48 



FUNDAMENTALS OF FRUIT PRODUCTION 



The maximum amount of water that a given soil may contain depends on 

 the volume of its pore space. Both may range from about 32 to a Uttle over 52 

 per cent.'^' This amount of water is equivalent to a 4 to 6 acre-inch precipitation 

 and would weigh from 20 to 32 pounds per cubic foot of soil. Naturally these 

 large amounts of water are not found in any soil except below the water table line 

 or immediately after heavy precipitation or irrigation and before drainage has had 

 an opportunity to carry away the surplus moisture. It is the amounts of capil- 

 lary and hygroscopic water that a soil will retain rather than its total water-holding 

 capacity as determined by pore space that are of interest in fruit growing, for the 

 reason that little or none of the free or gravitational water is utihzed by the 

 plants. For most purposes and under most conditions it is only the capillary 

 moisture that they use, though Loughridge**^ is responsible for the statement 

 that in some instances plants are able to remain alive, even though they cannot 

 grow, in soils whose water supply is reduced to the point where only hygroscopic 

 moisture is present. The total amount of capillary water that a soil will retain 

 depends not so much on the pore space as on the size of the soil particles 

 and the distance from the level of the ground water below. Tables 27 and 28 

 show the percentage of water that certain typical soils will hold as capillary 

 moisture against the force of gravity. These figures were obtained from soils 

 of undisturbed field texture several days after heavy rains so that gravitational 

 water had had ample opportunity to drain away. It should be stated in con- 

 nection with these tables, that in each case the soil became somewhat more sandy 

 at greater depths. 



It is interesting to note that the optimum condition for the growth 

 of plants is afforded by a soil when the capillary water amounts to between 

 40 and 60 per cent, of the total water-holding capacity of the soil, leaving 



