226 Irrigation and Drainage 



The moisture contained in the fallow ground, determined at 

 the same time, shows how much water such a soil may hold 

 against a drought and against percolation below root action. 



The amount of moisture, too, in this fallow ground happens 

 to stand just at the under limit for most vigorous plant -growth 

 in this type of soil, while the upper limit is given in the table 

 below for comparison : 



Showing upper and lower limits of best amount of soil moisture for one type of soil 



Kind and depth Lower limit of Upper limit of Available 



of soil soil moisture soil moisture soil moisture 



PER CENT PER CENT LBS. PER CU. FT. 



Clay loam, first foot 17.01 25-77 6.92 



Reddish clay, second foot 19.86 24.3 4.112 



Sandy clay, third foot 18.56 24.03 5.722 



Sand, fourth foot 15.9 22.29 6.786 



Total 23 55 



It will be seen from this table that to bring the surface four 

 feet of soil from the lower limit of the best productive stage of 

 water content to the upper limit requires an application of 23.55 

 pounds per square foot, or a depth of irrigation equal to 4.527 

 inches. 



It is quite certain that with a greater distance to standing 

 water in the ground, the 4th foot, and probably also the 3d foot, 

 could not have retained the amount of water shown by the table ; 

 and, hence, that an irrigation of 4.5 inches on such a soil would 

 have resulted in some loss by percolation below the depth of 

 root feeding. 



If it should happen that a soil like the one in question be- 

 came as dry as is shown in the table on page 225, then the depth 

 of irrigation required to bring the moisture content up to the 

 upper limit of productiveness would be for the maize 11.37 inches, 

 and for the clover 9.39 inches, supposing the ground- water to be 

 at the time not more than 7 feet below the surface. 



It follows, therefore, from the observations and data pre- 

 sented, that the amount of water required for one irrigation, 

 where the soil has not been permitted to become too dry, ; and 



