Food For Thought
 

...Apparently I don't have as much of a green thumb with thinner rooted orchids, i.e. oncidium alliance (all in bark). I've been feeding at a quarter-strength biweekly with Grow More 20-10-20 (urea free FWIW). I suspect frying roots... Interestingly, I don't have this issue with miltoniopses (but probably because most of mine are not in bark). Do you always pre water these guys regardless of how much you dilute? If so, how long do you wait?

I generally don't pre water (for a quarter strength feed) when it comes to paphs and phals in bark and they do just fine.

I had trouble with the thin root Oncidiums too - especially in bark. Turns out I wasn't watering enough. I feed mine pretty heavily especially when they are actively growing.

Could this be the problem for you too?

P.S. I never pre-water

Hmm...That was on my differential as a number of my thin rooted (and younger) oncidiums are in medium grade bark in particular. Somehow I'm inclined to water them less frequently (1x/week) as opposed to my miltoniopses in medium or fine bark. The oncidiums I have tend to get a slight curl at the end of their leaves very shortly after watering, so I attributed it to overwatering and root frying from what I saw of surface roots.

Hi. I almost always pre water my orchids before applying fertiliser or anything else. I rather be safe than sorry. My oncidiums are all budding now though they dont bud so easily...

If you do not 'pre-water', then you are fertilizing, but not watering. That's fine, as long as you do not mistake that feeding for a watering.

In my shared greenhouwe, we water (drench) twice a week After every 4th watering, we follow up with feeding. Nov-Feb we cut back a little on watering, and scale b ack on feeding as well.

I'm suspicious that nitrogen from nitrate can burn root tips if it's too strong. I noticed when I used Orchid Focus, which it 100% nitrate nitrogen, it burned the root tips on my vandas. I started using a lower dose and I didn't have the problem anymore. I do water my orchids before fertilizing but I don't know if that's necessary. It's just the way I've always done it.

For what it's worth, I always water before applying fertilizer. Fert on dry roots feels like it would be salt on a wound - to me, anyways.

I never pre-water. Ever. The way I see it, the plant is only going to take up so much of whatever you give it. If I pre-water before fertilizing, I'm not really fertilizing and I have therefore lost control of the nutrition, I plan to give the plant.

Food For Thought
 

I agree with Blackvine's pre-watering before fertilizing behavior. It is a habit I learned from reading labels on fertilizer products. Some say "pre-soak plants before applying."

Not everything you read on the web is true. For every advice you find to support your point of view, there is always other advice to the contrary. Here is advice from Garden Web, that espouses my opinion:

"Dry roots are slow to absorb so that is why watering ahead is good. I water, then go back around with the fert in a pump sprayer and spray any visible roots and the medium. I don't spray any leaves on purpose. Any time on the morning you water would be fine.

Orchids are slow-growing plants and don't need much. Some are considered "heavy feeders" but that is relative, nothing like what a fast growing tomato plant, for example, would need.

For the average, non-commercial / non-expert grower, watering your orchids before you fertilize is an excellent idea, and while it's not a vital piece of care for your plants, it's something you should strive to do. Water heavily and thoroughly immediately before feeding. Run plenty of water through the pot, so that several volumes of water have run out through the drainage holes. Give the mix and the roots plenty of time to get soaked and absorb the moisture. Then feed.

The justification? Well, orchid roots are very sensitive to environmental conditions in general, and they are especially sensitive to salts and minerals (resulting from fertilizer) that tend to build up in your pots over time. As stated, orchids just aren't heavy feeders in comparison to many other plants, and even though you are advised to feed at 1/4 to 1/2 strength of whatever the fertilizer directions say, your plant is still not going to be able to absorb all of the food.

So, it will build up and could threaten your orchid. Watering heavily and thoroughly before fertilizing helps to flush out any salts remaining in the pot. And, it can help act as a bit of a buffer if you happen to mix up a batch of fertilizer that's a bit too strong, since whatever you add to the pot will now be further diluted. Plus, pre-soaked roots are better at absorbing fertilizer."

Sorry, Matt. While that sounds like good advice, I believe that quote is written from the perspective of a terrestrial plant expert, intent on rationalizing his recommendations related to orchids. I do agree with the "heavy feeder" is a relative term, but it's still based upon an incorrect order of magnitude assumption. And that last statement about pre-wetted roots absorbing better is absurd.

If you think about orchid plants in nature, their water supplies are pure (rain, dew/fog), and the nutrition cascading out of the forest canopy is 1) very dilute - on the order of 15-25 ppm TDS - and 2) is only present for the first few seconds after a downpour begins, becoming essentially pure again almost immediately.

As an evolutionary adaptation, the velamen on the roots has the unique ability to become very rapidly saturated, and instantaneously trap and hold ionic species, so they won't be washed away by the continued rainfall.

Prewatering saturates the velamen, and any ions in the water supply will occupy the capture sites, both of which preclude the capture and uptake of any subsequent fertilizer application. Granted, if that fertilizer solution is concentrated enough, there will likely be some diffusion into the saturated velamen, but what the hell are we doing applying that strong of a fertilizer in the first place??? Not only is it wasteful, but it's bad for the plants and the environment, as the runoff ends up in the soil and water supply.

I believe a better approach - and one I've been employing for the last few years - is to start with a pure water supply, add a very small amount of fertilizer (I use 25 ppm N in RO), and use that solution at every watering - for me, about every other day in summer, a bit less in winter. I am seeing much better growth, better-looking plants with more growths, and fewer incidents of disease.

My fertilizer concentration is still 4x-5x what the plants would see in nature, but still a lot closer to what the plants have evolved to.