42 FIRST PRINCIPLES OF AGRICULTURE. 



The earth may be compared to a sponge full of water, 

 which rises towards the surface with heavy rainfalls, and 

 falls below as evaporation and percolation proceed. 



Drainage. If a hole dug into the soil partly fills 

 with water, and remains with slight fluctuations through 

 the season, the water contained in it is called "bottom 

 water," and the point to which it rises is called the 

 "water level." If the water level is constantly near 

 the surface, the soil is liable to be too wet ; for most 

 plants suffer if their, roots are immersed for any length 

 of time in stagnant water. Plants need air, both for 

 root and branch. Too much water in a soil prevents 

 the circulation of air, and also keeps it too cold for 

 most crops ; the cranberry and rice plants are prominent 

 exceptions to this rule. The soil may also be too wet, 

 even when the water level is deep into the earth, by 

 reason of absorbing too much of the rain that falls upon 

 it. Drainage corrects in the first case by lowering the 

 water level, and in the second by permitting a more 

 rapid passage of water through the soil. 



Land well drained is improved, not only by the removal 

 of water from it, but because the more rapid diffusion 

 and passage of the water through the soil carry the air 

 and warmth to lower levels, which are, as has already 

 been shown, important factors in making soil constitu- 

 ents soluble, and thus increasing the power of plants 

 to secure food. 



In too many cases half-developed crops are secured 

 year after year upon land, which, if properly drained, 

 would be capable of maximum production. Where springs 

 occur, and where the land is composed of clay overlying 



