CHAPTER XVI 

 SOILS AND SOIL WATER RELATIONS 



Most vascular plants are rooted in the soil from which they obtain both 

 water and mineral salts. The entrance of water from the environment into 

 any of the organs of a plant is called the absorption, intake, or uptake of 

 water. In most vascular species the quantity of water absorbed through 

 organs other than the roots is of negligible importance. Any thorough con- 

 sideration of the entrance of water into roots requires a consideration of 

 the properties of soils, particularly as they affect the movement of water 

 from the soil into the root. 



Constitution of the Soil. — The soil matrix which is the habitat of most 

 roots is an extremely complex system. In general five different components 

 of this system are distinguished : 



I. The Mineral Matter of the Soil. — The parent substance of practic- 

 ally all soils is rock, of which there are numerous varieties. By various 

 weathering processes rock strata are reduced to fragments of diverse sizes and 

 these compose the bulk of most soils. The rock particles in soils may vary 

 in size from stones and gravel down to sub-microscopic particles of colloidal 

 clay. The mineral portion of the soil is customarily classified into several 

 fractions depending upon the size of the particles. Such classifications usually 

 disregard the very large particles of the soil (rock fragments, pebbles, etc.). 

 The classification now generally used is as follows : 



The proportions of the different fractions present are very different in different 

 kinds of soils. 



The clay fraction of the mineral portion of soils requires special con- 



246 



