80 ELEMENTS OF AGRICULTURE 



a pebble is broken in two, it will have more surface area. 

 It can now hold more water. The finer the material is 

 broken, the more surface there will be, and the more water 

 it will hold. 



The finest soil particles are extremely small less than 

 four hundred-thousandths of an inch in diameter. The total 

 surface area in a cubic foot of such material would be very 

 great. Such fine particles do not always act as individ- 

 uals in holding water, some of the particles usually stick 

 together. A cubic foot of soil grains having a diameter 

 of one -thousandth of an inch (coarse silt) would have 

 a surface area of 37,700 square feet. Four feet in depth 

 of such a soil would have a water-holding surface of not 

 less than 3.4 acres for each column of soil with one square 

 foot of surface area. 1 



The water capacity of a soil is the amount of water 

 that it will hold when all the free water is allowed to 

 drain out. Some clay soils will retain about 40 per cent 

 of water, that is, 100 pounds of soil may retain 40 pounds 

 of water. A cubic foot of clay weighs about 80 pounds 

 and could, therefore, hold about 32 pounds of water. 

 Sandy soils may have a water capacity as low as 5 per 

 cent. 



Plants cannot remove all the water from a soil. They 

 die for lack of water long before the soil is absolutely dry. 

 They can use a larger proportion of the water from a sandy 

 soil than from a clay. King found that in a sandy soil 

 whose water capacity was 18 per cent, corn was able to 

 reduce the water to 4.17 per cent. In a clay soil whose 

 capacity was 26 per cent, it succeeded in using the water 



iKing, The Soil, p. 73. 



