2 NATURE AND PROPERTIES OF SOILS 



are active within its precincts. Such an equilibrium it never 

 attains and thus the evolution goes on and on. It is this 

 continual change and this endless response to environment 

 that makes the soil useful to plants. The disintegrating 

 rock and the decaying organic additions are thus converted 

 into a mechanical support for plants, while at the same 

 time they are forced to liberate the nutrients essential to 

 plant growth. 



In the light of its origin and function the soil may be 

 denned as a mixture of broken and weathered fragments of 

 rock and decaying organic matter, which covers the earth 

 in a thin layer and supplies mechanical support and in part 

 sustenance to plants. 



This debris of rock and plant residue, teeming with its 

 microscopic life and ever restless in its endless efforts at 

 equilibrium, is the arable soil from which man must obtain 

 his bread. As the light of investigation is thrown on it, 

 new changes, new functions and new and unsuspected re- 

 lationships are brought to view until the story of the soil 

 may be retold with a clearer insight into those processes 

 that render it useful to man. 



1. Composition of the soil. — The soil as denned is com- 

 posed of two general classes of material, mineral and organic. 

 The former in most cases makes up from 90 to 99 per 

 cent, by weight of the dry substance of a soil, the organic 

 matter, except in the case of peat and muck, being in rela- 

 tively smaller amounts. In spite of the low proportion of 

 organic matter its presence is vital, not only because of its 

 influence physically but because of the nutrients, especially 

 nitrogen, that it carries. The mineral portion of a soil 

 functions as a frame-work and as a source of certain chem- 

 ical elements, which are necessary to proper crop growth and 

 development. 



It must be realized at the very outset that the two main 

 constituents in a normal soil exist in very intimate relation- 



