THE CHEMICAL COMPOSITION OF SOILS 99 



109. Improving Alkali Soils. 27 When a large tract of 

 alkali is to be brought under cultivation, the amount and 

 kind of prevailing alkaline compound should be deter- 

 mined by chemical analysis. It frequently happens that 

 improved drainage, coupled with a judicious irrigation 

 system, is all the treatment necessary. If the prevail- 

 ing alkali is sodium carbonate, a dressing of land plaster 

 may be applied so as to change the alkali from sodium 

 carbonate to sodium sulphate, a less destructive form, 

 the reaction being 



Na 2 CO 3 + CaSO 4 = CaCO 3 + Na 2 SO 4 . 



Some shrubs, as greasewood, and weeds, as Russian 

 thistle, take from the soil large amounts of alkaline 

 matter, and 'it is sometimes advisable to remove a num- 

 ber of such crops so as to reduce the alkali. A slightly 

 beneficial effect is occasionally noticed on small ' alkali ' 

 spots where straw has been burned and the ashes used, 

 forming potassium silicate. As a rule, however, ashes 

 are more injurious than beneficial on an 'alkali' soil. 

 Irrigation and thorough drainage, if continued long 

 enough, will effect a permanent cure. Irrigation with- 

 out drainage causes a worse alkaline condition by bring- 

 ing to the surface subsoil alkali. All irrigated lands 

 should be provided with suitable drainage systems to 

 prevent accumulation of alkaline salts. The waters 

 from some streams and wells are unsuited for irrigation 

 because they contain too much alkaline matter. 



Mildly alkaline soils will usually repay in crop pro- 



