It has been shown in Part II that a glacial drift soil in a comparatively unweathered condition has a considerable power of absorption for ammonia, although it has sometimes been held that it is the weathered material in soils that is responsible for the absorptive power. Glacial drift, as exemplified in the Craibstone soil, however, though its weathering is geologically recent compared with that of many English soils, has been subjected to age-long weathering since the glacial period, and in the process of soil formation a certain amount of weathering with the resultant decomposition has necessarily taken place. Craibstone soil has also been shown (Table II, Part II) to contain a fairly large percentage of organic matter and it might be contended that the absorptive power was chiefly due to either or both of these factors.