RELATION TO ENVIRONMENT. 



ing vegetation, and the action of certain chemical solvents on 

 certain rocks, the finer and less porous soils are formed. The 

 variation in the proportion of these ingredients together with the 

 different degrees of disintegration of vegetable matter make the 

 soils of different physical conditions. 



922. Relation of physical condition of soil to plant growth. 

 The relation of these different physical conditions of soil to plant 

 growth lies in the means it affords the plant to get and maintain a 

 foothold, its power of absorption and retention of water, the cir- 

 culation of air, the ability with which the soil particles hold 

 mineral substances necessary for plant food, its power to absorb 

 or radiate heat, to reflect or absorb light, its tenacity or power of 

 resistance to washing by heavy rains, or drifting in winds, etc. 

 Thus the soils of a region having the same annual rainfall vary 

 in their capacity to hold water, so that one kind of soil may have 

 a vegetation which is largely xerophytic (see Chapter XLVII); 

 for example, fine ground lime soil poor in humus, because of its 

 low water-holding power, while on lime soil rich in humus a 

 mesophytic or hydrophytic vegetation may thrive. Sand has a 

 less water capacity than clay, and also parts with it much more 

 readily by evaporation. A fine-grained soil, rich in humus, is 

 most favorable for the retention of a suitable amount of moisture. 

 Forests and meadows here reach their highest development. 

 Sandy soil poor in humus underlaid with gravel becomes wet 

 with each rain, but quickly dries out. Clay surpasses all soils in 

 its power to take up and retain water. According to Schimper, 

 clay grounds in the dry regions of the Mediterranean are highly 

 prized because of this power to absorb and hold water, while in 

 west Europe where the precipitation is great, the clay soils are 

 often too wet. In this condition, clay soils, as well as lime soils 

 rich in humus, are apt to lack oxygen because of their non- 

 porous condition, and are, therefore, unfavorable for plant growth. 



923. The chemical condition of the soil often affects its 

 physical properties, so that clay is made less porous by adding 

 potash, ammonia, etc., or more porous by the addition of certain 

 acid salts, phosphoric acid, etc. 



