[Andrew]

As JLu implied, this topic is a big can of worms. While solid scientific studies of nutrient uptake have been carried out in non-orchid species, most of the info about orchid nutrient requirements is a mixture of anecdotes and studies by hobbests and horticulturists that, to be honest, lack experimental and statistical rigour (it could be worse, you should see some of the BS science that gets touted as fact in aquarium circles).

There are two schools of thought, one is that if you feed continuously with a single complete fertiliser that will supply sufficient nitrogen, phosphorus and potassium (why do you guys never discuss potassium?) the plant will use what it wants, grow strongly and flower of its own accord. This also has the benefit of making a mixed collection easier to fertilise.

The second school of thought is that seasonal flowering orchids have distinct nutrient requirements during different growth stages. "Green" growth is a nitrogen hungry process due to the heavy nitrogen demand in producing chlorophyll. Thus, when the plant is actively growing leaves increasing nitrogen will meet this nitrogen demand. During flowering phosphorus and potassium are supposedly in more demand so the proportional concentration of these macronutrients are increased. Phosphorus is utilised in nucleic acids, phopholipids etc. It's utilised in leaf, root and flower growth both structurally (DNA, cell walls) as well as in energy production. Why bloom fertilisers contain so much of it is beyond me. Presumably, it's a throw back to flowering annuals etc where flowers are produced on mass at a high energy cost to the plant. From both the scientific literature on typical perennials as well as those who have tried to look at orchid response to nutrient availability (as mentioned above, not by the most scientific means), phosphorous requirements are pretty low and grossly increased phosphorus contents in bloom boosters probably says more about the inexpensiveness of phosphorus than plant requirements. Be aware that mycorrhizal fungi are often sensitive to phosphates. If you are growing anything even mildly myco-heterotrophic (doesn't apply to most orchid growers) or if you are using a fungal agent like trichoderma (or worm tea for that matter) you'll want to take care that you're not oversupplying phosphate to the detriment of the fungi. The often overlooked macronutrient is potassium. It's a major transport and channelling ion in plants. I don't know why it's rarely discussed in orchid nutrition because its effects of flowering, fruiting and disease resistance (Cymbidiums flower in the cooler, more stressful months) are well documented for other plants. Most orchid bloom boosters in Australia have increased potassium compared to growth formulas, of course we don't used the oxidised states of nutrients to express NPK's so it may be more obvious when reading the fertiliser packets. To summarise the growth/bloom fertiliser method probably has less to do with starving or pushing growth as it has to do with meeting demand with minimal waste, although some orchids (eg Dendrobium nobile) do respond to nutrient starvation.

As for which method is better, take your pick. Both approaches have merits and I'll still happily listen to both the one and two fertiliser parties. Personally, I changed to using only one fertiliser for my Cymbidiums a few years ago and I'm now going back to two because I just haven't been getting the count and quality that I used to get (low K or high N?). Do orchids receive this kind of seasonal shift in nutrient availability in the wild? Most likely not but if you've ever seen orchids in the wild you'll find 99% are scrappy, sorry excuses for plants that have no place on the show bench.