IX.] GYPSUM 267 



that any regular use was made of gypsum. In this 

 latter case it is not easy to make out to what extent its 

 employment was the result of experience, or of a quasi- 

 scientific opinion which traced a connection between the 

 action of sulphur upon the mildew of the hop, a 

 supposed lack of sulphates in a mildewed leaf, and 

 the sulphates in the gypsum. In the latter part of 

 the eighteenth century the value of gypsum for legu- 

 minous crops like clover and lucerne became widely 

 recognised ; Benjamin Franklin, in America, is said to 

 have sown gypsum so as to form the word "plaster" on 

 clover crops by the wayside, in order that passers by 

 should learn by the eye what had so stimulated the 

 growth. It is, in fact, on leguminous and such other 

 crops as are specially dependent upon potash, that 

 gypsum has an effect. This is intelligible on the 

 principle set out above, that the solution of calcium 

 sulphate which arises from the gypsum will attack the 

 zeolites containing potash and will so bring some 

 potassium sulphate into solution in the soil water. 

 In confirmation of this view, Boussingault has shown 

 that when clover is manured with gypsum and improved 

 thereby, the ash of the crop contains a greater pro- 

 portion of potash but shows no increase in either the 

 lime or the sulphuric acid. 



The figures obtained by Boussingault are set out in 

 Table LXXXIV. 



Thus, while the variations in lime and sulphuric 

 acid — the constituents of the gypsum, are small, the 

 proportion of potash has been greatly increased by the 

 use of gypsum. 



Similarly, it has been found, in testing the action of 

 gypsum upon hops, that it has a beneficial effect only 

 upon the soils where hops also respond to dressings of 

 potash salts, and that the result of applications of 



