Fertilizer Change During Flower Spike

[Ray]

Originally Posted by Bloomer001

I'm feeding about 125 mg N weekly in 2 doses.

May I ask how you are determining the mass you're applying?

Don't forget that "exposure time", extent of root growth, and velamen volume are other important factors.

Starting at the extreme of a bare-root vanda in a slat basket: their "exposure time" is only during the actual application of the fertilizer solution, and the amount that can be absorbed is limited to the volume of the velamen layer on the roots.

Now go to a cattleya in a pot of coarse bark. The exposure time is the time of application, just like the vanda, but is also extended by the time that roots are in direct contact with solution held within the medium. Bark doesn't wick very well, so once those "contact points" have been extracted, that period ends. The total volume of velamen is probably reduced compared to the vanda, so that reduces the immediate uptake, even while the exposure time is extended.

Then there's an oncidium in sphagnum - the medium holds and wicks the fertilizer solution well, and the plant has lots an lots of much finer roots (and greater overall surface-to-volume ratio) to make contact with it, but the velamen is relatively thin. If we assume the velamen-to-vasculum transfer rate is similar for the three, it seems likely the oncidium will take up more of what is applied.