238 THE PRINCIPLES OF SOIL MANAGEMENT 



As the demand for produce of high value increases, 

 the maintenance of the moisture supply of the soil by 

 irrigation may well be extended on large areas of soil 

 in so-called humid regions, as well as in arid sections. 



The highest type of soil-management must seek to 

 utilize the available water-supply for crops in the three 

 ways outlined above, that is, by increasing the water 

 capacity of the soil, by eliminating as far as possible 



Fig. 76. An example of poor drainage on level clay soil. 



the losses by percolation and evaporation and, lastly, 

 by supplying any deficiency which may still exist by 



wise irrigation. 



107. Means of decreasing the water content of the 

 soil. The removal of water from the soil may be 

 accomplished in two general ways. These depend upon 

 facilitating the two types of loss, by percolation and 

 evaporation, described on page 191. They are: (1) 

 Drainage. (2) Surface culture, to hasten evaporation. 



108. Drainage by ditches. Drainage consists essen- 



