632 SOILS: PROPERTIES AND MANAGEMENT 



renewed and the excess water is permitted to pass away 

 quickly in the drainage channels. 



FIG. 63. Area of land nearly level, but having compact subsoil with 

 undulating subsurface, thereby causing wet pockets that force 

 plants to form short roots. Weeds are abundant in such areas. 

 Drainage removes the water and permits deeper penetration of the 

 plant roots, thus enlarging their feeding zone. 



7. The improved ventilation and higher temperature 

 due to drainage promote the activity of decay organisms, 

 by which fresh organic matter is changed into forms that 

 may be used as food by crops. This aids in the formation 

 of humus, with its beneficial physical effects on the soil. 



8. The higher temperature, better ventilation, better 

 distribution of moisture and of decayed organic matter, 

 together with the deeper penetration of roots, make avail- 

 able a larger amount of mineral elements from the soil 

 particles. 



9. It may now be recognized that there is a distinct 

 sanitary aspect to soil management. The accumulation 

 of materials of a toxic nature is promoted by poor drain- 

 age, and their destruction is hastened, and perhaps in 

 part their formation is prevented, by the conditions 

 that accompany good soil drainage. 



10. Drainage reduces heaving. Heaving, or the lifting 

 of crops by frost action in the soil, indicates the presence 



