80 SOILS 



CAPACITY OF DIFFERENT SOILS TO HOLD WATER 



The different forms in which water is found in 

 the soil have been mentioned in Chapter 

 II. The water that is most valuable to 

 the plant is that which is held by the 

 grains as film moisture, although a large 

 part of this may be drawn from the reser- 

 voir of free or standing water below. Soils 

 vary widely in their ability to hold film 

 water. In judging the value of a piece of 

 land for cropping, it is fully as important to 

 consider its water-holding capacity as its rich- 

 ness in plant food; a soil may be exceedingly 

 rich in the essential plant foods, yet if it does 

 not hold enough water to dissolve that food 

 and carry it to the plants, it will produce no 

 more than a very poor soil. Fertility consists as 

 much in an abundance of soil water as in an 

 abundance of plant food. 



The capacity of a soil to hold water depends 

 upon its composition and upon its texture. 

 The lighter a soil is, or the more sand it 

 contains, the less water it will hold. The 

 smaller the grains, the more water the soil 

 holds, since there is more surface for it to 

 cling to and less likelihood that it will leach 

 through. Each soil grain is surrounded by a 

 film of moisture ; if there are over 168,000,- 

 000,000 grains in an ounce of soil, as in some 

 alluvial soils, the amount of surface for the 

 water to cling to is much greater than if there are 

 but 56,000,000,000 grains in an ounce, as in some 

 truck soils. The more humus a soil contains the 

 greater is its water-holding capacity, for humus is 

 vegetable sponge. If small quantities of several 



