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its giving a fixed condition to the nitrogen or ammonia which 

 is brought into the soil, and which is indispensable to the nutri- 

 tion of plants. Water is absolutely necessary to effect the de- 

 composition of gypsum on accoimt of its difficult solubility, (I 

 part of gypsum requires 400 parts of water for solution) and 

 also to assist in the absorption of the sulphate of ammonia by 

 the plants: hence it happens that the influence of gypsum is 

 not observable on dry fields and meadows. The decomposition 

 of gypsum by carbonate of ammonia does not take place in- 

 stantaneously ; on the contrary it proceeds very gradually, and 

 this explains why the action of gypsum lasts for several 

 years." * 



Further inquiries and experiments are necessary in order to 

 establish this theory. Certain plants, such as clover and the 

 vines of potatoes, are found to contain a comparatively large 

 amount of this substance. So far as this fact appears, gypsum 

 would seem to constitute an essential food of the plant in which 

 it is thus found. On the seashore its application seems useless j 

 but its efficiency may be neutralized by the highly saline atmos- 

 phere which prevails in such situations. In the interior it op- 

 erates effectually upon some soils, while in the immediate vicin- 

 ity it may be unavailing. This would seem to imply that its 

 efficiency has a relation to the particular character of the soil. 

 On this subject we are still greatly in want of more light. One 

 point at present seems to be established, — that the particular 

 soils or places where it will operate well are to be ascertained 

 only by actual trial ; but this is not to be regarded as a hard- 

 ship, since the expense of such experiments need be only very 

 trifling. Such experiments, conducted with observation and 

 care, are exceedingly desirable. Where it operates well, no 

 cheaper manure'and. none more easily applied can be found. — 

 It was at one time supposed that, by the constant application 

 of it year after year to the same land, the productiveness of the 

 land would be diminished, and it would become, as it is termed, 



* Liebig's Organic Chemistry, Webster's edit. p. 144. 



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