12 Irrigation and Drainage 



well believe that one of the chief causes which has led 

 the higher plants to send their roots foraging so deeply 

 into the ground is this great need of water in the sur- 

 face layer, where the nitrogen suppliers dwell, and for 

 the express purpose of not drawing upon this supply 

 too extensively, and thus leaving the surface soil to 

 become too dry. It is true that when heavy rains 

 come, or when irrigation waters are applied which lead 

 to the percolation of water downward, the nitrates 

 which have been formed at and near the surface are 

 dissolved and more or less completely washed more 

 deeply into the ground, where the deep -running roots 

 are in position to take advantage of them and prevent 

 their being lost ; and thus a double gain is secured. 



Let us call attention to another important principle. 

 In the soils which have been highly manured, or which 

 are naturally well supplied with organic matter ready 

 for decay, large amounts of nitrates are rapidly formed. 

 Under such conditions the moisture which invests the 

 soil grains rapidly approaches saturation, and finally 

 reaches a point when it is carrying so many salts in 

 solution that the water is no longer suitable for the 

 use of the germs which have given rise to the salts, 

 and their activities are on this account brought to a 

 standstill. But let a rain come which produces perco- 

 lation, or let the field be irrigated sufficiently to pro- 

 duce the same effect, and at once the salts which have 

 been inhibiting the nitrate -forming process are washed 

 out and a fresh supply of water is left, which at once 

 becomes a stimulus for increased activity, while the 

 ready -formed salts containing nitric acid are carried 



