SOILS AND FERTILIZERS 73 



In the order of their importance to the agriculturist, 

 capillary water ranks first, ground water second, and 

 hygroscopic water last. 



Capacity of Soils for Water. It is evident that a 

 given soil can hold enough water to fill all the spaces 

 between its particles. Experiments have shown that 

 a cubic foot of coarse sand, completely saturated, will 

 hold about one third of a cubic foot of water, while the 

 same bulk of rich humus soil will hold about two thirds 

 of a cubic foot. Other soils will range between these 

 two extremes. But as saturation, or the complete 

 filling of the spaces between the particles, prevents the 

 air from penetrating the soil, the question of importance 

 to the farmer is, not how much water the different kinds 

 of soil can hold, but what are their different capacities 

 for capillary water, or how much water will they hold 

 when all the free water is allowed to drain out. 



As capillary water clings to the surface of soil par- 

 ticles, it is plain that the greater the number of particles, 

 the greater will be the extent of surface to be covered 

 with water. In other words, the finer the particles, the 

 greater the power of the soil to hold capillary water. 

 An understanding of this will enable us to see that 

 coarse sand will retain but little capillary water, while 

 a clay loam, rich in humus, will retain a large amount. 

 Obviously, the desirable soil for growing crops, other 

 things being equal, is one that will retain a large amount 

 of capillary water with a subsoil that holds the ground 

 water near enough to the surface to be made available 

 for replacing the loss by plant use and evaporation. 



Amount of Water needed by Plants. It requires 

 an enormous quantity of water to mature a crop. 

 Various estimates have been made of the exaet amount 



