CONDITIONS AFFECTING SOIL FEETILITY 47 



Water in the soil. A part of the water which falls in rain 

 immediately evaporates, a part sinks into the soil, and the rest 

 drains away into streams and ponds. This latter is known as 

 the run-off. The water that sinks into the soil is called the 

 percolating water. The run-off is greater in clay than in sand, 

 on slopes than on the level, and in cultivated soil than in 

 pasture or woodland. Clay gullies more easily than sand be- 

 cause the water cannot penetrate it so readily. Pastures and 

 woodlands protected by the close ground cover are scarcely 

 affected by a rain that may wash out the crops in cultivated 

 fields. The water that enters the soil exists there in three 

 forms, known as free, capillary, and hygroscopic water. Free 

 water responds to the pull of gravity and goes downward until 

 it reaches a point where the soil is saturated. Capillary water 

 is not affected by gravity, but moves from moist to dry regions 

 like a drop of water on blotting paper or the oil in a lamp 

 wick. Much dissolved plant food is brought up from the sub- 

 soil by the capillary water, and the alkali in certain soils is 

 due to the same cause. Hygroscopic water is not affected by 

 either gravity or dryness. It clings closely to the soil particles 

 and binds them into soil crumbs. It is present even in air-dry 

 soils and roadside dust, but is not available to plants. Plants 

 depend largely for their moisture on the capillary water in the 

 upper layers of soil, though some of the free water deeper in 

 the earth may return by capillarity. Water rises highest by 

 capillarity in fine-grained soils. Clays and silts can lift water 

 in this way from six to ten feet. In sandy soils water will 

 not rise more than two feet. 



The water table. The free water sinks downward until it 

 reaches a point where all the spaces between the particles are 

 filled with water, or where, as we say, the soil is saturated. 

 This point is known as the water table, or permanent ground 

 water. It now spreads out laterally and slowly drains off by 

 seeping out of the soil at the edge of streams, lakes, and 



