42 CHAPTER III 



quantity of salts that may be present without injury to crops 

 varies greatly, depending upon the kind of salts present, the 

 nature of the soil, the crops grown, and the drainage. In any 

 case it is highly important to recognise the brak tendency 

 before definite injury to crops occurs, so that measures can 

 be taken to prevent harmful accumulation at the surface. The 

 economy of methods of reclamation must be carefully studied ; 

 it may frequntly not pay to reclaim cheap land which is badly 

 affected. 



Briefly, the chief measures for preventing future harmful 

 accumulation in present mild cases, and for reclaiming badly 

 affected lands, are as follows : — 



(1) Reduction of Surface Evaporation. — Practise deep and 

 thorough tillage, and maintain an efficient surface mulch. 



(2) Changing the Form of the Soluble Salts. — This refers 

 particularly to carbonates of sodium and potassium, the most 

 harmful forms. 



(a) By application of finely ground gypsum (sulphate of 

 lime). The soluble carbonates are thereby at least partly 

 changed to less injurious sulphates. 



Na2C03 + CaS04— > Na.S04 + CaC0. 



In order to be effective, the application of gypsum must 

 be several times as great as the amount of NaoCO, present. 

 It is said that gypsum will have no effect if the soil or irriga- 

 tion water used contains appreciable quantities of sulphates. 



(b) Other methods suggested are based upon neutralisa- 

 tion of the soluble carbonates with acids. Symmonds has 

 used nitric acid with good results, and Lipman found sulphuric 

 acid also useful. The latter has suggested the use of applica- 

 tions of sulphur. This is oxidised in the soil by bacteria to 

 sulphuric a^id. 



(3) Removal of the Salts from the Surface Soil. — 



(a) By encouraging the rise of the salts to the surface 



and then scraping off the incrustation. 



(h) By flooding heavily and thus washing the soluble 



salts into the lower soil depths well beyond the reach of the 



roots of crops. The salts will, of course, rise to the surface 



again after a time. 



(c) By encouraging the rise of the salts to the surface, 

 and then flooding over the surface and running the water off 

 the land. 



