32 GENERAL CONSIDERATIONS 



depends quite as much on its consistency and water- 

 holding capacity as on its chemical composition. 

 During and after a heavy rain there is, of course, a 

 movement of the water downward through the soil. 

 This movement is due to the force of gravity. Its 

 amount and rapidity will depend on the character 

 and fineness of the soil particles and on their 

 arrangement. Water sinks much more readily into 

 a loose mellow soil than into one that is hard and 

 compact. Not all the water, however, that falls on a 

 soil passes through it. The attraction of each soil 

 particle is sufficiently strong to hold against the 

 force of gravity a thin film of water covering its 

 entire surface. The smaller the soil particles, the 

 greater will be their aggregate surface and the 

 greater, consequently, w^ill be the amount of moisture 

 so retained. When the rain ceases and the surplus 

 water has settled into the lower levels, a counter up- 

 ward movement of moisture begins. The surface 

 particles of the soil lose their film of moisture by 

 evaporation. The soil having been compacted by 

 the rain, these particles are in close contact with 

 those below and a portion of the film of water which 

 surrounds these lower particles arises by what is 

 called capillary attraction to replace the moisture lost 

 by evaporation, just as the oil in a lamp wick rises to 

 replace that which is consumed in the flame. These 

 lower particles that have now become partially dried 

 call in turn on those below and thus a capillary 

 pumping process is set up that if unchecked, will, in 

 time, return to the atmosphere all of tlie moisture 

 thus held in suspension in the soil. If, liowever, a 



