THE WATER OF THE HABITAT 19 



potassium, calcium, and magnesium. These occur usually as 

 nitrates, sulphates, phosphates, chlorides, carbonates, or oxides. 



The effect of alkalies and acids in the soil upon water con- 

 tent and absorption is not altogether understood. Experiments 

 indicate that alkalies hinder absorption, and acids promote it. 

 In other words, alkalies reduce the amount of available water, 

 while acids increase it. Alkaline soils are really dry soils, a fact 

 clearly established by the character of the plants that grow upon 

 them. On the contrary, acid soils are wet; usually indeed they 

 show maximum water content. They contain plants which are 

 adjusted to an excessive water supply. The majority of such 

 plants exhibit adaptations to water, but some of them have the 

 appearance of dry land plants, or xerophytes. A careful exam- 

 ination of the structure of the latter reveals modifications due to 

 water. Consequently, it seems almost certain that such "bog 

 xerophytes" are dry land plants, which in coming to grow in 

 water have retained certain superficial ear-marks of the original 

 habitats. 



24. Air content. In all soils, but particularly in acid ones, the 

 air content is a factor of considerable importance, owing to the 

 constant use of oxygen by the roots. The amount of air present 

 depends upon the water content and the compactness of the soil. 

 Air content varies inversely as the water content: it is large in dry 

 soils and very small in wet ones, especially those covered with 

 water. Water plants, i.e., hydrophytes, show characteristic modi- 

 fications called forth in response to a low air content. When a 

 soil becomes packed, the movement of the air into and through the 

 soil is impeded, and a very important task of cultivation is to keep 

 the soil so stirred that the air content does not fall too low. " Sour " 

 soils, including "sour " bogs, owe their nature to the production of 

 organic acids in the presence of a low suppl}^ of oxygen. When 

 stirring the soil is possible, "sourness " is easily remedied, since 

 complete decomposition prevents the formation of acids. 



25. Determination of soil properties. Since the soil is prima- 

 rily important because of its influence upon water content, the 

 determination of soil texture is the principal task in this connection. 

 In an elementary study of habitats it is sufficient to know that a 

 soil, or the soil water, is acid or alkaline, without further reference 

 to amount. Such a test is readily made in the field by means of 

 fresh strips of litmus paper. Air content is in large degree a result 



