154 GYPSUM ON ACID SOILS. 



to explain its occasional failures any more than its general success, 

 on the lands where it is profitably used in general but only why 

 it cannot act at all, on lands of a different kind. 



The same chemical action being supposed, explains why the 

 power of profiting by gypsum should be immediately awakened on 

 acid soils after making them calcareous ; and why that manure 

 should seldom fail, when applied mixed with much larger quantities 

 of calcareous earth. 



[When the foregoing attempt to explain the cause of the non- 

 action of gypsum on acid soils was written, and first published in 1832 

 (as it here appears distinguished from the later additions), the dis- 

 covery of Tiumic acid by European chemists was not known to me, 

 and its very general existence in soils, now universally recognised, 

 was scarcely known to any. Without pretending to identify the 

 acid of soil whose existence I maintained, as early as 1818, to be 

 almost universally present and injurious in this country, it is now 

 clear and unquestioned that the humic acid is thus plentifully and 

 generally diffused. The effects ascribed above to the supposed 

 oxalic acid, of decomposing and destroying sulphate of lime when 

 applied as manure, may be as much produced by the actually pre- 

 sent humic acid. For, not only is the latter convertible to the 

 former, as above argued of all vegetable acids, but, without the* 

 need of such conversion, the humic acid is now understood to have 

 the like power of decomposing sulphate of lime. This is stated 

 fully and distinctly in a very recent publication (Browne's Ame- 

 rican Muck Book, 1852), as follows : "Gypsum is decomposed by 

 carbonate and muriate of barytes, the carbonates of strontia, 

 potash, soda, and ammonia, as well as by oxalic and humic acids % 

 and where any of the four last named occur naturally in the soil, 

 or are applied by artificial means, new combinations take place, 

 which are attended in some cases with beneficial results, . . 

 If, however, it [the soil] contains too much free humic acid, it wilt 

 decompose the gypsum, so that humate of lime will be formed, and 

 the sulphuric acid set free, which may then act as a corrosive on the 

 roots of plants" (p. 71.) Nothing is wanting to the fullest and 

 clearest establishment of my doctrine as stated above, except that 

 the humic acid, like the oxalic, has stronger affinity for lime than 

 the sulphuric, and therefore will decompose sulphate of lime 

 (gypsum), and form instead humate of lime, of which the effect 

 as manure is altogether different. And that humic acid (or what- 

 ever may be the acid of soil) really has this stronger affinity for 

 lime, is sustained by enough agricultural facts within my persona} 

 observation, even if the proposition had no support whatever 

 chemical science. 1852.J 



