Field beans being grown as a green manure at the Centre for Alternative Technology, Pantperthog, Wales. Llwyngwern Quarry, Pantperthog, Machynlleth, Powys, Wales, UK. They can be sown in the late summer or early autumn, are fast growing and can cover bare soil in the winter. Their foliage smothers weeds and their roots prevent the erosion of the soil. They can then be dug into the ground when still green returning nutrients to the soil and improving soil structure. The bacteria that live in the roots of the bean plants capture nitrogen from the air which is used to make nutrients. The fibre of their roots help the soil to hold on to water.