THE PHYSICAL ENVIRONMENT OF ORGANISMS 559 



and agents that produce and modify it. The most important of these are 

 (1) the kind of rock from which it is largely derived; (2) the kind of climate 

 (temperature, rainfall, etc.) in which it has been formed; (3) the type of 

 vegetation that has grown upon it; (4) the length of time the weathering 

 processes have continued ; (5) its texture (kinds and sizes of particles) ; 

 (6) aeration and drainage; (7) the predominating processes that have 

 operated and the order in which they have occurred. We shall consider 

 only a few of these very briefly, to illustrate something of the possibilities 

 for variation in the resulting types of soils and to point out some of the 

 factors that result in the belts and zones of soil groups illustrated in 

 Fig. 32.11. 



When the surface of the land is first exposed to weathering processes, 

 it consists either of bedrock or (very locally) of unconsolidated marine 

 or fresh-water deposits. Under the action of wind, water, gravity, changes 

 in temperature, and chemical solution, the rock disintegrates and crum- 

 bles into various-sized particles that may be roughly classed as gravel, 

 sand, silt, and clay. 



Gravel particles are more than 1.0 mm. in diameter. 



Sand particles range in diameter from 1.0 to 0.05 mm. 



Silt particles range in diameter from 0.05 to 0.005 mm. 



Clay particles are less than 0.005 mm. in diameter. 



As these particles accumulate, they are subjected to the action of 

 soil water. If rainfall is distinctly greater than evaporation, the soluble 

 salts of calcium, magnesium, potassium, etc., will tend to be carried 

 down beyond the reach of plant roots and thus lessen the productivity 

 of the soil. On the other hand, if the rainfall is distinctly less than evap- 

 oration, the soil water will move upward, and the soluble salts will ac- 

 cumulate on the surface, producing alkali soils that are unproductive 

 because of the too great concentration of salts and the too low moisture 

 content. In regions where rainfall and evaporation rates are more nearly 

 balanced, the soluble salts produced by weathering tend to remain in 

 the soil and form a storehouse from which plants may freely draw. Here 

 plant life, and consequently animal life, is abundant, and the organic 

 accumulations from the dead bodies and the catabolic wastes of plants 

 and animals are added to and retained by the soil. 



Soil types and soil groups. Since the type of soil is largely deter- 

 mined by the climate, we find that different types of soils are charac- 

 teristic of different climatic belts and zones. This is illustrated by Fig. 

 32.11. Such a map can show only the very broad and generalized regions 

 of different soil types. Since there are many local peculiarities of climate 

 and even more local variations in rate and sequence of soil-forming 

 processes (because of drainage, local geological history, etc.), each of the 

 soil areas shown on the map may, and usually does, contain numerous 



