One area of orchid culture that has always been difficult to quantify is nutrient uptake.
Sure, we can mix up a certain concentration of fertilizer solution, and we often discuss “ppm N” as the control, but how do we know how much of the nutrients they actually absorb? Let’s start by looking at plants for which it is known, potted nursery stock like poinsettias.
It is well established that in order to raise a poinsettia from rooted cutting to mature plant for Christmas sales, the grower must supply 1 gram of nitrogen as part of their feeding regimen. Terrestrial plants, because they have such fine, hairy roots, grow to occupy essentially 100% of the container volume. The grower knows the volume of the containers, knows the liquid retention of the potting medium, the time until the desired maturity is to be reached, and the formula of their fertilizer. They can then use those factors to calculate the concentration and frequency of feeding that will allow the plant to get that one gram of nitrogen. For example, if the grower adds one liter of a 125 ppm N (125 mg/L) solution to a 1L container of medium that retains 50% of the solution, then 500 ml of the solution will be retained and available to the plant, so we can estimate that the plant will get 62.5 mg of nitrogen per feeding. That simply doesn’t work for orchids, due to the media used, if any, and the form and structure of the root system.
When we water, most of the solution just pours through the potting medium. “A bit” is absorbed by the plant and the potting medium, but we must understand that, due to the structure of an orchid’s root system, nutrients held in portions of the medium that are not in direct contact with the roots will not be absorbed. In other words, it is pretty much only the solution that comes into direct contact with the velamen that carries the nutrients to the plant.
Think of a vanda, which is often grown in slat baskets with no medium, with the roots dangling below. For them, feeding only occurs at the instant the solution hits them and saturates the velamen. That has led to the concept of “instantaneous uptake”, and we can calculate the mass of nutrients absorbed by making a few assumptions that lead to calculating the total volume of the velamen on the root system. The calculator below does the calculations: