50 IRRIGATION PRACTICE 



to less than half of that from a non-cultivated soil. Scores 

 of other investigators have demonstrated that the mulch 

 formed by cultivation reduces largely the evaporation. 



This saving, due to mulching, is easily understood. 

 As has been explained, soil moisture is held as a film 

 around the soil particles. Water moving toward the soil 

 surface must pass from particle to particle through the 

 narrow films at the points of contact of the soil particles. 

 The smaller or the fewer these points of contact, the more 

 difficult is the upward movement of the water. If water 

 were passing through a large tube into several smaller 

 tubes, the flow of water would be retarded. When the top 

 soil is loosened, the points of contact between the loose soil 

 above and the compacted soil below become reduced. 

 At the zone of loose earth, the ascending water finds it 

 difficult to pass through the fewer points of contact, and 

 at the same time to maintain its rate of flow. The more 

 thoroughly the soil is cultivated, that is, the fewer the 

 points of contact, the more difficult will the movement 

 become, and the more greatly will the evaporation 

 be reduced. 



Likewise, as a soil becomes dry, the flow of moisture 

 through it is lessened. This is clearly understood when it 

 is recalled that a dry soil means a soil with very thin 

 moisture films around the particles. Water passing 

 through thin films encounters much friction, and the 

 rate of flow is diminished. Stirring the top soil tends to 

 dry it out very rapidly, and to leave a very dry mulch, 

 through which water can pass only with difficulty. Culti- 

 vation, therefore, retards evaporation by breaking the 

 points of contact between the upper and lower soil layers 

 and by drying out the loosened top layer. 



It is true that from the surface of every soil particle, 



