26 TOBACCO. 



guano, &c., cannot suffer waste by washing out, and will 

 come to use when grain or grass shall follow in the 

 rotation." 



He observes of gypsum (lime sulphate) that it is "a 

 valuable application to tobacco, not because it is very 

 largely taken up by the crop, for the greatest export of 

 sulphuric acid, viz. 20 Ib. per acre, is restored by 50 Ib. of 

 plaster, and the greatest export of lime, 120 Ib., is made 

 good by 400 Ib. of the sulphate, but because lime sulphate 

 dissolves in 400 times its weight of water, and may 

 rapidly wash out of the porous tobacco lands, and espe- 

 cially because the solution of lime sulphate in the soil is 

 a very effective agent in rendering soluble and accessible 

 to crops the potash and magnesia, which too often exist in 

 close-locked combinations. The average annual rainfall 

 (snow included) in our latitudes, is no less than 10,000,000 

 Ib. per acre. This enormous quantity of water would be 

 enough to dissolve and wash out of the soil 25,000 Ib. of 

 gypsum per acre if it had time to saturate itself, and 

 then flowed off. In fact, but a small proportion of the 

 rainfall runs through and out of the soil, not more than 

 10 to 20 per cent., according to its porosity and situation ; 

 but it is plain that there is nothing to hinder the waste 

 of a hundred pounds or more of gypsum per acre yearly, 

 since all investigations go to show that the soil has no 

 retaining power for lime sulphate as it has for potash and 

 for phosphoric acid. In Nessler's experiments, gypsum 

 had an excellent effect on the burning quality of the 

 tobacco raised under its application, an effect attributable, 

 he believes, to the fact that this fertilizer often liberates 

 potash in the soil, as Liebig and Deherain have demon- 



