SOIL WATER 93 



a single day. If the sun is very hot, the amount of 

 water lost by evaporation is large; even thorough 

 tillage cannot entirely prevent the escape of water. 

 This means that the supply of film water in the 

 surface soil must be quietly replenished from be- 

 low, else the plants will suffer. 



In some soils the water table is many feet 

 below the soil in which the roots of plants 

 feed; in such cases there is especial need 

 that the water be able to move rapidly through 

 the soil. Very sandy soils not only do not hold 

 much water, but also have little power to transport 

 it by capillary action. Very stiff clay soils, on the 

 other hand, while they hold a large amount of 

 water, regain water very slowly after having be- 

 come dry, so that they frequently suffer much in a 

 drought. When a clay soil dries it shrinks, and 

 cracks appear not merely on the surface, but also to 

 a depth of several feet. These cracks let in air 

 which still further dries the soil ; the roots of plants 

 may also be torn apart and exposed, All kinds 

 of loams have excellent water-carrying power; 

 this fact, together with their mellowness and fer- 

 tility, makes them among the most valuable of 

 farm soils. 



How to Test the Water-holding Capacity of 

 Farm Soils. The points that have been brought 

 out in the preceding paragraphs will be made 

 more concrete to the reader if he will try a few 

 simple experiments. Get a quart each of stiff clay, 

 sana, and the black, spongy humus beneath 

 forest trees. Put the three samples into a slow 

 oven and dry them for two or three hours, or until 

 they appear perfectly dry. Stand three lamp 

 chimneys in pans and fill one with the dry humus, 

 one with the dry sand and one with the dry clay. 



