45 8 SOILS. 



present. The amount required per acre will, of course, vary 

 with the amount of salts in the soil, all the way from a few 

 hundred pounds to several tons in the case of strong alkali 

 spots ; but it is not usually necessary to add the entire quantity 

 at once, provided that sufficient be used to neutralize the sodic 

 carbonate near the surface, and enough time be allowed for the 

 action to take place. In very wet soil, and when much gypsum 

 is used, this may occur within a few days ; in merely damp soils 

 in the course of months; but usually the effect increases for 

 years, as the salts rise from below. 



The effect of gypsum on black-alkali land is often very strik- 

 ing, even to the eye. The blackish puddles and spots disappear, 

 because the gypsum renders the dissolved humus insoluble and 

 thus restores it to the soil. The latter soon loses its hard, 

 puddled condition and crumbles and bulges into a loose mass, 

 into which water now soaks freely, bringing up the previously 

 depressed spots to the general level of the land. On the sur- 

 face thus changed, seeds now germinate and grow without hin- 

 drance ; and as the injury from alkali occurs at or near the sur- 

 face, it is usually best to simply harrow in the plaster, leaving 

 the water to carry it down in solution. Soluble phosphates 

 present are decomposed so as to retain finely divided, but less 

 soluble earth phosphates in the soil. 



It must not be forgotten that this beneficial change may go 

 backward if the land thus treated is permitted to be swamped 

 by irrigation water or otherwise. Under the same conditions 

 naturally white alkali may turn black (see above, chapter 22, 

 p. 451). Of course, gypsum is of no benefit whatever on soils 

 containing no "black" alkali, but only ("white") Glauber's 

 and common salt. 



Removing the Salts from the Soil. In case the amount of 

 salts in the soil should be so great that even the change worked 

 by gypsum is insufficient to render it available for useful crops, 

 the only remedy left is to remove the salts, partially or wholly, 

 at least from the surface of the land. Three chief methods are 

 available for this purpose. One is to remove the salts, with 

 more or less earth, from the surface at the end of the dry 

 season, either by sweeping or by means of a horse scraper set 

 so as to carry off a certain depth of soil. Thus sometimes in a 

 single season one-third or one-half of the total salts may be got 



