228 MOVEMENTS OF SALTS IN THE SOIL 



subsoil water, the formation of alkali crusts on the surface 

 during summer drought has considerably increased, and in 

 many cases land is now far less fertile than it was before the 

 expensive irrigation systems were introduced. We may lay 

 it down then as a general rule that irrigation, as ordinarily 

 practised, is a very unsuitable treatment for alkali lands, unless 

 free drainage is at the same time established through the soil. 

 There are several ways by which the fertility of an alkali 

 soil may be considerably improved. When the land is con- 

 taminated with salts containing sodium carbonate, it is quite 

 possible by applications of gypsum to convert this into sodium 

 sulphate, which we have seen is far less harmful to crops. 

 Where gypsum can be obtained at a moderate cost, it is 

 often possible to reclaim land from sterility by this applica- 

 tion only, and this has been actually done to a considerable 

 extent in California. In India, unfortunately, gypsum is 

 frequently not easily obtained. The gypsum should be ap- 

 plied in dressings of from 500 Ib. to i ton or more per acre. 

 On unirrigated land the application will probably succeed 

 best when applied just before the commencement of the wet 

 season. Gypsum has no action on sodium carbonate in a 

 dry soil. Where gypsum is costly, it may be usefully spread 

 in smaller quantity shortly before sowing the seed, thus pro- 

 tecting the young plant in its tenderest condition ; or local 

 applications may be made to alkali patches, or round the 

 roots of fruit trees. The effect of gypsum is seen in the 

 greatly improved tilth of the land, as well as in the absence 

 of the corrosive action on plant tissues. 



Gypsum may sometimes be of great use for the destruction 

 of the hard pan common in alkali lands. On a calcareous 

 hard pan it has no effect. In soils containing much sodium 



