A chemical process in which nitrogen (mostly in the form of ammonia) in plant and animal wastes and dead remains is oxidized at first to nitrites and then to nitrates. These reactions are effected mainly by nitrifying archaea and bacteria, such as Nitrosomonas and Nitrobacter respectively. Unlike ammonia, nitrates are readily taken up by plant roots; nitrification is therefore a crucial part of the nitrogen cycle. Nitrogen-containing compounds are often applied to soils deficient in this element, as fertilizer. Compare denitrification.