268 Irrigation and Drainage 



It sometimes happens that waters draining from 

 swamp, lands where there has been considerable stag- 

 nation, or where there are too strong solutions of 

 humic acids or salts of iron, are not suitable for irri- 

 gation purposes, and must be avoided. In portions 

 of Europe, too, there are streams used for irrigation 

 which are known as "good" streams and "bad" 

 streams. Crops irrigated from one produce heavier 

 yields than when irrigated from the other, and cases 

 are cited where the differences in yield are so large 

 that they can hardly be assigned entirely to difference 

 in the amount of plant -food carried by the two. 



