178 THE SOIL: INORGANIC MATTER 



pull of gravity which carries it downward through the pore 

 spaces and channels in the soil. This bodily movement of 

 water carries with it the substances in solution which are 

 not absorbed as they go. The water may go directly down 

 in a vertical direction or more nearly horizontally if any- 

 thing like an impervious subsoil or entrapped air deflects 

 its course. Gravitational movements cease as soon as the 

 excess water drains off and the surface tension overcomes the 

 pull of gravity. It is by this movement that soils lose plant 

 foods which are not absorbed. 



The surface of a liquid acts as if it exerted at all times a 

 certain tension or pressure on the liquid below. If a drop 

 of liquid is free to take any position, it assumes the spherical 

 form in which the surface tension is uniform throughout, 

 that is, it is in equilibrium. When the surface of a liquid 

 is forced to assume various shapes, as in the case of film 

 water covering soil particles, there is exerted an unequal 

 tension. The more curved the surface the greater the 

 tension. In a soil the surfaces at the top are more curved 

 than they are lower down where the films are thicker. This 

 means that the pull of the surface tension at the top will 

 tend to raise water until the gravity pull balances the ten- 

 sion pull. As water evaporates, or plant roots absorb it, 

 the increasing tension due to thinner films and consequent 

 greater curvatures, draws up more water. This movement 

 of water, of course, carries with it dissolved plant food. 

 Theoretically this movement can take place in any direction 

 and does under some circumstances, but under ordinary 

 circumstances the increasing surface tension occurs on top 

 of the soil due to evaporation, and hence the water move- 

 ment is upward. 



(6) Movement by Diffusion. — It is a property of sub- 

 stances, or rather of the molecules of such substances in 

 solution, to move within the solvent from a region of greater 

 concentration to one of less concentration in that particular 

 compound. That is, sodium nitrate, for example, tends to 

 move from that portion of the soil moisture where its con- 

 centration is considerable to other portions where there is 

 little or no sodium nitrate. This movement is called diffu- 



