22 



PRINCIPLES OF FARM PRACTICE 



restricting their roots to a region near the surface above 

 the water table. If the soil, in such condition, is undrained, 

 there is no way for the surplus water to escape except through 

 evaporation. But evaporation requires heat; consequently, 

 the heat which otherwise might be used in warming the soil 

 is used to evaporate water. Furthermore, the water table 

 during the process is lowered so slowly that the roots of the 



young plants are kept 

 near the surface of the 

 ground, because they 

 will not grow to any ex- 

 tent in the free water 

 below the water table. 

 Later in the season, the 

 water table drops to 

 lower levels, but a gap 

 is left between the free 

 water below and the 

 feeding area of the roots 

 above. The distance is 

 often too great for a 

 supply of water to be 

 lifted by capillary action 

 to the region of the roots. Hence, plants in undrained soils 

 frequently suffer from drought. 



Now, -if the land is well drained, the water table is soon 

 lowered by the removal of the surplus water through drainage 

 instead of through evaporation. Thus the heat, which in un- 

 drained land must be used to lower the water table by evapo- 

 ration, is saved. At the same time, the water table is quickly 

 lowered and plants are able to send their roots deeper into 

 the soil. When the dry weather of the later season comes, 



Diagram of section of undrained soil. 



A. Early in season. Roots are kept near 

 the surface. 



B. Late in season. Plant suffers from 

 drought because of shallow roots not being able 

 to reach area of capillary water. 



