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-colour being due to the organic matter dissolved by the alkaline carbonate, 

 while the chloride and sulphate, which do not dissolve organic matter, 

 form what is called white brack. In small quantities these alkaline salts 

 are contained in nearly all soils, being derived from the insoluble sodium 

 silicates, which slowly decompose, under the influence of air, water, and 

 the solar heat, into the soluble salts above mentioned. Hence there is 

 ver going on a constant addition to the soil of soluble sodium salts, and, 

 unless means exist for a removal of these salts, as constant and as rapid 

 as their addition, they would in time so accumulate in the soil as to 

 render it incapable of cultivation. 



As a rule, natural drainage is sufficient to remove the noxious salts 

 from the soil, and they are thus continually being carried down to the 

 ocean by the rain water in which they are dissolved. Exceptions to this 

 rule, however, exist: for, when the surface soil is porous but has an im- 

 permeable layer underneath, the alkaline salts accumulate as it were in 

 a basin from which there is no outlet; or the soil may itself be so com- 

 pact, or otherwise constituted, as to prevent proper drainage;* lastly, 

 a high water table in the subsoil may prevent the exit of the injurious 

 compounds f : in all these cases, unless a remedy be provided, there can 

 be but one result, an accumulation in the soil of the salts which render 

 it brack. 



When, therefore, drainage of a soil intended to be irrigated, becomes 

 impossible, the question to be considered is whether the alkaline salts are 

 collected at any particular level, or whether, on the other hand, they are 

 mere or less evenly distributed throughout a mass of soil several feet in 

 thickness. In such localities where rain is frequent and the temperature 

 not excessive, the probability is that the salts will be fairly evenly dis- 

 tributed throughout the soil, so that comparatively little, if indeed any, 

 injurious effect on the vegetation ensues. If, however, the rainfall is 

 scanty and the climate warm, or if there even be a brief period of heavy 

 rainfall succeeded by a dry season, time and opportunity are afforded for 

 the harmful salts to rise to the surface, by capillary attraction, in the 

 way already outlined, and accordingly the surface soil is charged with 

 such an accumulation of salts that cultivation is greatly hindered, if not 

 rendered absolutely impracticable. 



Under these circumstances it becomes an important matter to con- 

 sider what proportion of sodium salts a soil may contain and still be 

 capable of successful cultivation. Professor Hilgard has devoted con- 

 siderable time and attention to investigating this point in connection with 

 the alkali soils of California, and he states (Bulletin No. 128) : " Barley 

 failed to grow where the total salts were '203 per cent., but gave a full 

 crop where they were '159 per cent, (half of which was carbonate). Wheat 

 is rather more sensitive; maize fails on slightly alkaline land; but certain 

 sorghums do well on mild white alkali (i.e., not containing much car- 

 bonate)." 



In India, Dr. Leather's experiments have led him to conclude that 

 germination was possible where the percentage of carbonate did not ex- 

 ceed '4, nor the sulphate and chloride 1 per cent. In after growth '2 



* Such a possibility demonstrates the importance of properly ascertaining, by 

 mechanical analysis or otherwise, the physical condition of the soil all the way 

 from the surface, down to a depth of from four to six feet, wherever an irrigation 

 project is under consideration. 



t This again shows that if a soil is likely to prove brack under irrigation, no 

 investigation can be considered complete unless a series of determinations of the 

 height of the water table in the subsoil has been made. 



