16 THE OLIVER PLOW BOOK 



those in dry climates. The reason for this is the amount 

 of evaporation from the leaves and stems. The more 

 humid the atmosphere the less moisture it takes from 

 the plants, the dryer the atmosphere the more moisture 

 it draws from the plants. If the atmosphere were 

 thoroughly saturated at all times the water require- 

 ments for the plants would be very small because of the 

 light transpiration of water from the soil. 



Another peculiar fact in connection with the water 

 required is shade. Shade increases the amount of 

 water required for plant growth. It retards the 

 process by which the plant constructs its tissues because 

 the rays of sunlight necessary for this process are dimin- 

 ished. 



Soil fertility has a great deal to do with the amount 

 of moisture required to grow a crop. A poor soil requires 

 more water than a rich soil for the simple reason that 

 the more fertility there is in the soil the stronger the 

 water holding content. A soil may be fertile in all the 

 elements but one. The lack of this one causes the soil 

 to require more moisture because growth is retarded 

 when a plant fails to get any one of the elements neces- 

 sary. The plant keeps on using and giving off water 

 exactly the same as though all the elements were in the 

 soil. 



The great problem is to catch and save as much of the 

 rainfall as pdssible. The ground below the seed bed 

 must act as a reservoir to hold enough water for it to 

 come upward by capillary attraction to the root bed 

 and not escape into the air. 



Capillary water is that which adheres or clings to the 

 surface of the soil grains and to the roots of plants in 

 films thick enough to allow surface tension to move it 



