152 THEORY OF GYPSUM ON ACID SOILS. 



Such I suppose to be the circumstances of those lands of this 

 country on which gypsum exerts the greatest power. But in Eng- 

 land, though clover culture is universally extended, gypsum has 

 shown scarcely any benefit -as manure, and though extensively 

 experimented with, has not been found sufficiently operative to be 

 brought into ordinary practice on any one farm in the kingdom. 

 This may be accounted for by supposing the soils generally to be 

 supplied by nature abundantly with gypsum, so that no more is re- 

 quired. Davy found gypsum in the soil itself of four farms, 

 examined with this view, and in one of them the very large propor- 

 tion of nearly one per cent. (Agricultural Chemistry, Lecture vii.) 

 But there is another and numerous class of cases in which gypsum 

 cannot be supposed to be present, and yet when applied shows no 

 benefit. These are the poor acid soils of lower Virginia (and else- 

 where), and the cause of which it seems to me not difficult to 

 explain. 



However wonderful and inscrutable the fertilizing power of this 

 manure may be, and admitting its cause as yet to be hidden, and 

 entirely beyond our reach, still it is possible to show reasons why 

 gypsum cannot act in many situations, where all experience has 

 proved it to be worthless. If this only can be satisfactorily ex- 

 plained, it will remove much of the uncertainty as to the effects to 

 be expected ; and the farmer may thence learn on which soils he 

 may hope for benefit for this manure, on which it will certainly 

 be thrown away, and by what means the circumstances adverse to 

 its action may be removed, and its efficacy thereby secured. This 

 is the explanation that I shall attempt. 



If the vegetable acid, which I suppose to exist in what I have 

 called acid soils, is not in part the oxalic (which is the particular 

 acid in sorrel), at least, every vegetable acid, being composed of 

 different proportions of the same three elements, may easily change 

 to any other, and all to the oxalic acid. This, of all bodies known 

 by chemists, has the strongest attraction for lime, and will take it 

 from any other acid which was before combined with it ; and for 

 that purpose, the oxalic acid will let go any other earth or metal, 

 which it had before held in combination. Let us then observe 

 what would be the effect of the known chemical action of these 

 substances, on their meeting in soils. If oxalic acid were produced 

 in any soil, its immediate effect would be to unite with its proper 

 proportion of lime, if enough were in the soil in any combination 

 whatever. If the lime were in such small quantity as to leave an 

 excess of oxalic acid, that excess would seize on the other substances 

 in the soil, in the order of their mutual attractive forces ; and one 

 or more of such substances are always present, as magnesia, or, more 

 certainly, iron and alumina. The soil then would not only contain 

 some proportion of the oxalate of Unie^ but also the oxalate of either 



