Chick, in her interesting paper (1906) on the study of nitrification with reference to sewage purification, states that it had been long upheld that a most important preliminary to nitrification both in soil and in sewage filters was an absorption of NH+4 or ammonium salts on surfaces of soil particles, or filtering medium. In the case of soil, a long controversy had taken place as to whether a physical or chemical process was involved, and the former view on the whole prevailed. Chick, however, concluded against an absorption theory, since nitrification took place in sewage filtering through coke, although no absorption of NH+4 took place on the coke. The problem of the site of nitrification in soil has not been seriously considered for many years, and it has been probably taken for granted that since nitrification takes place in isolated culture media,soil nitrification also occurs at the expense of NH+4 held in solution in the soil moisture. Albrecht & McCalla (1937), however, have obtained evidence that NH+4 held in the base-exchange complex of clay may be oxidized by bacteria, indicating that NH+4 need not be in the aqueous phase in order to be, attacked by the nitrifying bacteria. McCalla (1941) has suggested that the bacteria themselves act as base-exchangers and can exchange cations with those taken up in base-exchange complexes (such as clay).This theory is a development of the recent views of Jenny & Overstreet (1940) who postulate a base-exchange between plant roots and soil complexes. Waksman, Renszer, Carey, Hotchkiss & Renn (1933), Peele (1936), Rubentschik, Roisin & Bieljinsky (1936), Conn & Conn (1940) and ZoBell (1943) have all either shown that bacteria can be adsorbed on various surfaces or have investigated the effects of surfaces on bacterial activity. Waksman et al. (1933), for example, found that nitrifying bacteria of the sea water in the Gulf of Maine were largely adsorbed on to bottom-mud. Little, however, has been done to determine whether nitrification as it occurs normally in soil takes place wholly or partly at soil surfaces, whether NH+4 in soil solution or adsorbed in the base-exchange complexes of soil forms the actual substrate of the nitrifying bacteria in soil, or whether the kinetics of nitrification in soil are dependent on the physico-chemical conditions obtaining in soil. We have attempted to answer these questions by experiments carried out with the soil-perfusion technique described previously (Lees & Quastel, 1946). The following paper describes the results obtained.