|Since 1978, the author has been conducting research into the theory and practice of biological husbandry in collaboration with a farmer who stopped using pesticides and mineral fertilizers in 1976. Eggs are exported from the farm. About 60% of feed is grown on the farm in a legume-cereal rotation (faba beans-oats-clover-winter wheat), and plant and animal residues are recycled. Annual weeds function as a self-seeding cover crop, protecting the soil, conserving nutrients and fixing carbon where and when cultivated crops are not present. Yields average about 25% lower than those on conventional farms, but the farm is more profitable because of lower input costs. A nitrogen budget suggests that inputs of nitrogen are sufficient to sustain cereal yields equivalent to those of conventional systems. However, much of the annual input of N to cereal fields, in manure, is not available in the short term. Various laboratory and field studies suggest that as fertility or the biological activity of soils increases, problems related to immobilization of N by straw, phytoxicity and annual weeds decline, and that less manure is required to augment the N supply by a given amount. While N might be identified as the "limiting factor" for cereal production, alleviation of N shortages is dependent on intensifying cycling, rather than on increasing N inputs. This intensification is achieved by augmenting natural rhythms on the farm through appropriate tillage techniques, and by ensuring an abundance and high activity of the catalysts of the N cycle, i.e. of all of the farm biota.