76 Agricultural Chemistry. 



thrown-up earth ridges. On these ridges various crops, especially 

 vegetables, are grown, with the resultant utilization of the 

 manurial constituents of the sewage. 



The amount of suspended matter in river water varies enor- 

 mously, depending upon the rain fall, the character of the sur- 

 rounding soil, and other circumstances. Soft waters or those con- 

 taining carbonate of soda, are often muddy, while hard waters 

 tend to deposit their suspended clay and become clear. In some 

 cases the quantity of suspended matter is very great, and a dense 

 muddy river, if it over-flows its banks, deposits upon the soil 

 a layer of finely divided particles of materials brought down 

 from higher up the valley. The sediment is often rich in plant 

 food and forms an important fertilizer. In some places in Eng- 

 land, land is systematically treated with the flood water in order 

 to increase the thickness of the soil. The process is known as 

 "warping" and the "warp" soils are extremely rich and fertile. 

 The Nile river in Egypt affords, on a large scale, a still better 

 example of a river used in this manner. 



In countries of limited or unevenly distributed rainfall, as in 

 many of our western states, irrigation is often practiced. In 

 this case, since there is very little drainage, the composition of 

 the water used is of importance. If the water is charged with 

 common salt, sodium sulphate or sodium carbonate, there is grave 

 danger of the surface soil, through the prolonged evaporation 

 and concentration of the water, becoming charged with the sol- 

 uble matter to such an extent as to seriously interfere with plant 

 growth. The soil is then said to become "alkali." This con- 

 dition may also arise from accumulation-in-place of the salts, 

 produced by the weathering of the rocks. The slight rain fall is 

 insufficient to produce percolation through the soil arid carry 

 the accumulating salts into the under ground water system. This 

 produces 3 sterile condition which may be caused by sodium 

 sulphate and chloride (white alkali), or by sodium carbonate 

 (black alkali). 



