Foliar Feeding
 

Hi, good day. Do we need to aim for pH 5.5 to 6.5 for foliar feedings?

Also, do we also spray the underside of the leaves?

The utility of foliar feeding may be low for some types or orchids. Any orchids with a thick waxy cuticle, like Phalaenopsis or Cattleyas, probably won't get much out of foliar feeding. Maybe it would work better with thin-leaved genera like Oncidium or Ludisia?

I usually assume that most nutrients need to get into the plants through the roots, and apply nutrients accordingly.

This ^

Roots are more efficient for Nitrogen uptake, but leaves aren't too bad, neither. We don't completely know the mechanisms of foliar uptakes of nutrients. But there are some structures which are used for this purpose.

It is difficult to quantify/compare the uptake rate of roots vs leaves. But in one study of Phal. hybrid, N uptake rate by leaves can be up to 30-50% of the uptake rate by root. I would consider foliar uptake to be a significant part of nutrient uptake.

Also uptake is a part of the story. The N absorbed from roots are readily redistributed, but N from leaf uptake get redistributed less.

Susilo, H., Peng, Y.C., Lee, S.C., Chen, Y.C. and Chang, Y.C.A., 2013. The uptake and partitioning of nitrogen in Phalaenopsis Sogo Yukidian ‘V3’as shown by 15N as a tracer. Journal of the American Society for Horticultural Science, 138(3), pp.229-237. (abstract)

I think where OW was going, and definitely the part I agree with, it that the waxy cuticle layers on some plants - especially phals - prevent water from wetting the plant well, and that's going to really quash the likelihood of foliar nutrient uptake.

If the fertilizer can't contact the leaf, you might be right. But isn't it easy to add surfactant? The Phal experiment uses surfactant.

They use isotope labelled N, and painted upper (UL) or bottom (BL) surface of leaves, young (YR) or older roots (OR).

Estimates of N which went into the plant is

UL: 34.1 microgram (c)
BL: 25.2 microgram (c)
YR: 82.8 microgram (a)
OR: 60.3 microgram (b)

Letters in the parenthes indicates significant differences from post-hoc statistics.

Apparently young roots are most efficient, but leaves can absorb quite a bit (evem with phals with thick cuticles).

Foliar absorption can occur via stomata area or non stomata area (cuticular region). In Phal, it appears that absorption isn't via stomata (no diff. in absorption via upper and lower surface). In the cuticles, polysacchride strands can be sticking out. These are hypothesized to be a possible route (not completely understood). So some leaves look glossy doesn't mean they can't absorb nutrients.

A couple of reviews:

Fernández, V. and Eichert, T., 2009. Uptake of hydrophilic solutes through plant leaves: current state of knowledge and perspectives of foliar fertilization. Critical Reviews in Plant Sciences, 28(1-2), pp.36-68.

Pandey, R., Krishnapriya, V. and Bindraban, P.S., 2013. Biochemical nutrient pathways in plants applied as foliar spray: Phosphorus and Iron. Washington, DC: VFRC (Virtual Fertilizer Research Center) report, 1. (PDF)

An observation... Foliar feeding (except in a controlled lab experiment, perhaps) is pretty hard to do without the solution ending up in the medium (therefore in the root zone) anyway... so why work so hard, when we know that feeding the roots works?

I do both since I have air plants and other plants I just spray once a week lightly to make sure everyone gets some food. so far they have not complained (G)

Why foliar feedings because the plants we got have no or little roots after repotting.