Was gifted five tomato plants for the allotment but toms like a richer, more fertile soil than we have here. So I'm making a point of feeding them and the simplest, no cost way to do that is Comfrey Tea. There are various Symphytums on the allotment, notably Creeping Comfrey, White Comfrey, Common Comfrey and Russian Comfrey.
 Plants need NPK i.e. nitrogen, phosphorous, potassium. Synthetic fertilisers can provide these but Comfrey leaves are an excellent source of NPK. The other consideration with these elements is that they have to be in a form that is readily available for the plants to absorb i.e. water soluble.
 There is no mystery to making Comfrey Tea. Put a quantity of the leaves in a bucket then fill it up till the leaves are submerged. A lid is not essential but the resulting liquid stinks! The leaves break down entirely within a few weeks resulting in a very smelly brew.
 Actually I don't wait that long. I pour it over the toms every few days (it's still pretty smelly) then top up the leaves and the liquid. I put the leaves in a basket in the bucket so they're not floating around half decomposed when I pour.
 Reader, I should also point out that you have another source of well balanced NPK in a form that is already water soluble for further dilution. Moreover you have a lifetime supply, absolutely free of charge. Yes, I am referring to urine.