116 IRRIGATION PRACTICE 



days; and 17 per cent the last four days of the two-week 

 period. Similar proportional figures were found for 

 longer periods. On a deep soil, of good water-holding 

 power, during fourteen days after irrigation, 62 per cent 

 of the total loss occurred during the first seven days, and 

 only 38 per cent during the second week. Such figures, 

 which might be multiplied by drawing from many experi- 

 ments on the subject, show that methods designed to 

 conserve soil moisture should be put into operation as 

 soon as possible after irrigation. Especially to prevent 

 direct evaporation, the soil should be cultivated as soon 

 as possible after irrigation in fact, as soon as the soil is 

 dry enough to support the cultivator without injuring the 

 structure of the soil. 



82. The depth of soil. The deeper the soil, the aver- 

 age percentage of soil moisture being the same, the larger 

 is the loss of water in a given period of time. This law is 

 easily understood. If two soils weighing 100 pounds to 

 the cubic foot are 1 and 2 feet deep respectively, and both 

 contain an average of 20 per cent of moisture, they will 

 contain respectively to the square foot of surface, and to 

 their full depth, twenty and forty pounds of water. Dur- 

 ing the first day, each soil will lose, say, two pounds of 

 water. There will remain, at the beginning of the second 

 day, in the shallow soil, eighteen pounds, and in the deep 

 soil, thirty-eight pounds of water, or 18 per cent and 19 

 per cent the deeper soil having a higher percentage of 

 moisture. During the second day, then, in accordance with 

 the law of the initial percentage, the deeper soil will lose 

 more water than will the shallow soil. The difference 

 will become more marked with each passing day. Other 

 factors enter in, as the fuller development of plant roots 

 in deep soil, but, assuming all other factors to be the same, 



