NATURE OF PLANTS 49 



Water is the most important force in starting these changes. 

 It dissolves, that is removes, several of the elements that appear 

 in the silicates. Note also when water has taken up certain of 

 these elements and compounds that its dissolving power is greatly 

 increased. For example when water has absorbed carbon dioxide 

 its power of dissolving calcium is increased 25 times. Further- 

 more the removal of an element by the water means that new 

 chemical compounds are formed and these are frequently of such 

 a nature as to lead to further chemical reactions. We say that 

 the dissolving action of water upsets the chemical stability of 

 the substances composing the rocks. Certain gases of the air> 

 particularly oxygen, also enter into union with some of the rock 

 elements, making them more soluble. So you can think of a 

 series of chemical struggles going on between the silicon and 

 other elements and compounds for the possession of the potas- 

 sium, calcium, etc., and that in this way these latter elements 

 are slowly removed from the silicon and united into new com- 

 pounds suitable for plants. 



The removal of these elements from the rock minerals forms 

 cavities and as a consequence the rock tends to become honey- 

 combed and finally begins to crumble and so approaches a soil 

 condition. Two forces are therefore involved in soil making: 

 the one chemical which is termed decomposition and the other 

 mechanical, termed disintegration. Disintegration or the crumb- 

 ling of the rocks is hastened by a number of factors, especially 

 the force of running water. The power of water to break up the 

 rock substance and distribute the particles thus formed is one 

 of the most important factors in soil formation. Changes in 

 temperature also constantly cause the contraction and expansion 

 of the rock material and of the water occupying any of the spaces 

 in the rock and so assist in the disintegration. Finally the force 

 of the wind and the grinding action of moving ice are important 

 factors in wearing away the rock surface and reducing it to a soil 

 condition. By these slow and intricate processes compounds 

 for plant uses are formed and the rocks are reduced to the condi- 

 tion of a soil. Such a soil, however, does not contain sufficient 

 amounts of the crude food material to support the ordinary 



