ENVIRONMENTAL FACTORS 3OI 



soil and its ability to hold water and salts. Sandv soils are porous, 

 ' light ' to work, easily penetrated by roots, and thev dry readily ; 

 in contrast, clayey soils are retentive of water, hea\T, poorly aerated 

 and sticky when wet, and hard when dry. For most working and 

 plant-growing purposes, mixed ' loams ' are best. 



(2) Soil icoter containing dissolved substances is also of funda- 

 mental importance, being commonly the chief source of water for 

 plants, ^^'ater is of course essential to plants as usually their main 

 constituent by weight, as the medium for phvsical and chemical 

 changes, and because large quantities must be absorbed to cover 

 the continual loss by transpiration from their surfaces. Except in 

 extremely dry soils and below the level of permanent ground-water, 

 the soil water mainly forms films around the component particles 

 of the soil. The amount of such water and thickness of the film 

 depends on such factors as the soil's mechanical constitution, on the 

 recency of precipitation and tendency to run-otf, on subsequent 

 weather conditions, on humus content, on the covering of vegetation 

 and litter, etc., and on the effect of this covering on water loss 

 by transpiration and evaporation. Especially does water percolate 

 through and evaporate from coarse gravelly or sandy soils, and remain 

 suspended in fine clayey and humous ones with their high capillary 

 action and immense aggregate surface of microscopic or colloidal 

 particles. However, in such retentive soils much of the water may 

 be so strongly held that it cannot be abstracted and used by the plants, 

 so that it is often necessary to distinguish between water which is 

 available to plants (the so-called chresard, above the point of per- 

 manent wilting, though this may vary somewhat with different 

 species) and that which is so strongly held as to be unavailable 

 [echard). The entire water content of the soil may conveniently 

 be termed the ' holard '. Water tends to promote the stratification 

 of soils, and the soil's content of available water is often the chief 

 factor causing local differences between plant communities. 



Another important factor affecting vegetation is the soil water's 

 content of dissolved inorganic salts, etc., which are derived from 

 the mineral matter present and from organic breakdown. Certain 

 of these salts' constituent elements are essential to the plants' con- 

 tinued well-being and even to their life, while others are apt to be 

 obnoxious or actually poisonous. Outstanding are the extremely 

 saline soils which are inhabited only by specially adapted plants 

 (halophytes). In these connections the various needs and abilities 

 of different plants lead to correlations with the chemical content of 



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