Irrigation. 161 



Brak and Irrigation Relationships. 



Wlieii rocks become weathered into soil a certaiu amount of 

 soluble substance is formed. Part of this soluble material is 

 useful to plants. jNTow in regions of abundant rainfall most of 

 these soluble materials are washed out of the soil and carried 

 by percolating waters into the natural drainage channels (the 

 rivers), and thence to the sea. In the arid and semiarid regions 

 the natural precipitation is not sufficient to bring about this 

 leaching- out of soluble substances. There the water passes down 

 ;i short distance into the soil, where part of it is taken up by plant 

 Kxjts, while a small amount rises to the surface and is evaporated. 

 Soluble substances tend to accumulate, therel^'ore, iu soils of jjry 

 ipg-ions, with the result that man finds these soils very productive 

 when first he starts to irrig-ate. Frequently the yields start to decrease 

 ;ifter a few years. Let us inquire into the reason why this decrease 

 lakes jdace on irrigated lands. We shall find usually that the diop 

 is due either to the deterioration of the physical condition of the soil, 

 due largely to the use of excessive quantities of water, or to the 

 accumulation of brak or to both. AVhen the farmer applies only 

 medium quantities of fresh water to an arid soil containing the usual 

 amount of soluble material, he cannot run much risk of causing the 

 soil to become brak, provided he cultivates the surface or grows a crop 

 like lucerne, both of which will tend to reduce evaporation. Usi; of 

 large quantities of fresh water : What usually happens, though, is 

 that the farmer, believing that yield will be proportional to the quan- 

 tity of water applied, uses excessive amounts of water at frequent 

 intervals and pays no heed to the drainage nor does he troal)le much 

 about surface evaporation, (a) Where drainage is poor: If ihe water 

 ill excess of that sufficient to satisfy the maximum capilbny capariiy 

 of the soil is unable to escape into some natural drainage channel, it 

 accumulates as free water in the soil and rises into the root zone, with 

 Ihe result that deep-rooted"" plants like lucerne are drowned out, as it 

 were. Also, on account of the surface soil being more or less kept con- 

 stantly moist, a large quantity of water is evaporated at the surface, 

 leaving behind the soluble materials that have been dissolved out of 

 the soil. As time passes this accumulation of sails at the suiface 

 becomes so great that the plants are injured. The water-table rises 

 close up to the surface, and as a result of these two the soil is 

 more or less ruined. Use of large quantities of allmline or 

 hrak water : If under the above conditions brak water be used, then 

 the time taken to injure the soil will be much shorter, because the 

 salts already contained in the water are added to those dissolved out 

 of the soil, so that brak accumulates more quickly. 



If the brak water contains carbonate of soda, not only do we "et 

 accumulations of brak, but also the rapid destruction of the desirable 

 crumb structure of the soil, with the result that the soil becomes moie 

 or less puddled, so that even air and water can penetrate only with 

 extreme difficulty into the soil. Here the complete ruin of the soil 

 would be brought about, (h) Excessive use of fresli water where the 

 sail lias good vnderdrainage : The danger in this case is that associ- 

 ated with the Ineaking down of the desirable crumb structure of the 

 soil and the leading out of solulile [ilant food materials. But while 

 the injury to the farmer using ex<-essive quantities of water i.i:i\ not 



