FeedMe Fertilizer

[Roberta]

Mine doesn't have a brand on it other than "Sunset Valley Orchids" but the label looks similar to this one"
Amazon.com
Here is the label of mine

[Ray]

@Alan Sailer

The FeedMe fertilizer you purchased is simply Greencare MSU fertilizer - the original - that has been repackaged and relabeled (and repriced exorbitantly, with excessive shipping costs added). It is blended from individual ingredients that are purchased as needed, from different sources, as the market dictates. Sometimes all of the ingredients are quite uniform in size and shape, and sometimes, if the pricing varies, the blend can be quite heterogeneous (Edited due to now being awake enough - thanks ES). It was really bad when they first started making it, and it still varies a bit, but has become much better over time.

Because of the heterogeneity you noticed, 1 teaspoon of the dry material can be chemically different than another 1 teaspoon taken from the same batch. That’s one reason I recommend people make their own liquid concentrates - larger “samplings” of the powder will be closer to identical in chemistry, making the concentrates closer from batch to batch (and easier to dispense).

Your TDS meter is waaaaayy off! For a 100 ppm N solution (a reasonable concentration for weekly feeding), that particular fertilizer formula requires 0.74g of powder per liter, which is a true TDS of 740 ppm. At the 1 tsp/gal you mixed, the true TDS is around 1500 ppm, not 50.

Don’t worry about the pH of the fertilizer solution, as it plays very little role in the (important) rhizosphere pH. The plant and microbes present control that far more than most realize. If you want to prove that for yourself, follow the “pour through” test described in THIS ARTICLE.

[Dimples]

I have really hard tap water. I add 1/5 to 1/4 tap water to my RO and fertilizer. The minerals in the tap help prevent the extreme pH swing from adding fertilizer and also add a little more Ca and Mg. I’ve found all RO + Dyna Gro FP will have a pH nearing 3. I agree with Ray, that the pH in a non-soil environment isn’t as critical, but I still don’t like the idea of pouring vinegar-level acidic solution on their roots, and I also use the same fertilizer solution for my other house plants in a traditional soil-less media. Adding a little tap water keeps the pH around 6.

[Alan Sailer]

Dimples,

Thanks for the information.

You know what's funny was I was just driving back from an appointment and was thinking that adding tap water (mine is alkaline) to the RO/fertilizer might be a good idea for pH and buffering.

Then I see your reply.

Thanks again.

[estación seca]

The potting material has a huge buffering capacity. Do what Ray suggested - test the pH of the effluent. It's probably fine to use the fertilizer you have.

[Alan Sailer]

Ray,

I did not see your post until now. The information about the true composition of the FeedMe! is valuable.

AS far as homogeneity, making a liquid concentrate sounds like a good idea. I'll try it out.

I am a bit puzzled about the TDS comment. I have not been measuring TDS just pH. I do have an old conductivity meter but so far I have only used it to measure my tap, RO and DI water, not fertilizer concentration.

Am I misunderstanding that part of your post?

seca,

I'll try measuring the pH of the drained water. Science!

Cheers.

[Ray]

The second paragraph of your original post mentioned TDS. I just went and reread it, so I gather it’s the RO with a TDS of 50, not the 1 tsp/gal fertilizer solution.

If that’s the case, either you meter is still way off, or you need to replace the filters and membrane in that system.

Here are some useful links:

fertilizer concentrate calculator

don’t trust your tds meter

TDS & EC

[Alan Sailer]

Ray,

Thanks for the clarification.

The bottom line for me on three issues.

First, I decided to just dissolve the whole mess of FeedMe! into a 4liter container of RO water. I calculate that this means that I need to add ~50ml of this liquid to one gallon of water to get the recommended fertilizer concentration.

Second, I found if I add this concentrate to a mixture of 50/50 RO/tap water I get a pH of about six. I suspect that the tap water will buffer the mixture.

Third, my TDS meter is (of course) an old conductivity meter. I don't know if it is perfect but when I use it on three sources of water I get believable results. The tap water in our area (according to the City) is about 400 TDS which is close to what the meter tells me. My RO water from this fairly hard water (with a few year old RO membrane) gives me ~50. Finally the DI water from where I work (semiconductor processing) gives me about 7 TDS.

These numbers are not gold standard traceable results
but they tell a consistent story.

Incidentally, there is a local professional grower in our area that uses well water. I took some of their water and ran it through my conductivity meter. The TDS was over 800. And they grow nice orchids. I used to wonder at all the salt deposits on their pots but no more. Their only treatment is citric acid for pH.

Everyone,

Thanks for all the input and help. It has clarified a lot of issues, including the fact (already suspected) that I am over thinking this stuff.

Cheers.

[estación seca]

This is a terminology issue. There is no such thing as a TDS meter. There are electrical conductivity meters mislabeled as though they give TDS readings.

To measure total dissolved solids one evaporates a known amount of water and determines the mass of minerals left behind. The usual unit is milligrams per liter, parts per million or something comparable. The measure does not involve moles because the number of atoms of different chemical species is not known. This can't be done by putting a meter into water.

An electrical conductivity meter measures the amount of current transmitted through the water, which can be converted to the total number of charged particles in solution. Some common ions in ground and tap water have one positive charge, and others two. These ions can be chemical species with different masses. Unless you know exactly the minerals in your solution and their proportional amounts, you cannot derive a measure of total dissolved solids from an electrical conductivity measurement. Furthermore, non-charged particles can contribute to total dissolved solids, and they are not measured by conductivity meters at all.

One of these meters can be useful if you already know the electrical conductivity of the desired amount of powdered fertilizer added to the desired amount of water. To get different volumes with the same fertilizer concentration, you just add fertilizer to water until the electrical conductivity (perhaps mislabeled TDS) reads the same. But a conductivity meter cannot provide any useful information about total dissolved solids in a novel water solution.