THE SOIL AS A HABITAT FOR LIFE — RUSSELL 451 



6 inches. The total number of arthropods per square yard to a depth 

 of 12 inches was about 220,000, some 70 percent being in the top 6 

 inches. The springtails are less numerous than the mites but are 

 of particular interest because of their antiquity. They are wingless 

 and appear to be survivors of the days before insects developed 

 wings — a primeval race left stranded in the march of evolution. A 

 fossil insect closely resembling a present-day springtail has been 

 found in the Lower Devonian rocks in Scotland going back some 

 300 million years, and 40 or 50 million years before the first winged 

 insects appeared in Upper Carboniferous times. Some species dwell 

 near the surface of the soil and are usually brightly colored; others 

 live several inches lower down and are colorless, without eyes, and 

 very sluggish in their movements; the Protura are also low-level 

 dwellers. 



But the soil population comprises many other kinds of animals, such 

 as enchytraeids, earthworms, myriapods, larvae, etc. Kiilinelt in his 

 recent book^ lists some 700 species in all, but there are certainly 

 more: as methods improve the list is extended. A. Macfadyen puts 

 the number at up to 1,000. 



STRUCTURE AND PROPERTIES OF SOIL 



In order that these varied organisms can live in the soil, these con- 

 ditions must be satisfied : they must have living space, air, water, food, 

 suitable temperature, and sufficient freedom from harmful factors to 

 live long enough to reproduce themselves in numbers adequate for race 

 survival. Two important properties of the soil make this possible: 

 its content of organic matter and its peculiar structure. The organic 

 matter is derived from the remains of plants that grew on the soil 

 and died there; they were drawn into the soil by worms and other 

 animals and served as food and energy suppliers for the soil popula- 

 tion — the energy being ultimately derived from sunlight by photo- 

 synthesis ; F. Raw has studied this in detail. The quantity and nature 

 of the organic matter determine the size and character of the soil 

 population; nonacid soils of deciduous forest carry the largest and 

 most varied populations grassland soils may be lower in the scale 

 and arable soils lower still. 



The unique structure of soil results from the properties of its con- 

 stituents. Most of it — often 90 to 97 percent of dry soil — consists 

 of mineral particles derived by weathering or disintegration from the 

 parent rocks. Those between 2 mm. and 0.02 mm. in equivalent diam- 

 eter, usually chemically inert and unchangeable, are called sand and 

 constitute the framework of the soil. Those lying between 0.02 and 



2 Kiihnelt, E. W., Soil biology with special reference to the animal kingdom. Faber 

 and Faber, 1961. 



