WATER IN THE SOIL 13 



Black soil is not necessarily, as is popularly supposed, a 

 rich soil. However, a black soil is likely to contain a good 

 deal of humus, which aids greatly in plant growth. 



The red color of some soils is due to the red iron oxide 

 they contain. Another form of iron oxide is yellow, and 

 this gives the yellow color so common to clays. 



Water in the Soil. Suppose that we have before us a 

 flowerpot filled with soil that is saturated with water. In 

 this condition the spaces between the soil particles are 

 filled with water and the soil air is thus excluded. Through 

 the opening in the bottom of 

 the flowerpot a large part of 

 the water will drain away. 

 This is called free or gravita- 

 tional water. After all the 

 gravitational water has dripped 

 away part of the water re- 

 mains. It exists as thin films 

 around the particles of soil 



Fig. 2. Capillary water. 



and is called film or capillary 



water. The capillary water moves in any direction, but 

 always toward the driest portion of the soil. In dry 

 weather the roots of the crops are supplied with moisture 

 from the saturated zone of the ground, which may be sev- 

 eral feet down, by the force which we call capillary attrac- 

 tion. Thus the capillary water moves through the soil 

 very much as oil moves through the wick of a lamp. 



It is the capillary water in the soil that is useful to plants. 

 In soil that is saturated with water, crops cannot thrive 

 because the soil air is excluded. Therefore, it is important 

 that the supply of capillary water be maintained by proper 

 methods of tillage. The ground must be kept in such a 



