As a matter of course, the several organic compounds contained in plants may continue to exist in soils for some time, varying according to conditions of temperature and moisture. Thus dextrin, glucose, and even lecithin and nuclein have been reported to be found. The activity of the numerous fungous and bacterial ferments under favoring conditions will, of course, limit the continued existence of such compounds somewhat narrowly, so that they can hardly be considered as active soil ingredients save in so far as they favor the development of the bacterial flora. _Porosity of Humus._—One of the essential features of natural humus is its great porosity, whereby it not only becomes highly absorbent of water and gases, but is also gradually oxidized, probably under the influence of bacteria. For this oxidation, as measured by the evolution of carbonic gas, progresses most rapidly under the same conditions as to moisture, temperature and access of air, that are known to be most favorable to fungous and bacterial growth. Hence the formation of carbonic dioxid in the soil is assumed to be the measure of the intensity of such activity. _Physical and Chemical Nature of the Humus Substances._—The humus substances are gelatinous when moist, but are neither markedly adhesive or plastic. Like the other colloidal substances of the soil, they serve to retain both gases and vapors, including moisture, liquid water, and its dissolved solids. In the natural, porous condition they are powerfully absorbent of gases, including especially aqueous vapor. Dry humus swells up visibly when wetted, the volume-weight increasing to the extent of two to eight times; so that humus stands foremost in this respect among the soil constituents. The _density_ of natural humus is about 1.4, being the lightest of the soil constituents. Hence soils rich in humus are “light” not only in the farmer’s sense of being easily tilled when not too wet, but also of light weight for equal volumes when compared with clayey and sandy soils. Some data bearing upon these points are given in the table[39] below, for the substances moderately and uniformly packed: VOLUME-WEIGHTS OF Humus.[40] Clay. Quartz Sand. .3349 1.0108 1.4485 When saturated with water, the same substances gave the following figures: Air-dry. Saturated Increase. with water. % Humus[41] .3565 1.1024 209.2 Clay 1.0395 1.6268 55.9 Quartz sand 1.4508 1.8270 25.9 [39] Wollny, Zersetzung der Organischen Stoffe, pp. 242, 243. [40] Peat pulverized and extracted with alcohol and ether to remove resinous substances. [41] Peat pulverized and extracted with alcohol and ether to remove resinous substances.