120 



THE BOOK OF WHEAT 



ments by ridges. The highest compartment is flooded to the 

 top of the ridge, which is then opened on the lower side. The 

 water thus passes into the next compartment, and this pro- 

 cedure is continued until all the compartments are irrigated. 

 If the land is properly prepared and irrigated before the 

 wheat is sown, two subsequent irrigations will make a good 

 crop. When the soil is thus used as a storage reservoir, in 

 parts of Kansas and California no irrigation is needed between 

 planting and harvesting. 



Alkali. Arid region soils are usually rich in mineral in- 

 gredients. This is because such soils originated in the de- 



THE FURROW METHOD OF IRRIGATING 



composition of rocks in regions where the rainfall is too scanty 

 to wash out the soluble elements as in humid regions. The 

 soluble salts are naturally distributed throughout the soil, and 

 are not harmful until the application of irrigation water. They 

 are then leached out of the higher grounds and concentrated 

 in the lower lands. Evaporation tends to bring them to the 

 surface. Many irrigation waters also contain much salt in so- 

 lution, which results in a further deposition of salt. The result 

 of these factors is often ruinous to vegetation. Many thou- 

 sands of acres have been thus rendered unfit for cultivation in 

 the United States, and the agricultural industries of 59 vil- 

 lages in India were wholly or partly destroyed by the rise of 



