84 THE FACTORS [Part I 



power of capillary conduction and its permeability , and these variations 

 occasion variations in the vegetation. 



The amount of water that is taken up by the soil is termed by Mayer 

 the full water-capacity of the soil, and the amount of water which remains 

 after any excess has been drained away he terms absolute ivatcr-capacity. 

 The latter, which is most important for plant-life, depends chiefly on the 

 size of the particles of soil. A coarse sand, for instance, has an absolute 

 water-capacity of 13-7 % of its volume, a true clay soil one of 409 °/ c . 

 The air-capacity is inversely proportional to the water-capacity, as all 

 pores not filled with water are filled with air. 



The permeability of a soil is closely connected with its water-capacity. 

 Coarsely grained soils are very permeable, whilst finely grained soils, 

 and especially clay, are characterized by a great resistance to filtration, 

 and take up therefore an amount of water in excess of their absolute 

 capacity. 



Capillary conduction of water, or power of absorbing water, a not less 

 important property of the soil, is measured by the rapidity with which 

 dried earth becomes soaked up to a definite height with water when placed 

 in contact with it. Clay possesses the greatest power of conducting water, 

 next come humus and sandy soils, whilst gypsum and chalk exhibit the 

 lowest power of absorption. 



The importance of the physical differences in soils in relation to plant- 

 life may be summed up in the following sentences : — 



1. Finely grained soils rich in humus and with a sufficiently permeable 

 substratum possess a moderate degree of humidity that is generally 

 favourable for plant-life. On such a soil trees and field- plants attain their 

 full development. 



2. A sandy soil poor in humus with a permeable subsoil such as gravel 

 admits indeed of being thoroughly soaked to some depth at each fall 

 of rain, but dries quickly when the rain is over. On such a soil, therefore, 

 in a climate of medium humidity, only xerophilous plants that require 

 little moisture can grow. 



3. A finely grained calcareous soil that is poor in humus offers even 

 less favourable conditions, since it has very little power of absorbing 

 water. On such soil, in fact, the vegetation is markedly xerophilous, 

 whilst on a calcareous soil that is rich in humus, if the climate is appro- 

 priate, hygrophilous vegetation may appear without risk. 



4. Of all soils clay absorbs and retains water best. In dry regions, 

 for instance that of the Mediterranean, clay is highly esteemed on account 

 of these qualities, whilst in moist regions like that of Western Europe, 

 soils of exactly the opposite character are preferred, because a clay soil 

 under a heavy rainfall absorbs water beyond its absolute capacity. 



5. Absorption beyond its absolute capacity by a soil conduces to szvampi- 



