Taking a hint from saille, I thought tomyself what is it that the trees would want from me. Light? Water? Food? As much as I'd like to invent a small renewable outddor resistant lighting source or a mini C02 generating terraformer, neither of those are really practical. But food? Food, I can do. Specifically fertilizer. And since basic fertilizer is Nitrogen, Potassium and Phosphate that was my inspiration. Since procuring large amounts of nitrogen might get me added to the Do Not Fly list, I figured potassium and Phosphate would be best.

Behold, fake salt! A combination of Potassium Chloride and Calcium Phosphate, that just needs to be liquified, concentrated into a super-saturated salt solution and Potassium Phosphate crystals will form. (Calcium chloride will absorb water from the air, so if I'm getting my inorganic chemistry right, it should stay in solution.) A few weeks later, I've got 10 unique crystals full of botanically appreciated goodness. Each time it rains or snows a small amount of the crystal will dissolve and fertilize the tree.