EXTERNAL CONDITIONS OF ABSORPTION, ETC. 



511 



plant. In this way the ability of the soil to support a prolific vegetation 

 is dependant on those climatic influences which rapidly determine the 

 death of plants, or the parts of plants, and converts them into humus. 

 Herein we see the foundation of the variety of the vegetation in different 

 parts of the earth, and the determining cause of the richness of a 

 tropical vegetation. 



The industrious Schiibler * has made a series of experiments, in order 

 to reduce to number the capacity of various kinds of soil to absorb 

 water from the atmosphere. The results are contained in the following 

 Table : 



These experiments were performed in an atmosphere saturated with 

 moisture at a temperature of from 12 to 16 R. (59 to 64 Fahr.). 

 In order to apply these results, we want three other series of experi- 

 ments on the absorbing power of these substances. 1. According to 

 varieties of temperature ; 2. The thickness of the layer of earth ; 

 3. The degree of moisture of the air. Should we now attempt to apply 

 Schiibler's results (which are certainly unsatisfactory) to a soil 12" deep, 

 we should find that plants were supplied during a period of 120 days 

 with the enormous quantity of eighteen millions of pounds of water. 



But water is not the only nor the most important portion of the food 

 of plants. They require carbonic acid gas and the volatile salts of 

 ammonia, which must be derived from the atmosphere ; they are ab- 

 sorbed, the carbonic acid partly, and the ammoniacal compounds probably 

 entirely, by means of the roots. The greater proportion of these sub- 

 stances which are brought down by the rain are also evaporated with 

 the water ; hence the necessity of the presence in the soil, in the form of 

 clay and humus, of media to convey them to the plant. In all agricultural 

 estimates of the value of the soil, the entire decision turns upon the 

 contents of clay and humus. Some of the best wheat soils often do not 



Agrikulturchcmie. 2d edition. By Krutzscli. pp. 2, 84. 



