Conditions Modifying Duty of Water 205 



in still more humid climates, but where there are frequent 

 recurrences of intervals of drought, the amount of water which 

 must be used in order to secure full yields will be relatively 

 larger than would be required in rainless countries, because the 

 surface losses of moisture will be relatively greater, as well as 

 those from percolation and drainage. 



4. The character of the subsoil, as well as that of the surface 

 soil, is an important factor in determining the duty of water, 

 especially in the hands of the unskillful irrigator, and par- 

 ticularly so if he possesses no knowledge, or execcises poor 

 judgment, regarding the water-holding power of the soil to 

 which the water is being applied. Where the texture of the 

 subsoil is coarse and its water -holding power small, it requires 

 the best of judgment, both in regard to the amount of water 

 which may be applied at one time and as to the rate at which it 

 should be led over the surface or along the furrows, in order 

 that there shall be no waste by percolation below the depth of 

 root feeding. 



It has been pointed out that even moderately fine sands 8 

 feet above the ground water quickly lose by percolation all but 4 

 per cent, or less, of their dry weight, of the water given to them. 

 Since plants will suffer for water when such soils have lost all 

 but 2 to 3 per cent of their dry weight of the soil moisture, it 

 follows that in 4 feet in depth of such a subsoil there is room for 

 only 1.5 to 2 per cent of water, or 1 to 1.5 inches, to be applied 

 at one time, without loss taking place by percolation below the 

 depth of root action. It is plain, therefore, that on open soils 

 the duty of water will be relatively small, unless great skill and 

 rare judgment are exercised in its application. 



5. The frequency and thoroughness of cultivation after irriga- 

 tion is another factor which will modify the duty of water. For 

 the effectiveness of soil mulches is modified as well by the fre- 

 quency of stirring as by its depth. The force of this statement 

 will be better appreciated when the results given in the table 

 which follows have been considered; 



