SOIL CHANGES DUE TO IRRIGATION 93 



acids, are add waters. This classification may be extended 

 to cover any water as soon as its predominating con- 

 stituent is known. The above are the leading classes. A 

 typical analysis of a water in several of the above classes 

 will be found in the last four columns of the preceding 

 table. 



This classification of natural waters is very useful in 

 irrigation practice, and especially important in consider- 

 ing the alkali question. For the purposes of this chapter 

 it is sufficient to make clear that practically all known 

 natural waters, unless rain-water and water coming 

 immediately from the melted snow be excepted, contain 

 varying quantities of all the essential elements of plant- 

 growth. Moreover, the variations in the proportions of 

 the constituents of water are so great that while the waters 

 may be roughly classified as chloride, sulfate or car- 

 bonate waters, there is a host of intermediate kinds which 

 overlap two or more groups. For an exact understanding 

 of the chemical behavior of an irrigation water on the 

 soil or crop, an analysis of the water in question must 

 be available. 



67. Plant-food value of irrigation water. The infor- 

 mation found in the preceding table makes possible some 

 interesting calculations. The quantity of plant nutrients, 

 such as nitrogen, potassium, phosphorus and lime removed 

 from an acre of soil by some of the common crops, has been 

 computed by Warington. His results, obtained under 

 humid conditions, do not differ greatly from those that 

 might be obtained under irrigated conditions, and, until 

 data from irrigated crops are obtained, may be used with 

 approximate accuracy. A crop of wheat yielding thirty 

 bushels to the acre requires at least about thirty pounds of 

 potash, ten pounds of lime, twenty pounds of phosphoric 



