Online Much theory, practice and inspiration can be gleaned from books and pamphlets first published decades ago. The Waste Products of Agriculture by Sir Albert Howard. Biodynamic Agriculture, An Introduction by Herbert Koepf et al Chemicals, Humus and the Soil by Donald Hopkins. The Story of the Soil by Cyril Hopkins. Extensive compost and soil analyses, including an inexpensive compost maturity and soil microbial respiration test kits.
Site includes interesting articles and an excellent photo gallery of soil critters at work.
SFI evolved from the pioneering work of soil ecologist Elaine Ingham at Oregon State University. The Drought of '88 was still fresh in the mind of the author, Walter Goldstein, agronomist at the Michael Fields Agricultural Institute, East Troy, Wis. when he wrote this in '89. Focuses heavily on the structure-improving role of sweetclovers. Targeted primarily at farmers in the Northern Plains. But any farmer who fears drought will take something away from reading this.