When you start thinking of soil as a living organism that has to be fed, your whole view of gardening changes. But keep in mind that soil building is not a one-time operation but an ongoing process. Learn the tricks of the trade from a longtime horticultural expert who explains that the key to well-fed and happy plants lies in constantly replacing the humus, or the organic material, that is essential for releasing nutrients in a form that's easily absorbed by plant roots. Chemical fertilizers