A biological process involving the conversion of nitrogen-containing organic compounds into nitrates and nitrites. It is accomplished by two groups of chemo-synthetic bacteria that utilize the energy produced. The first step involves the oxidation of ammonia to nitrite, and is accomplished by Nitrosomas in the soil and Nitrosoccus in the marine environment. The second step involves the oxidation of nitrites into nitrates, releasing 18 kcal of energy. It is accomplished by Nitrobacter in the soil and Nitrococcus in salt water. Nitrification is an integral part of the nitrogen cycle, and is usually considered a beneficial process, since it converts organic nitrogen compounds into nitrates which can be absorbed by green plants. The reverse process of nitrification, occurring in oxygen-deprived environments, is called denitrification and is accomplished by other species of bacteria.