CAMERON, A. E., “General Survey of the Insect Fauna of the Soil,” Journ. Econ. Biol., 1913, viii. “Insect Association of a Local Environmental Complex in the District of Holmes Chapel, Cheshire,” Trans. Roy. Soc. Edin., 1917, lii. DARWIN, C., “Vegetable Mould and Earthworms,” London, 1881. HAMILTON, C. C., “The Behaviour of some Soil Insects in Gradients of Evaporating Power of Air, etc.,” Biol. Bull., 1917, xxxii. MORRIS, H. M., “Observations on the Insect Fauna of Permanent Pasture in Cheshire,” Ann. App. Biol., 1921, vii. “On a Method of Separating Insects and other Arthropods from Soil,” Bull. Entom. Res., 1922, xiii. “The Insect and Other Invertebrate Fauna of Arable Land at Rothamsted,” Ann. App. Biol., 1922 A, ix. REH, L., In Sorauer’s “Pflanzenkrankheiten,” 1913, iii. RICHARDSON, C. H., “The Attraction of Diptera to Ammonia,” Ann. Ent. Soc. Amer., 1916, ix. RUSSELL, E. J., “The Effect of Earthworms on Soil Productiveness,” Journ. Agric. Sci., 1910, iii. SHELFORD, V. E., “Animal Communities in Temperate America,” Chicago. 1914, “The Importance of the Measure of Evaporation in Economic Studies of Insects,” Journ. Econ. Entom., 1912, vii. CHAPTER X. THE CHEMICAL ACTIVITIES OF THE SOIL POPULATION AND THEIR RELATION TO THE GROWING PLANT. In the preceding chapters it is shown that the soil is normally inhabited by a very mixed population of organisms, varying in size from the smallest bacteria up to nematodes and others just visible to the unaided eye, on to larger animals, and finally earthworms, which can be readily seen and handled. These organisms all live in the soil, and therefore must find in it the conditions necessary for their growth. We have dealt in the first chapter with the supplies of water, air, and heat, without which life is clearly impossible. Equally necessary is the source of energy, for the organism requires energy material as surely as the motor engine requires petrol, and it ceases to function unless an adequate supply is forthcoming. All the energy comes in the first instance from the sun, if we exclude the unknown but probably small fraction coming from radio-active elements. But this radiant energy is not utilisable by the soil population, excepting surface algæ; it has to be transformed into another kind. So far, chlorophyll is the only known transformer; it fixes the energy of sunlight and stores it up in bodies like hemicellulose, sugar, starch, protein, etc. The transformation is imperfect; even the heaviest yielding crops grown under glass, in conditions made as favourable as our knowledge permits, utilise only about 4 per cent. of the total energy available during their period of growth; in natural conditions not more than 0·4 per cent. is utilised. Such as it is, however, the energy fixed in the plant represents all, indeed more than all, that the soil organisms can obtain.