Diatomaceous earth for calcium?
 

HI!
I have a couple of catts that are purpling-blotching-black striping on pseudobulbs: looking Ca+2 deficient by my google-search-diagnosis.

I have a bunch of Diatomaceous Earth i'd bought for different purposes before and that's supposed to be 19% Ca. It's pretty fine ground so i guess would maybe be like a suspension wash? Has anyone tried using it for calcium source? I'd trid gypsum top dressing in the past as well, for the large part kinda washes out and makes a brown mess, but maybe they don't need that much?

Thanks in advance

Calcium in various animal shells is almost completely insoluble in water, except in acidic solutions. That's why limestone exists. Limestone consists of layer after layer of those diatom shells. They have been sitting there for millions of years, after spending many years on the bottom of seas. Adding any kind of shell to plant medium will not provide enough extra calcium to matter to the plants.

Calcium quickly binds to many ions commonly found in water, and forms almost-insoluble salts. That is why people use commercial calcium supplements, and mix them in pure water (rain, distilled, reverse osmosis.) They are in forms in which the calcium is soluble, so long as you don't dissolve them in low-pH water that already has a lot of insoluble calcium compounds.

You can look up your water quality on the Web page of your local water supplier. Look for their Annual Water Quality Report. It will show whether there is much calcium in your water. If there is any, acidifying your tap water to the 5.5-6.5 range will make plenty of calcium available to your plants. I don't recall where Seattle gets its water; if it's from wells, there is a good chance your have plenty of calcium available.

Edit: Here's the 2023 Seattle water quality report. Unfortunately and regrettably, it only provides information about toxins, and not the things important to gardeners. You could telephone them. Ask what is the pH, total dissolved solids, and calcium content. They measure those but didn't report them.

Last quarter report is between 9210 and 9600 micrograms/L. I’m not quite sure where that falls on amount scale or what to do with that info?
I think Seattle supply is mostly snow melt and rain water ?

9210-9600 mcg/L of calcium? That's 9.2-9.6 mg/L or around 9.5 parts per million. That is very little. You can use your tap water for your orchids, with proper fertilizing and supplementation.

But to supplement Ca for the catts? Would the tap be enough without supplementation? Given that they started making those changes in pseudobulbs and leaves within last year I thought meant they needed more?

Can you post a picture of the issue?

Most nutrient minerals are easily moved from older tissues to newly growing ones. Calcium, once absorbed, is not mobile within the plant, so must be supplied regularly.

The primary symptom of calcium deficincy is the death of new growths, not spotting or streaking elsewhere on the plant.

With your tap water you either need to use something including calcium and magnesium, like an MSU formula, or you need to alternate a fertilizer with a calcium/magnesium supplement. Don't mix fertilizers with calcium/magnesium supplements at the same watering, because those two minerals might precipitate out and not be available.

Ray has a lot of information on fertilizing plants in the free information section of his Web site. Check it out.

And I agree with Ray that calcium deficiency in Cattleyas manifests as blackening and dying back of new shoots.