Fertilizer and Mag/Ca Questions

[estación seca]

I think the point is not to use MSU formulated for one kind of water in the other kind of water or pH may be undesirable.

[Rothrock42]

I agree that the meter probably isn't terribly accurate, but it is better than I can do by sight or taste. Mine does say that it corrects for temperature and does have a calibration function (although I haven't done that.)

Our city's website reports 3 to 8 ppm and 80 ppm if/when they use supplemental well water.

I use it as a baseline. It tells me when I did something stupid like put two scoops in when I meant to put in only one (or the other way around) and I'm hoping it will tell me if something starts to go wrong with my pipes or if the city has started using supplemental wells.

I look at it more from a Bayesian perspective -- the prior is the city's website and the cheap meter reading is a small amount of evidence and the happy orchids are a bigger amount of evidence in support of the theory that Portland has relatively pure and suitable water for growing orchids.

[wintergirl]

I bought a cheap TDS meter off ebay and it read rain water as 0 and my well water was the same reading I got from the lab report when my well was dug.

[Ray]

Originally Posted by estación seca

I think the point is not to use MSU formulated for one kind of water in the other kind of water or pH may be undesirable.

First, if you use the MSURO formula in water already containing minerals, it might not have any significant impact on the pH at all (depending on just how much-, and what's already in it). Using MSUWW formula in pure water may result in low pH, agreed.

However, the pH of the applied solution has far less impact on the pH of the rhizosphere than we might think, as the potting medium, the microorganisms that live in it, and plant also play huge roles in that. Try this:

Water and feed your plants normally, so the potting medium is saturated, then wait about 30 minutes or more for the pots to completely drain. Then, trickle a small amount of pure (distilled, RO, etc.) water over the surface of the medium - just enough to allow you to collect the small amount of "pour through" to test. That gives you the best assessment of the medium pH.

Do the same, but trickle a bit of your fertilizer solution through, and the result will be very close, if not identical. That's why I stopped bothering to test my solution, unless trying a new additive.

What will also shock you (it did, me) is that testing a variety of plants in the same medium can give you a wide range of results.