If you read the sister article about your "Feeding Regimen", you know that I advocate basing that upon the needs of the plant, and to manage that regimen in terms of nutrient formula, feeding frequency, and nutrient concentration. In reality, to gain an even better understanding, what we should be doing is focusing on the mass of nutrients that the plant is receiving. We do so with our own diets - counting calories or grams of carbs - so why not with our plants?
Why not? Because it is complex and very difficult to do well! (In reality, it's practically impossible for a hobby grower with a varied collection of plants, but still worthwhile to understand. I'll explain later.)
Most professionals control the feeding rate by the parts-per-million (ppm) of nitrogen in their nutrient program, having selected a formula that has the right ratio of other ingredients to keep their particular crops healthy and strong. In the poinsettia industry - the largest dollar value of ornamental crop grown in the US (orchids are second) - there is a well-established feeding target of one-half gram of nitrogen per plant, to be fed from the planting of the cutting to the time of "blooming" for the Holidays, a period of about 4 months. But what exactly, does that mean? Simply put, it means that the volume of nutrient solution, multiplied by the concentration of nitrogen in that solution, multiplied by the number of times the plant will be watered over that growth period, will provide one-half gram of nitrogen.
Sounds simple enough, but how do we apply that to our collections?
First, we have some simple "knowns": the fertilizer makeup and how strong we mix it. In my own case, I use the GreenCare Orchid Special for Pure Water (a.k.a., the "MSU" fertilizer), which is 13% nitrogen. I mix it at approximately 3/4 teaspoon per gallon to give me about 125 ppm N. As a part-per-million is equivalent to a milligram per kilogram, and a liter of water is one kilogram in mass, I know one liter of my solution contains approximately 125 mg (125 thousandths) of a gram of nitrogen. OK, great. Now what? Let's look at some of the other variables and less-know entities.
Let's start with the simplified situation of an enclosed, well-controlled, indoor grow room with artificial lighting. In that environment, the light level, temperature, and humidity can be controlled to be very consistent, and that, in turn, means that watering can likely be done on a very regular, periodic basis. Why don't we use every 5 days as our example?
If we pour 500 ml (half a liter, and a bit more than a pint) of our nutrient solution into the pot, we have applied 62.5 milligrams of nitrogen. Doing that every 5 days means that it will take us almost 7 months to apply that same one-half gram of nitrogen as the poinsettia growers. However, what is applied and what is retained for the plant to use are totally different.
In the case of the poinsettia, the grower likely uses a peat-based growing medium, and waters it just to the point of it saturating the medium and starting to drain. The medium soaks up the nutrient solution like a sponge, so the liquid retention is very high, and the fine, hair-like roots of the plant essentially fill the pot, allowing them to absorb a large percentage of what is in the medium. For an orchid grower, the amount of moisture retention in the medium is far less, and the plants' roots are far more sparsely distributed in the pot, meaning that the amount absorbed by the plant is a mere fraction of the amount applied. Even in the case of semi-hydroponics, where that amount can be more easily determined, it's tough.
A 4.5" S/H pot of coarse PrimeAgra, no matter how much we pour into it, will retain between 125- and 150 grams of liquid, absorbed in the medium and held in the reservoir. That means that we have about 17 mg of nitrogen available for the plant to absorb. If we water every 5 days, as in our example above, it takes about 5 months to have the half-gram available for the plant. But how much is actually absorbed by the plant? In the case of the peat-potted poinsettia, we can assume a very high percentage due to the medium and root structure, but in orchids...who knows?
To make matters more complex, unlike the poinsettia grower, all of our plants are not likely in the same medium and in the same size pots - if they're in pots at all! Further, as we tend to grow a variety of plants under many different conditions, there is little data as to the true needs of any of them, which is totally different from mass-markets ones. The fact that we are typically under the influence of seasonal changes to our conditions magnifies the complexity.
So does this mean we should just give up? ABSOLUTELY NOT! While we may not be able to pinpoint the values well, there are several good concepts that we can use to improve our culture:
Armed with those concepts, we can establish a "base" feeding program, then through observation of plant growth, medium drying rates, and the like, tailor that regimen to optimize the regimen for individual plants.