«?%■»'. 



Soil Conservation Service 



Poor drainage in the soil has restricted efficient use of this entire field. 



Natural Drainage 



Good soil drainage is important to the farmer. It is important for a 

 soil to hold some moisture for use by plants, but it is also important for the 

 soil to contain air as well as water in the pores. To get this condition, 

 the soil must be able to allow excess water to move downward readily. When 

 the soil is too wet, the air is replaced by water. If this condition continues 

 for several days, many plants suffer or die. Poor drainage limits the kinds 

 of plants which will grow successfully. To the farmer, poor drainage pre- 

 sents another handicap, too. It is much more difficult for him to work the 

 wet, muddy fields, and for this reason crops are often planted too late. 



Poor natural drainage in the soil may result from several causes. 

 One of the principal causes is a high water table. Shallowness to bedrock 

 or to a compact layer markedly influences natural drainage. If the surface 

 soil takes in water faster than the subsoil can transmit it, a temporary 

 perched water table may develop, entirely separate from the regular ground 

 water table. A perched water table has the same effect, however, in drown- 

 ing the roots of the plants. Surface slope is another condition affecting 

 drainage. On sloping land some of the excess water can run off over the 

 surface. On level areas or in basins, however, any water falling on the land 

 in the form of rain or snow, or running onto the land from adjacent hills 

 must either soak away through the soil or lie there until it evaporates. In 

 sandy soils with favorable soil structure, such water may disappear rather 

 quickly, but on silty or clayey soils, temporary ponds may stay for weeks 

 and the soil beneath them will be saturated. 



Another condition which may cause poor natural drainage is seepage. 

 Water may be moving downhill through some deeply buried layer so that 

 plants at the ground surface are not affected until the water breaks out to 

 the surface, forming a seep spot or wet-weather spring. Such wet spots 

 interfere with the use and tillage of a field. 



In this bulletin, soils are considered in four classes according to wetness. 

 These include: (1) zvell-d rained soils; (2) moderately well-drained soils 

 which are saturated to within 18 inches of the surface for periods of a 



