Soluble Salts in Soils. 95 



continued rock decay under conditions where little or no 

 leaching has taken place. Rains enough fall to produce 

 decay, but not enough to carry the salts formed into the 

 drainage channels and out of the country. This is why 

 alkali lands are largely peculiar to desert or semi-arid 

 climates. 



109. Leaching Necessary to Fertile Soils. It is clear 

 from 106 and 108 that if there was not some leaching to 

 take up and carry away the extremely soluble salts not 

 available as plant food all soils would in time become "al- 

 kali lands ;" so that while excessive leaching is undesirable, 

 a sufficient amount is indispensable. 



The prevention of the accumulation of undesirable solu- 

 ble salts in the soil of irrigated lands in dry climates is one 

 of the most serious of practical problems. 



110. Soluble Salts in Marsh Soils. The black marsh soils 

 of humid climates often contain unusually large amounts 

 of soluble salts, sometimes reaching 2,366 parts per mil- 

 lion of the dry soil in the surface 6 inches after maturing 

 a crop. This would make the water contain 1.18 per cent, 

 of salts if the water content of the soil was 20 Ibs. per 100 

 of dry soil. Many of these soils behave much like alkali 

 lands, being unproductive, the crops often dying when 

 there is no evident reason for it. 



111. Correction for Alkali Lands. It has been found that 

 when a soil is unproductive from too high a per cent, of 

 sodium carbonate or black alkali and there is not enough 

 of other soluble salts to be injurious, this may be corrected 

 in part by the use of gypsum, or land plaster, which has the 

 effect of converting the carbonate into the sulphate or 

 "white alkali," like amounts of which are less harmful. 



It often happens that waters which must be used in irri- 

 gation contain black alkali, and where this is the case it is 

 well to correct the water by using land plaster in the reser- 

 voirs or distributing canals, for the water to run over or 

 through, before reaching the field. 



