14 Irrigation and Drainage 



water in the soil to the direct vital processes which 

 are going on there whenever steady growth is taking- 

 place. But there are other processes which are purely 

 physical, to which attention needs to be called before 

 we have brought into view the full line of operations 

 to which this great agent, water, leads. 



Other plant -foods, 1 those which contain the phos- 

 phoric acid, potash, lime, magnesia, iron and sulfur, 

 must be taken from the inert solid form in the soil 

 into solution in water before they can be of any service 

 in plant growth, and this is another of the important 

 roles which water has to play in the life processes of 

 the soil. Then, too, all water used in irrigation, and 

 even rain water, contains^ 1 larger or smaller quantities 

 of plant -food, either directly in solution or borne in 

 suspension, which adds so much to the ' fertility of the 

 soil itself. 



So, too, all waters which have been exposed to the 

 atmosphere have become charged with oxygen, carbonic 

 acid and nitrogen, which they carry with them into 

 the soil, and these always aid, in one way or another, 

 both the physical and the life processes which make 

 for fertility of the land. And, again, when a large 

 volume of warm water falls upon or is applied to the 

 soil, and it sinks deeply into it, it carries with it not 

 'only its own warmth, but also the heat which it may 

 have absorbed from the surface of the ground ; and 

 this warmth, carried deeply into the ground, makes 

 the root action stronger and at the same time increases 

 the rate of solution of plant -food from the soil grains. 

 When we have made this brief survey of what warm 



