1. Dried cell substance of various micro-organisms—nine fungi, one actinomycetes, two bacteria—were subjected to decomposition in soil and sand. In neutral garden soil, the addition of microbial substance gave rise to a more or less abundant, but always temporary, development of bacteria and actinomycetes; the latter group of organisms was often stimulated to a very conspicuous degree.2. The development of micro-organisms was accompanied by a more or less abundant production of nitrate. After 60 days 19–61 per cent. of the added N had been nitrified, and after 120 days the proportion had in most cases not increased very materially.