260 ELEMENTARY STUDIES IN BOTANY 



that the chemical composition of all soils is appropriate for 

 plants, and that the differences between soils is not so much 

 a question of proper or improper chemical constitution as of 

 something else. The chemical composition of soils is uni- 

 formly appropriate for plants in the same general sense that 

 the air is uniformly appropriate for them. It is sometimes 

 said that the chemical composition of soils " makes no dif- 

 ference," but this does not mean that it is not extremely 

 important, any more than the statement that the air " makes 

 no difference " would indicate that the air is not important. 



It makes no difference sim- 

 ply because it is always 

 present. 



The great differences 

 among soils have to do 

 with their power to handle 

 water, and this is a physi- 

 cal property of the soil 

 rather than a question of 

 chemical composition. The 



FIG. 226. Roots of scarlet runner bean power of a Soil to receive 

 marked with lines one millimeter apart , . 



and photographed after forty-eight hours. and to retain Water IS a 



very important consider- 

 ation in connection with plants. For example, it is 

 evident that the receptive power of sand is high, but 

 its retentive power is low; while in the case of clay the 

 reverse is true. One of the great advantages of humus is 

 that its receptive and retentive powers are better balanced 

 than in sand and clay. It is easy to devise a series of experi- 

 ments that will show in a rough way the comparative recep- 

 tive and retentive powers of these three types of soil. It has 

 been shown also that, for any given soil, the more finely the 

 particles are divided the better it is for plants. When the soil 

 is turned up with plough or spade, it is dried by the air and 

 pulverized and so put in better physical condition for hand- 



