122 ACTION ON THE GREEN LEAF. 



Davy, and to demand that the question should be further con- 

 sidered and explained. Among these I may mention, 



a That it produces the most sensible effect on a clover field 

 well covered with herbage, when it is strewed on a calm day 

 so as to rest on the surface of the leaf. In reference to this 

 fact, Peschier of Geneva states, that the green leaf decomposes 

 and converts into carbonate the sulphate of lime which rests 

 upon it. 



It is certain, however, that it does much good in many dis- 

 tricts, when harrowed in with the seed, or put in, as is so 

 extensively done in North America, in spoonfuls with the 

 potato seed and with the Indian corn at the period of planting. 



b That heavy rain, after the application of gypsum, often 

 prevents any visible action for the season. 



c That it succeeds better on light, dry, open, and sandy 

 soils on heavy land doing little good. 



d That clover in reality contains no more sulphur than other 

 crops. This is the result of analyses made by Messrs Way and 

 Ogston ; and, if confirmed, ought to form an important guide to 

 us in accounting for the special action of gypsum upon this crop. 



I shall therefore briefly discuss these two questions 



1. Theory of the action of gypsum. In explaining its action, 

 in a satisfactory manner, it is of consequence to bear in mind 



a That upon soils which already contain much lime, even 

 upon chalky soils, plaster often greatly improves the crop. In 

 such cases, it can scarcely do so, one would suppose, because of 

 the lime it contains. Besides, it requires a comparatively large 

 application even of quicklime to produce any sensible effect 

 upon a field ; and yet half a hundredweight of gypsum upon 

 the acre will not unfrequently produce a distinct increase in 

 the luxuriance and ultimate weight of a crop. 



Our attention, therefore, is directed to the other constituent, 

 the sulphuric acid, as the active ingredient in such cases. 

 Direct experiments, also, though not usually made with the 

 requisite accuracy, appear to confirm the conclusion to which 

 we are thus led. Thus Mr Tinzman, whom I have previously 

 quoted, made in 1842 a comparative experiment upon clover 

 with gypsum and sulphate of baryta, applied in equal weights, 



