The traditional thing is you mix urine with wood ash. This returns all the nutrient salts to the soil, and nitrogen. My one concern is sodium accumulation. The

The traditional thing is you mix urine with wood ash. This returns all the nutrient salts to the soil, and nitrogen. My one concern is sodium accumulation. There are some thermal approaches based on solubility curves that sort of work. I'd also be interested in electrochemical and, ideally, biological approaches (can we use biological cell membranes and ion pumps instead of manufactured ones?). Weighing difficulty against impact, this seems under-researched.