Calcium and magnesium supplements?

[Papillon24601]

Oh! Secondary question! Does this count as fertilizer for the orchids we are winter-resting?

[Orchid Whisperer]

Originally Posted by Ray

But gypsum isn't very water soluble. That's fine, sitting in soil where it can slowly dissociate, but in the transient exposure of orchid watering, it's not going to be as effective as other products.

Ray learned his chemistry at Georgia Tech, so he's forgiven for not knowing that gypsum is moderately soluble in water (roughly 2 cc/liter, or more, at room temperature, close to 2 tsp/gallon).

I dissolve a teaspoon of gypsum (for lawns and gardens) and a teaspoon of Epsom salts in a gallon of water. I add several ounces of this solution (maybe 8 oz., I don't measure precisely) per gallon of water when I'm providing Ca and Mg. Seems to work well for me, and it is cheap too. BTW, because plaster of Paris is gypsum with the water driven off by heat, you can use plaster of Paris instead of lawn gypsum.

For some orchids that are strong calciphiles, I incorporate chunks of broken concrete and eggshell into the potting medium as well. Bletia patula is a species that has responded really well for me after these additions to the medium.

[Ray]

OK. You got me (and my apparently failing memory).

[Orchid Whisperer]

Originally Posted by Ray

OK. You got me (and my apparently failing memory).

I've lost my memory too. One of these days I'll remember where I put it.

[estación seca]

Originally Posted by Papillon24601

Oh! Secondary question! Does this count as fertilizer for the orchids we are winter-resting?

When people speak of withholding fertilizer during the winter, they are speaking mainly of nitrogen. But, plants that aren't growing don't use many minerals, either, so it isn't necessary to give calcium nor magnesium during winter unless deficiency is suspected.

[Ray]

If you know you have a pure water source, you need to add stuff, but you're right about the demand being very low.

If you think about the true scale of things, it helps: a plant is about 95% water, and about 4% carbon, which it gets primarily from the air. The remaining 1% is ALL of the rest of the minerals.

For a plant to gain one pound of mass therefore, it must take up and process a mere 4.5 grams of nutrients. Then if you consider that a complete, powdered fertilizer might be what? 25% nutrient ions, an weight maybe 5 grams per teaspoon, that means that takes somewhere in the range of 3- to 4 teaspoons of fertilizer over the time it gains that pound.

So how long does that take? That depends on the plant. A little masedvallia might not gain a full pound in a lifetime. A standard cymbidium may take a couple or three growths to do that, so what? 12-18 months?