Regarding MSU fertilizer and deposits on medium

[Twelve]

I'm relatively new to the hobby and I've been fertilizing my orchids with the MSU fertilizer from repotme.com. I believe it's 13-3-15 8Ca-2Mg. The recommended ratio of fertilizer is 3/4 teaspoon per gallon of water and I was doing this for 3 weeks then on the 4th I would leech the medium with water to remove all the salt deposits. It seemed like it was working fairly well I didn't really have any noticeable burn on any leaves or roots from the fertilizer but there was a lot of salt deposits on the top of the medium and this concerned me so I cut back to fertilizing every other week. I'm still noticing salt deposits on the top of the medium after just one week of fertilizing so I'm wondering if maybe I'm adding too much fertilizer to the water? Any insight is greatly appreciated!

[Roberta]

That fertilizer formula is designed for pure water - rainwater, or reverse osmosis or deionized. (It probably is OK if you live on one of the areas with wonderful tap water such as New York City or San Francisco) If you are using it on tap water that has significant solids, you may be creating a problem. There is another formulation for well water/city water, that doesn't have the Ca-Mg part. Also, what is the pH of your fertilizer solution? Starting with pure water, this MSU formulation will give you the near-ideal pH of somewhere around 5.5-6.2 . If you use it on very alkaline tap water, it isn't designed to adjust the pH downward and you can get unwanted deposits. So you need to look at the rest of the picture to figure out what you're actually putting on the plants.

[Twelve]

I'm using distilled water, would I need the other MSU fertilizer for use with that? I am unsure of what the pH of my solution is but I'll check that out as soon as I can. So ideally I want the pH of my solution to be 5.5-6.2 and if it is too high or too low I can just adjust the amount of fertilizer until it's in that range?

[Roberta]

That MSU formula should be fine with distilled water. 3/4 teaspoon per gallon once a week shouldn't be leaving deposits (I use 1/2 teaspoon per gallon except on the Catasetinae in the summer when they get 1 teaspoon per gallon and don't have any deposit build-up) So something else may be going on . Ray is the one who is really knowledgeable about the fine points of fertilizers and their use. Hopefully he'll jump on on this one.

[Twelve]

I think I'm making the mistake of letting the orchids soak in the solution a little bit rather than letting it run through it. Wasn't 100% sure if that was the issue or if I was fertilizing too frequently with too much fertilizer but it makes more sense now that I know the ratio is correct. Yikes I hope I didn't damage the root systems too bad. Is it also true that it's better to water first before fertilizing?

[Roberta]

I fertilize once a week (if that much), with a pump sprayer (so I am really not putting that much fertilizer solution on each plant.) But especially in summer, I'm watering every day or every 2 days (depending on which plants) so any excess is certainly being washed away. Orchids don't need very much fertilizer since they grow slowly. (The growing Catasetinae get a bit more because this is their rapid-growth time, they would be happy with much more than I'm giving them) but they also would want both fertilizer solution and water running through the pot, not sitting in it.

[Ray]

While 3/4 tsp/gal (~130 ppm N) weekly is a little high (in my opinion), it's certainly not outrageous or harmful.

The bark has been absorbing minerals since the first time you fed the plant, and they don't all come out when you flush. If the bark dries, the minerals will precipitate.

I think the better way to handle it is, rather than using the same concentration less frequently, apply a lower concentration more frequently.

[Twelve]

Thank you both for all of your help and insight!!