SOIL CHANGES DUE TO IRRIGATION 69 



becomes smaller and smaller. This point of largest vol- 

 ume coincides almost exactly with the point at which the 

 penetration of the soil is easy; which, as has been said, 

 is the point of optimum water content. The farmer who 

 desires to keep the soil in the best tilth, from top to lower 

 depths, in order to increase the air space in the soil and 

 to permit the easy penetration of roots, can do so by main- 

 taining in the soil a moderate quantity of water, between 

 the point of lento-capillarity and maximum capacity, 

 somewhere in the neighborhood of the field capacity. 

 The farmer who depends upon the rainfall and, therefore, 

 cannot control his water supply, cannot well maintain 

 the soil in this good condition. The irrigation farmer, on 

 the other hand, who may, usually, apply water at will, 

 can so plan his irrigation, when he knows his soil, as to 

 maintain the land during the larger part of the season 

 in the most desirable condition for plant-growth. (Fig. 16.) 



46. Effect on top soil. Through the top soil, whether 

 under irrigation or rainfall, all water added to a soil 

 ordinarily passes. The top soil first becomes completely 

 saturated, then it dries out quite thoroughly, and the 

 process is frequently repeated. It follows, therefore, that 

 the top soil is subject, almost from day to day, to the 

 greatest changes, physical, chemical and bacteriological. 

 In the greater depths, more water is held over from irri- 

 gation to irrigation, and consequently the changes due 

 to varying moisture content do not go on to the same 

 degree. It is interesting to note that, in a soil properly 

 irrigated, the lower layers of soil to the depth of 10 to 12 

 feet are kept, from irrigation to irrigation, within 1 to 4 

 per cent of the point at which the structure of the soil 

 is the most desirable. 



It is a common observation that irrigation tends to 



