ECOLOGICAL RELATIONS OF SOIL 



225 



less organic. The amount of organic material 

 in English soils varies from 3 to 10 per cent. 

 The organic content of soils is usually low 

 in hot climates and lowest in hot, arid ones. 

 Humus is typically colloidal in structure 

 and helps to retard erosion. Like hme, in- 

 creased organic content tends to make 

 heavy soils more granular and make it 

 easier to keep them in good tilth. Humus 

 acts to conserve mineral plant nutiients and 

 to regulate their liberation. It modifies 

 structure, color, consistency, moisture-hold- 

 ing power, and other physical and physico- 



varied group spend a part of their Ufe un- 

 derground, and many of these organisms be- 

 come an integral part of the soil if the lat- 

 ter is broadly rather than narrowly defined. 

 Few land animals burrow into rock. A 

 Colorado bee, Perdita opuntiae, regularly 

 excavates its own holes in sandstone (Cus- 

 ter, 1928). Many animals burrow among 

 rock slides or live in the natural openings 

 between rocks. These petrocoles include, 

 particularly, snails, spiders and other ar- 

 thropods, ants, and various small mammals 

 In favorable locations, lizards are likely to 



OH 



1.0 



2.0^ 



^MBM^^' 



Conifer forest Tall- grass prairie 



Semiarid short- 

 grass prairie 



m 



Poorly drained 

 meadow 



Fig, 57. Selected soil types (see legends in Figure 54 for soil horizons). (Redrawn from 



NikiforoflF.) 



chemical properties of the soil. Like soil it- 

 self, humus is not stable, but is in constant 

 change; the older humus decomposes and 

 in part minerahzes. The amount present at 

 any place and time is the algebraic sum of 

 decomposition and formation (Nildforoff, 

 1938). 



BIOTA OF THE SOIL 



The organic matter in the soil supports 

 a complex microflora and fauna and often 

 a complex biota of higher organisms, an 

 adequate discussion of which would require 

 a book in itself. The soil in the root zone of 

 growing plants— the so-called rhizosphere— 

 contains various root excretions, includ- 

 ing vitamin-hke growth-promoting factors. 

 These permit growth of bacteria that are 

 unable to synthesize such materials. Less 

 specialized bacteria can live outside the 

 rhizosphere (Knight, 1945). Myriads of 

 bacteria, protozoans, worms (especially 

 nematodes and earthworms), crustaceans, a 

 long series of arachnids, insects, and many 

 vertebrates five in the soil. An equally 



be found. Flattening is characteristic of pe- 

 trocole lizards, and even of a turtle that has 

 become adapted to fife in rock crevices. 

 Animals are still more numerous under 

 stones that He somewhat loosely in contact 

 with the earth. 



A series of burrowing mammals dig out 

 their dens in ground studded with rocks or 

 make the openings to their burrows among 

 the large roots of trees, particularly those 

 near the forest margin. The common fox, 

 Vulpes fulva, has this habit. 



Animals of another ecological series dig 

 in moist to wet soil, where the burrows ex- 

 tend to or below water level; These include 

 the ant, Formica iilkei, mound-building ter- 

 mites, and numerous crayfish. Still others, 

 like the muskrat, burrow into the banks or 

 dykes of streams, placing the opening of 

 the burrow under water. 



Soil may act as a barrier for burrowing 

 animals. Certain north-south distributions of 

 well-drained soils in the Gulf Coast region 

 appear to act as barriers to crayfish disper- 

 sal. For example, a lobe of drier soil with 



