Review of Essays on Calcareous Manures. 153 



It is upon the same principle only, namely, that it serves as the 

 food of plants, that we can account for the effects of gypsum upon 

 certain crops, which in any other mode of viewing them would ap- 

 pear miraculous. This sulphate of lime, if merely ground, has no 

 attraction for moisture, and if burnt would rather absorb it from the 

 soil than the air; it cannot therefore act, as some have supposed, to 

 increase the deposit of dew ; it is so sparingly soluble in water, and 

 so inert, that it cannot act as a stimulus, nor is it certainly followed 

 by exhaustion, as all stimuli must be. But although sparingly soluble, 

 it is still conveyed by water into the bodies of plants, although in 

 small quantities, and as that water undergoes the chemical changes, 

 which we know are induced by the vital action of plants, the sul- 

 phate of lime must be deposited in them, injuriously perhaps to 

 some, but as a necessary food to others. Thus, it has been found, 

 that the ashes of clover, lucern, and many of those plants whose 

 growth is known to be most certainly promoted by gypsum, uniform- 

 ly contain sulphate of lime ; it is, therefore, their essential and ap^ 

 propriate food. Some soils may contain it naturally — here an addi- 

 tion will not increase the crop of such plants ; some may contain a 

 substance which will decompose the sulphate, and thus prevent its 

 action. The earth baryta will separate the sulphuric acid from lime, 

 and thus may produce this effect ; but this is so rare, that injury 

 from this cause can hardly be anticipated. Oxalic acid will separate 

 the lime from the sulphuric acid, and thus will not only render the 

 application of gypsum inefficient, but will also set the acid free to 

 act injuriously. But if the oxalic acid has been previously neutral- 

 ized by lime, the sulphate remains unaltered, and is capable of aid- 

 ing the growth of clover and other plants of the sort. In confirma- 

 tion of this view of the subject, we may quote the experience of 

 Mr. Ruffin. His soil would not produce clover, even with the aid 

 of plaister ; and this being known, he did not attempt to use it, until 

 encouraged by a spontaneous and luxuriant growth of white clover. 

 We may also state a fact, within our own knowledge. It is a gen- 

 eral belief, that in the neighborhood of the sea plaister is useless, 

 and it has been so found in general on the Island of New York and 

 in the adjacent county of Westchester, yet upon a narrow ridge of 

 magnesiau marble, which lies between the layers of gneiss and mica 

 slate which form the greater part of this region, we have seen plais- 



VoL. XXX.— No. 1. 20 



