Basic Fertilization Questions
 

I bought some Miracle Gro Orchid Food, recommended by a seller, and also something I had to pick up on short notice.
I want to be sure to follow the instructions correctly. It says 'For Orchids and other indoor acid loving plants' apply 1/4 teaspoon per gallon, and apply. While it says for outdoor plants, a 1 tablespoon over soil, on the same schedule. So would an orchid kept outside, still in a typical mix such as orchid bark, receive any more than the 1/4 teaspoon? Are there different guidelines for Orchids kept in moss, or fiber?

In the long run, am I right to think I'd be better off with a strong organic mix? And one of my sellers suggests they found a great slow release mix that's better than the Miracle Gro they had been using. Are there advantages/disadvantages to slow release?

Finally, this seems tricky, how to know you have watered enough preceding the fertilizer, and not too much so that fertilizer is not absorbed, and then how to know you have thoroughly fertilized your plant, without burning? (especially with inorganic)

In agriculture people measure how much fertilizer is in a solution by the concentration of nitrogen, in parts per million (ppm.)

A lot of people think orchids do better with more frequent fertilization at a lower concentration. But not everybody has the time to mix fertilizer every time they water, so many people fertilize only once a week, or once every two weeks, or once a month. Some people like to fertilize with a low concentration at every watering (10-50 ppm N.) Others like to fertilize only once a week (50-125 ppm N), or once a month (more), with a higher concentration.

Most (not all) orchids need a lot less fertilizer than most other plants. And few orchids need much fertilizer during the cooler winter period. Some do because they grow all year.

You can learn everything you need to know on the First Rays Web site. Look at the top for Free Information, then the section on fertilizer. Ray provides a nitrogen calculator. You input the nitrogen number from your fertilizer and the desired PPM of N. The calculator tells you how much to add to water to get that number. I don't know off the top of my head what the nitrogen number is in your fertilizer.

Orchids are orchids. Inside or out doesn't matter. I use the same concentration in or out.

Bark, moss, LECA doesn't really make much difference.

So long as the fertilizer provides all the minerals and nutrients, the difference between fertilizers in how your orchids grow is minuscule compared to whether you provide the proper growing conditions. The fertilizer you bought is fine. Pay attention to learning how to provide proper temperatures, light, watering and humidity before worrying about which fertilizer to use. Note many fertilizers don't have any calcium nor magnesium. If the fertilizer lacks calcium or magnesium and the water lacks these you need to use a calcium/magnesium supplement.

Don't water with plain water first. That idea was debunked in commercial agriculture over 75 years ago. You want your plants to take up as much fertilizer as possible. Water dry orchids with your dilute liquid fertilizer. If you water with plain water the roots will soak up plain water and have little capacity for absorbing the fertilizer solution. Your fertilizer solution should be dilute enough not to harm roots, and you shouldn't rely on tricks so you can use a solution that is too concentrated. This is the big reason why people like to use a lower fertilizer concentration but more often.

Most of the larger epiphytes don't have any trouble with fertilizer concentrations of nitrogen around 150ppm. Cloud forest orchids and slipper orchids prefer much less than this. If you go to the Sunset Valley Orchids cultural information page, Fred Clarke tells you how he applies MSU fertilizer to his plants. If your Miracle Gro has a different nitrogen number, you can use Ray's calculator to determine how to mix your fertilizer to get numbers similar to what Fred uses.

Could you clarify where Ray's calculator is?

Fertilizer PPM Calculator

Or, if you just want to be in the ballpark, use the flowing estimates:

For 25 ppm N, 2/%N = teaspoons/gal, or 2.3/%N = ml/L

So if I want a 100 ppm N solution of MSU RO @ 13.2% N, that would be 2/13.2=0.1515. Multiplied by 4 = 0.6 teaspoon (I’d just round down to 1/2), or if you want metric measures, 2.3/13.2=0.1742 x 4 = 0.7 ml/L

It seems like slow release fertilizer would just wash out of an open bark medium long before it had a chance to release its nutrients.
I'm not sure what a "strong organic mix" would be, but you don't want to add anything that would clog up the air spaces in your potting medium.
The Miracle Grow Orchid food (and there are others) is formulated to meet the needs of the plants. I'd use the 1/4 tsp per gallon, weekly. I doubt you'll see any signs of deficiencies in your plants, but if you do you can adjust accordingly at that time.
Thanks ES and Ray for the more detailed explanations, but just following instructions on the orchid food container has actually worked pretty well for me!

What about the amount of application? Using the solution instead of water in that watering including soaking method, or just add enough by spraying to wet the top of the soil?

I am a proponent of flooding the potting medium at each watering, as it flushes, saturates, and aerates all in one step. Over winter, when the plants are indoors, I mostly use a heavy spray on the media surface until drainage starts, which is admittedly less thorough.

We really don't know how much of what we apply is actually absorbed, because we don't know the retention of all potting media, we don't know how well solutions are spread around the pot as the water evaporates, and we don't know the contact area of the root system in relation to those. My recommendations are just the culmination of years of growing and observation, using the techniques I employ.

I did a calculation (AKA - "sheer folly") based upon the chemical reactions that fix carbon and the resulting nitrogen and water requirements, based upon an assumption that a plant gains one pound of mass in four years (Phals, maybe?), and if it was 100% absorbed, we could water 1 pint (16 fluid ounces, or about 570 ml) a week using a 55 ppm solution (assuming about 50 of that was N).

I would water thoroughly each time. Just wetting the surface means dissolved minerals dry on the upper part of the medium.
This shows as powdery white efflorescence, which may damage roots.

Thorough running-out-of-the-pot watering has the additional benefit of pulling fresh air into the root zone, along with flushing out any buildup of fertilizer salts and other crud,