Comfrey tea

Liquid fertilizer, high in potash and nitrogen:

comfrey leaves from the allotment steeped in water over the winter,

bottled and ready to dilute with water (c. 1:10 ratio);

dredges in the compost bin as an 'activant'

Next to try: 'nettle tea', similarly high in potassium, nitrogen, etc.

Plus, Comfrey is a perennial, so this years' stenchy brew is effectively *free*