74 IRRIGATION PRACTICE 



make their constitutents more easily available to plants. 

 Under humid conditions, with a high annual rainfall, 

 the soluble soil constituents thus formed have been 

 largely washed out of the soil into the country drainage 

 and finally into the ocean. In arid districts, with a 

 scanty rainfall and less ample drainage, most of the soluble 

 soil constituents remain in the soil. Humid soils, there- 

 fore, contain little soluble matter; arid soils, relatively 

 much. This is one of the chief differences between the 

 two classes of soils. Normal arid soils do not, however, 

 contain large proportions of soluble matter. In an investi- 

 gation of a great variety of fertile Utah soils, 50 grams of 

 soil were shaken with 500 cc. of distilled water for twenty- 

 four hours. The soluble matter thus extracted varied 

 from .2 per cent to .48 per cent. Under more abnormal 

 conditions, as will be explained in the chapter on alkali, 

 soluble matter may be present to the extent of several 

 per cent, and then the soil must be subjected to special 

 treatment before it can serve the farmer. 



52. Continuous solubility of soils. It is practically 

 impossible to wash the soil so thoroughly as to remove 

 from it all substances capable of going into solution. 

 Many experiments have been made on this subject, all 

 with fairly concordant results. For example, Schultze 

 treated a given weight of soil with a definite quantity of 

 water for six days, after which the solution was filtered 

 off and analyzed. This was repeated every six days dur- 

 ing six periods. During the first treatment, 1,000,000 parts 

 of solution contained 535 parts of mineral matter dis- 

 solved from the soil; during the second, 120; then 261, 203, 

 260, and 200 parts during the sixth period. That is, 

 while the first treatment dissolved most, every successive 

 treatment dissolved considerable quantities of soil con- 



