262 



SOIL PHYSICS AND MANAGEMENT 



the water of streams carries but little soluble material, but in arid 

 and semi-arid regions, where the great necessity for irrigation 

 exists, both soil and water may contain alkali in considerable abun- 

 dance. While the excess of alkali in irrigated lands is due usually 

 to the salts in the soil, yet it is in many cases due in part, and some- 

 times wholly, to the salinity of the water which is being used for 

 irrigation. The salts thus carried accumulate in the soil, pro- 

 ducing very injurious results. Forty grains of salts per gallon is 

 usually assigned as the limit for irrigation waters. This, however, 

 depends upon the character of the substances in solution. In Cali- 

 fornia the limit lies in all cases below 70 grains. The clanger of 

 using irrigation water containing considerable salts depends very 

 largely upon the drainage of the land irrigated or the methods of 

 preventing their accumulation. 



Suspended Matter in River Waters l 



Many streams whose waters are used for irrigation carry more 

 or less material in suspension which becomes a very important factor 

 in maintaining the fertility of the soil. The amount of sediment 

 carried in suspension by various streams is given in the above 

 table. 



Composition of River Sediments. Many river sediments have 

 been analyzed in the United States, in Europe, and in Egypt. The 

 results show that river muds are somewhat richer in the essential 

 plant food elements than the ordinary fertile soils from which the 

 water comes. It has been estimated by Forbes that the market 

 value of the fertilizing constituents in three samples of Salt TJivor 

 mud, to the acre-foot of water, varied from $7.08 to $25.51. 2 When 

 the fertilizing value of these sediments is considered in connection 



