Spinach, beet, and chard leaves seem particularly sensitive to
foliars (and even to organic insecticides) and may be damaged by
even half-strength applications. And the cabbage family coats its
leaf surfaces with a waxy, moisture-retentive sealant that makes
sprays bead up and run off rather than stick and be absorbed. Mixing
foliar feed solutions with a little spreader/sticker, Safer's Soap,
or, if bugs are also a problem, with a liquid organic insecticide
like Red Arrow (a pyrethrum-rotenone mix), eliminates surface
tension and allows the fertilizer to have an effect on brassicas.

Sadly, in terms of nutrient balance, the poorest foliar sprays are
organic. That's because it is nearly impossible to get significant
quantities of phosphorus or calcium into solution using any
combination of fish emulsion and seaweed or liquid kelp. The most
useful possible organic foliar is 1/2 to 1 tablespoon each of fish
emulsion and liquid seaweed concentrate per gallon of water.

Foliar spraying and fertigation are two occasions when I am
comfortable supplementing my organic fertilizers with water-soluble
chemical fertilizers. The best and most expensive brand is
Rapid-Gro. Less costly concoctions such as Peters 20-20-20 or the
other "Grows," don't provide as complete trace mineral support or
use as many sources of nutrition. One thing fertilizer makers find
expensive to accomplish is concocting a mixture of soluble nutrients
that also contains calcium, a vital plant food. If you dissolve
calcium nitrate into a solution containing other soluble plant
nutrients, many of them will precipitate out because few calcium
compounds are soluble. Even Rapid-Gro doesn't attempt to supply
calcium. Recently I've discovered better-quality hydroponic nutrient
solutions that do use chemicals that provide soluble calcium. These