SOIL AMENDMENTS 543 



years, and its effectiveness has apparently decreased as 

 the soils on which it was used have been longer under 

 cultivation. Possibly this is due to the tendency of these 

 soils to become more acid, which has caused the gypsum 

 to be less effective in liberating potassium a property 

 with which it has generally been credited. At present 

 gypsum is not very generally used on soils. It must be re- 

 membered, however, that superphosphates always contain 

 a considerable proportion of this material, and it may add 

 appreciably to the beneficial effects of that fertilizer. 



Aside from its action in liberating potassium (the actual 

 extent of which has never been very clearly demon- 

 strated), gypsum serves to supply sulfur to the soil. The 

 sulfur, while it may be needed in some soils, has the dis- 

 advantage of being present as an acid ; and if the acid 

 is added in larger quantity than is removed by plants, 

 there is a resulting loss of basic material in the drainage 

 water and a tendency for the soil to become sour. 



The action of gypsum in improving tilth is less marked 

 than that of caustic lime or of the carbonate. As a source 

 of calcium it is of no moment, as, if applied in such quan- 

 tities as those in which the other forms are used, the 

 sulfate would be very injurious. Ordinarily it is applied 

 at the rate of only a few hundred pounds to the acre at the 

 most. On the whole, gypsum is not an adequate substi- 

 tute for, nor so desirable a form of, calcium as the oxide, 

 the hydroxide, or the carbonate. 



464. Common salt. Sodium chloride has a marked 

 effect on some soils, but wherein its effectiveness lies 

 is not well understood. The addition of sodium and of 

 chlorine as plant constituents is clearly not the reason, 

 as these substances are always present in soils in avail- 

 able form far in excess of their requirements. 



