78 
ELEMENTS OF AGRICULTURE. 
7. Upon what soils are ashts of the most value ? 
8. With what kind of land do they best agree T 
9. Should they be used as manures ? 
10. What is the effect of ashes applied to meadow-land ? 
LESSON IX. 
PLASTER. 
62. Plaster, as we have seen, is composed of sulphuric 
acid and lime. It is also called gypsum; but this last 
name is that by which the stone or rock is designated, be¬ 
fore it is ground into plaster, 
63. The manner in which plaster acts, and its value, 
have long been subjects of controversy among agricultural 
chemists; some contending that it serves as a direct food 
to certain plants (Johnson), others that its utility consists 
in its power of absorbing the gases, and holding them in 
contact with the roots of vegetables (Liebig). 
64. It is sufficient, however, to state here, that it is a 
forcible stimulant to many cultivated crops, quickening, in 
a remarkable degree, the vital energies of plants. 
65. On some soils the action of plaster is scarcely per¬ 
ceptible ; but as it is a cheap substance, and of easy trans¬ 
portation and application, it should always be tried. Pro¬ 
fessor Johnson has ascertained, by analysis, that an ordi¬ 
nary crop of clover or sainfoin vvill yield per acre from 
one and a half to two hundred weight of sulphate of lime. 
This is precisely the quantity usually applied per acre in 
those parts of the country where plaster is in most genera,, 
use. 
66. Plaster should be sown broadcast in calm weather, 
when the dew is still upon the grass, at the rate of a bushel 
to the acre. 
37. Plaster seems to act most readily upon corn, cloves 
