This can be a particularly complex subject, and there are more opinions than there are growers out there, but I believe that some scientific evaluation of plants' conditions in the wild can help simplify it. The following is my "take" on the subject, but keep in mind that everything (and I do mean everything) is not "black and white", but a matter of degree:
Foliar versus Root Feeding:
- Because many orchids have evolved to be very conservative with their water, they have developed thick, waxy, cuticle layers on their leaves to retard water loss. It's particularly evident in phalaenopsis, for example. Those layers tend to be very water repellent, so would prevent the rapid absorption of the aqueous solutions.
- The areas of the leaf surface that DO absorb are closely associated with stomata (which don't absorb liquids), but again, as a water conservation adaptation, they tend to be on the undersides of leaves, which tend to see less solution exposure.
- Orchids vary all over the map, so some plants may absorb better than others.
Water First, Then Feed:
- In nature, epiphytes attached to trees and shrubs basically only get fed only when it rains, and the exudates and collected detritus gets washed down to them out of the canopy.
- That happen almost immediately upon the commencement of rain, then it's nothing but pure water after that. The velamen on the roots has been shown to be particularly good at trapping the nutrient ions immediately, and holding onto them, so they won't be washed away by the continuing downpour - another evolutionary adaptation for their "niche" lives.
- If you water first, you saturate the velamen, and the plant can no longer absorb the nutrients from the later-applied solutions as well.
Urea versus Non-Urea Nitrogen Sources: There is a lot of discussion as to whether urea may be directly absorbed by orchid plants, and the "common knowledge" is that it must be decomposed into ammonium compounds for absorption, but that's not strictly true.
- Nitrates and ammonium compounds are poorly absorbed through the leaves, but are preferentially absorbed through the roots.
- Urea has just the opposite affinity, which is why many "Green Up" products sold for lawns are loaded with urea.
Formula & Concentration:
- Orchids are about 85% water, 14% carbon and nitrogen, and 1% everything else, combined.
- Analyses of the "throughfall" and "trunk flow" of water cascading from the leaf canopy and down branched to epiphytes has been shown to be <25 ppm total dissolved solids (TDS), with almost all of that being nitrogen.
- There is no such thing as a "bloom booster". A plant is genetically programmed to bloom to its maximum potential, and a well-grown plant will do so. The best we can try to attain is to not do something that detracts from that.
- Rotating fertilizer formulas - unless you have poor ones - is a waste of time. That plant in nature sees a nearly identical "diet" at every feeding for its entire life.
So summing all of that up, I have adopted the following regimen:
- Use a high-nitrogen fertilizer formula. I like K-Lite (12-1-1-10 Ca-3 Mg)
- Make sure the formula is complete, with all of the trace elements.
- Apply it at a very low concentration (I use 25 ppm N) at every watering.
- Select a potting medium and container that allows frequent watering without suffocating the roots.
- Flood the pot at each watering. Not only is it irrigation and feeding, but it aerates the pot and flushes away fertilizer residues and plant waste products.
I have been growing orchids for over 40 years, and my regimen evolved to that about 5-6 years ago, and I have never seen my plants do better.
Originally Posted by raybark
Another thing to consider is the importance of watering, as that is well above feeding in the plants' "Maslow's heirarchy" of needs.
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