I wouldn’t do it, and I’ll explain why.
Let me preface this by stating different plants tolerate salt differently. I can tell you first hand from Hurricane Sandy deciding to allow ocean water to visit my town for a few hours, to a day or so, that some plants died at the thought of salt. Hues (a type of evergreen) that were exposed for an hour or two to the flood waters…..dead! I don’t think I found a single hue that survived the flooding. Surprisingly to me, some plants that were under water for longer periods lived. You would see 80 – 90 percent of the plants dead around a house, and one or two plants growing like nothing happened. Grass did surprising well, maybe because it was already saturated by the freshwater rain.
Back to the skimmate…….First off if we are talking about “fertilizing†some poor little house plant with a cup or two of skimmate, definitely not. Too much salt for a small volume of soil. If we are talking about dumping this cup of saltwater on a compost pile once a week that eventually will be spread over a couple acres, you might get away with it.
But here’s the clincher. If you do this skimmate thing and now spread some of this amended soil around your prize 10 foot rhododendron, and it’s not looking to good a year or two later…… I wouldn’t want to chance it.
And one final note, eat one more banana a week and add that peel to your compost pile, and that would surpass the amount of nutrient benefit to your plants then from a cup of skimmate.
Dump it down the toilet!