WATER-CONTENT 3I 



habitat. All soils contain more water than can be absorbed by the plants 

 which grow in them. This residual water, which is not available for use, 

 varies for different soils. It is greatest in. the compact soils, such as clay 

 and loam, and least in the loose ones, as sand and gravel. It differs, but to a 

 much less degree, from one species to another. A plant of xerophytic tend- 

 ency is naturally able to remove more water from the same soil than one of 

 mesophytic or hydrophytic character. As the species of a particular forma- 

 tion owe their association chiefly to their common relation to the water- 

 content of the habitat, this difference is of little importance in the field. In 

 comparing the structure of formations, and especially that of the plants which 

 are found in thern, the need to distinguish the available water from the total 

 amount is imperative. Thus, water-contents of 15 per cent in gravel and in 

 clay are in no wise con'parable. A coarse gravel containing 15 per cent 

 of water is practically saturated. The plants which grow upon it are 

 mesophytes of a strong hydrophytic tendency, and they are able to use 143^ 

 or all but .5 out of the 15 per cent of water. In a compact clay, only 33^ 

 of the 15 per cent are available, and the plants growing in it are marked 

 xerophytes. It is evident that a knowledge merely of the physical water- 

 content is actually misleading in such cases, and this holds true of com- 

 parisons of any soils which differ considerably in texture. After one has 

 determined the physiological water for the great groups of soils, it is more 

 or less possible to estimate the amounts in the various types of each. As 

 an analysis is necessary to show how close soils are in texture, this is 

 little better than a guess, and for accurate work it is indispensable that the 

 available water be determined for each habitat. Within the same formation, 

 however, after this has once been carefully ascertained, it is perfectly satis- 

 factory to convert physical water-content into available by subtracting the 

 non-available water, which under normal conditions in the field remains 

 practically the same. 



The importance of knowing the available water is even greater in those 

 habitats in which salts, acids, cold, or other factors than the molecular at- 

 traction of soil-particles increase the amount of water which the plant can 

 not absorb. Few careful investigations of such soils have yet been made, 

 and the relation of available to non-available water in them is almost en- 

 tirely unknown. It is probable that the phenomena in some of these will be 

 found to be produced by other factors. 



51. Terms. The terms, physiological water-content, and physical water- 

 content, are awkward and not altogether clear in their application. It is 

 here proposed to replace them by short words which will refer directly to 

 the availability of the soil water for absorption by the plant. Accordingly, 



