38 



Further than this, if irrigation is practised during the growing 

 season only, and this water also is evaporated from the soil in addition 

 to the natural rainfall, it is plain that the amount of soluble salts in 

 the soil must increase, both on account of that which may have been 

 in the water applied, and that which this additional water may have 

 been instrumental in producing from the soil on the spot through the 

 processes of weathering. 



Indeed, the more we study and reflect upon this problem, the more 

 we are led to fear that in all arid climates, where irrigation is practised 

 it will not be found sufficient to apply simply enough water to the soil 

 to meet the needs of the crop growing upon the ground at the time, 

 but, on the contrary, there must be enough more water applied to take 

 tip and carry away into drainage channels and out of the country to 

 the sea not only the soluble salts which the irrigation waters carry, but 

 also those which it causes to be produced from the soil and subsoil. 

 In other words, it appears that an excess of soluble salts in a thoroughly 

 irrigated field is not only a normal but an inevitable condition, unless 

 sufficient leaching takes place ; and if this is true, the sparing use of 

 water can only increase the number of years required to bring the salts 

 up to the danger point of concentration. 



CONDITIONS WHICH MODIFY THE DISTRIBUTION OF ALKALIES 



IN SOIL. 



If the surface of the ground is kept naked and compact, so that the 

 rate of evaporation may be strong, the alkalies will necessarily be 

 brought to the surface and become concentrated there, hence in posi- 

 tion to do the greatest harm to growing crops. 



If thorough tillage is practised early, so that but little water is 

 evaporated except that which passes through the roots of the crop, 

 then the salts cannot become concentrated in a narrow zone, but, on 

 the contrary, will be left all through the soil where the roots which 

 are taking water are distributed. In those cases, therefore, where the 

 general soil water is not too highly concentrated to permit normal 

 growth, crops may prosper so long as the surface is kept shaded and 

 thoroughly tilled. 



It must be observed, however, and kept in mind, that the roots of 

 plants cannot withdraw moisture from a soil without at the same time 

 tending to concentrate the salts in solution in the zone where the roots 

 do their feeding; hence, that if alkali waters are being used for irri- 

 gation, and in the long run if the purest waters are being used under 

 conditions of no drainage, sooner or later the soil of the root zone 

 must become so highly charged with the alkali salts that reduced 

 yields are inevitable. . . 



KINDS OF SOIL WHICH SOONEST DEVELOP ALKALI. 



Where alkali waters are used for purposes of irrigation, and where 

 eweet waters are being used under conditions of little or no drainage, 

 the clayey soils are the ones which soonest begin to show the bad 

 effects of concentrated salts. This is so for many reasons. 



In the first place, the soils of clayey texture, are not as effective 

 mulches as the sandy soils, hence, even where thorough tillage and 

 ehade are resorted to, there must necessarily be a larger rise of salt- 

 bearing water to the surface to produce accumulation than is the case 

 with the coarse sandy soils. 



