Green Manures & plant with fastest biomass growth.

Can anyone tell me what plant has the fastest biomass growth (ex hemp)? If I were to take common comfrey and let it go to seed could it be an ecological problem for farmers? Is bamboo a good idea? Is it possible to sycthe off the growth and leave it on the ground to add organic matter?

With green manures, I've seen field beans being sold by the 25kg sack full, is it possible to get Alfalfa, Red Clover and Lupins in simliar quantities from farmers or wholesalers?

When sowing is it possible to sow onto bare clay and cover with just netting or a small amount of compost and have most of the plants survive? Is there a green manure that can tolerate exposure to the elements? Are seed balls a viable option?

Thank you for any advice.

You could try this

http://www.recrops.com/miscanthus

With green manures, I've seen field beans being sold by the 25kg sack full, is it possible to get Alfalfa, Red Clover and Lupins in simliar quantities from farmers or wholesalers?

Yes! eg: from here:

http://www.wholesale.molesseeds.co.uk/search/green%20manure/