SOIL CHANGES DUE TO IRRIGATION 103 



of a very fine texture. When dry, it crusts and forms a 

 hard covering, which does not readily admit water or air 

 into the soil. This necessarily interferes seriously with 

 plant-growth. One season's irrigation is not greatly 

 injurious, but if repeated year after year, unless proper 

 cultural treatments are employed it may result disas- 

 trously. 



Another danger, of less importance, resulting from the 

 use of water containing much suspended matter, is that 

 occasionally the finely suspended matter clings closely 

 around the roots of the plant, and, as it dries and con- 

 tracts, injures the plant mechanically; or it may produce 

 a type of sun-scald, not yet clearly understood. It is 

 not wise to apply to young plants during a period of 

 high temperature an abundance of water heavily charged 

 with suspended matter. (Fig. 22.) 



74. Cultural treatment of sediments. It is not, 

 however, a very difficult problem to meet and overcome 

 this condition. The annual silt deposit should be plowed 

 into the soil thoroughly each fall or spring, and, to keep 

 the top soil open, thorough cultivation should be prac- 

 tised throughout the growing season. It has been observed 

 that fields of wheat, irrigated with water rich in mud, 

 have produced unusually large crops wherever the sedi- 

 ment was plowed in from year to year, and the soil thor- 

 oughly disked or harrowed in the spring after the high- 

 water irrigation, with its load of silt, had been applied. 

 The young wheat is not injured materially by such early 

 harrowing, and the advantages resulting from the breaking 

 of the silt crust are shown in unusually large crops. On 

 the other hand, an alfalfa field, cultivated in the old- 

 fashioned way, that is, which receives no cultural help 

 throughout the season, is soon made to suffer severely 



