This invention includes uses and methods of preparing phosphites and/or hypophosphites (and their polymeric forms and/or their salts) and phosphite-based agrochemicals (fertilizers and biocides) from phosphate or other naturally occurring or synthetic phosphorus sources using anaerobic microbial fermentation. The fermentation process produces both a liquid and solid phase phosphite/hypophosphite salts which can be used as an agrochemical product.