THEORY OF THE ACTION OF THESE SULPHATES. 33J 



of the substance of the pea, the bean, the vetch, and of the seeds of 

 other leguminous plants. This sulphur they must obtain from the soil, 

 and one cause of the efficacy of the above sulphates is unquestionably 

 tliat they are fitted easily to yield to the growing plant the supply 

 of sulphur they necessarily require — while, if they are more efficacious 

 upon the leguminous than upon other kinds of plants, it is because 

 the latter produce a larger proportion of that kind of organic matter in 

 which sulphur is constantly pre^^nt. 



That such is really the true explanation of their general action is 

 proved by the observation — that sulphuri: acid applied to the land in a 

 very diluted state exerts an influence upon the crops precisely similar to 

 that observed when jjypsum or sulphate of soda is used. (See Appendix, 

 Nos. I. and II.) 



In reference to this mode of action it is of consequence to know the 

 relative efficiency of the several salts. This will obviously depend upon 

 the relative proportions of sulphur or sulphuric acid they contain — sup- 

 posing the circumstances in which they are apphed to be equally favour- 

 able to the introduction of each into the circulation of the plant. Their 

 relative value upon this view is as follow^s : — 



100 lbs. of burned gypsum are equal to, or contain as much sulphuric 

 acid, as 



126 lbs. of common or unburned gypsum. 



128 lbs. of sulphate of [)otash. 



104 lbs. of sulphate of soda — dry. 



235 lbs. of sulphate of soda — crystallized. 



180 lbs. of sulphate of magnesia — crystallized. 

 And as of all these the gypsum is by far the cheapest, it should form, in 

 reference to this general action of the above sulphates, in all cases, the 

 most economical application to the land. 



h. But they have each also their special a-Ciion dependent partly upon 

 their physical properties, and partly on their chemical constitution. 



Thusit will be of little use mixing any of them with the soil, unless 

 they become capable of entering into the roots of the plants which are 

 growing upon it. The facility with which this can be effected depends 

 upon their solubility in water, which is very unlike. Thus an imperial 

 gallon of pure water at the ordinary temperature will dissolve of 



Gypsum (burned,) about \ lb. 



Gypsum (unburned,) 1 lb. 



Sulphate of Potash, IJ lbs. 



Sulphate of Soda, t^ry, l| lbs. 



Sulphate of Soda, crystallized, 3| lbs. 



Sulphate of Magnesia, 4 lbs. 



In rainy weather, therefore, and in moist climates, it would still be 

 most economical to apply the gypsum, since, though very sparingly 

 soluble, water would be sufficiently abundant to dissolve as much as the 

 plant might retjuire. But in times of only moderate rain, and especially 

 in dry seasons, the use of the sulphates of soda and magnesia, which 

 are also low in price, is recommended by the comparative ease with 

 which they may be taken up by water and conveyed to the roots. 



c. Again, the chemical constitution of these sulphates — the nature of 

 the substance with which the sulphuri: acid is combined — determines in 



