The apparatus is a simplification and an improvement on one devised by the author for the study of soil metabolism (Lees & Quastel, 1944) Its function is to percolate a column of sieved soil automatically and regularly with small quantities of a well-aerated solution of metabolites and to return the percolated solution, now modified by the activities of the soil microflora, to the reservoir whence it came As this circulatory process is continuous, metabolic activities in the soil are reflected as chemical changes in the reservoir solution and can be followed by analyses of it.