66 SOILS AND FERTILIZERS 



70. Height of water column and capillary movement. — 

 Gravity opposes the upward movement of water and conse- 

 quently the higher water rises the more slowly it moves. 

 This has been demonstrated by measuring the quantity of 

 water that evaporated from the surface of columns of sand 

 of different heights, the rate of loss by evaporation indicat- 

 ing the degree of rapidity of movement. 



Table 13. — Evaporation from the Surface of Sand Columns 



of Different Heights, their Bases being in Contact with 



Free Water 



This has a practical significance in dry weather when the 

 moisture supply for plants is drawn largely from the water 

 stored in the lower soil. The lower the water level becomes, 

 the more slowly does the moisture rise to the surface soil where 

 are to be found the larger part of the roots of many plants. 

 Fortunately, however, as the soil dries out, the roots go some- 

 what deeper, so that they in part overcome this difficulty. 



71. Gravitational water. — It has already been said that 

 gravitational, or free water, is the water in excess of the cap- 

 illary water and is constantly moving downward, thus pre- 

 venting the soil from becoming saturated owing to the 

 inability of the water to escape. It is very desirable that 

 the gravitational water shall not remain in that part of 

 the soil in which plants have their roots. A saturated 

 condition of the surface soil is very injurious to most agri- 

 cultural plants. In this respect there is a great difference 



