplement?

Thanks!

[PaphMadMan]

If you're using the MSU 13-3-15 8Ca 2Mg you do not need any other cal/mag supplementation, with or without tap water. In most areas you can get water test information from the municipal water authority online if you look for it, or if you get a water bill there may be instructions on how to get test results.

[Rothrock42]

I use MSU-type and tap water, but I live in Portland, Oregon and our water is mostly snow-melt and run off and contains very little mineral/salts content.

I've got a simple dissolved solids meter from the local hydroponic store. It is marked as +- 50 ppm and out of the tap it always says zero. It would be interesting to know what your water is starting off as.

Depending upon how I mix it up I get numbers between 100 and 200 after adding the MSU-type and some pH down (more acid).

For my vandaceous orchids I do use a different fertilizer, but add a little of the MSU-Type to get some calcium in there for them while they are in active growth.

I'd be interested to hear what the experts say.

[wintergirl]

Personally I do not fertilize much but all my plants have ground up egg shells in the media and many have added mag once a month, especially my vandas and catts. I use both well and rain water and reg fert and MSU when I do it.

[Orchid Whisperer]

Rothrock, I would not put too much faith in inexpensive TDS meters. If you paid less than about $250 for it, or if you can't calibrate it, the data from it is unlikely to be reliable. Best to ask for data from your local water department.

[PaphMadMan]

In defense of cheap TDS meters, they can be pretty good. Since periodic regulatory testing of bottled water for TDS is part of my job I have access to plenty of samples of known TDS. I know the freebie meter I got with a water filter pitcher is more than adequate. But in general you just don't know, and few people have the opportunity I do to check thoroughly.

But almost anyone in the US has access to water supply test records online, so it doesn't take much effort to find them.

[estación seca]

Welcome to the Orchid Board!

South Florida is well-known for having extremely alkaline tap and well water that is high in dissolved minerals, especially calcium. Some things I have read say it is even worse for orchids than is my mineral-laden, alkaline desert water. People in Florida certainly use it on their orchids when it hasn't rained in a while, but I suspect most orchids there get rain water most of the time.

I would suggest you go to your water utility's Web site and find their most recent water quality report. If your total dissolved solids are over about 250 parts per million, consider using reverse osmosis water, or collecting rain.

There are two different kinds of MSU fertilizer, for pure water and for tap/well water. It is not a good idea to interchange them. The fertilizer for tap/well water has chemicals added to lower the pH of the final dilution to something closer to what orchids like. The fertilizer for pure water does not have as many pH lowering compounds. If you use alkaline tap water, the pH lowering ability of the MSU fertilizer formulation intended for tap/well water will be much better for your orchids than the pure water blend. If you use the blend intended for tap/well water with pure water, the pH may be far too acid to be ideal for orchids.

If you're willing to do some math, and you know how much calcium and magnesium are in your tap water from reading your water quality report, you can mix your tap water with pure water to get something with an acceptable level of total dissolved solids for orchids, yet not need to buy a calcium-magnesium supplement. This kind of blend will be closer to pure water than tap/well water, so I would use the pure water MSU formulation if you do this. This is the method I use to make sure my plants get enough calcium and magnesium.

By the way, using Epsom salts from your drug store is the cheapest way to supplement magnesium.

[Orchid Whisperer]

Originally Posted by PaphMadMan

In defense of cheap TDS meters, they can be pretty good. Since periodic regulatory testing of bottled water for TDS is part of my job I have access to plenty of samples of known TDS. I know the freebie meter I got with a water filter pitcher is more than adequate. But in general you just don't know, and few people have the opportunity I do to check thoroughly.

But almost anyone in the US has access to water supply test records online, so it doesn't take much effort to find them.

We'll have to "agree to disagree" regarding the inexpensive TDS meters.

I have about 30 years experience in environmental science, focusing on water quality. My experience is that any TDS value obtained from any meter (even an excellent meter) should be treated as an estimate only. Reduce the quality of the meter, remove the ability to calibrate or correct for temperature and that estimate can get pretty dodgy.

Any TDS value from any meter is in fact only an estimate, based on electrical conductivity. The number you get can double (or be cut in half) depending on the types of solids you have in solution. If non-ionic solids are present, the TDS meter will not detect them at all (fortunately most natural waters have mostly ionic solids, but there are exceptions). The following link explains TDS meters and their limitations pretty effectively: Measuring Total Dissolved Solids (TDS) With a TDS Meter -

This next link goes into a bit more detail:http://www.tau.ac.il/~chemlaba/Files...e_EN%20(2).pdf Note that for two meter-measured TDS values to be comparable, the ionic composition of the samples need to be similar. Start adding fertilizer, Epsom salts, kelp extract, etc. to the water, and the ionic composition of the water will change a lot vs. water without these additives. To get somewhat accurate/comparable TDS readings for a set of natural waters (all from the same river, for example), the values need to be corrected to a standard or a laboratory-determined gravimetric TDS value (based on evaporating a filtered residue and weighing the solids that are left behind).

My concern in Rothrock's case is that his meter is indicating his tap water TDS is always zero. That is extremely unlikely, given the number of chemicals added to water during treatment and transport of the water to consumers through pipes that likely are lined with some form of mineral deposit or sediment. it leads me think that his meter is probably significantly underestimating his TDS.

[Ray]

I agree with OW on this one. A cheap TDS meter is a great way to monitor changes in the dissolved solids content, but the absolute values given are of little value.

I disagree with the ES interpretation of the differences between the MSU formulas: Nothing is added to the "Well Water" formula to drive the pH down. That happens naturally when you throw typical fertilizer salts into unbuffered water. The "RO" formula has those same chemicals in it, but also contains significant levels of calcium and magnesium compounds, which counteracts that to some degree, in effect, providing some buffering.

The Well Water formula has been changed, adding a small amount of calcium, reducing the effect.

[PaphMadMan]

I didn't make a general statement that cheap TDS meters are good, I said the one I have checked is good (> 0.99 correlation for 20 samples 0 - 660ppm, with < 10% coefficient of variation relative to standard method, and almost perfect reproducibility day to day), so it is possible. And I think that many of them would be fine for this purpose if you could check them against some known samples and come up with a conversion factor. Known samples could be as simple as RO water, and a couple carefully made concentrations of table salt or your fertilizer.