114 SULPHATES MAY BE DECOMPOSED. 



Second, But both salts will also yield alkaline matter, potash, 

 and soda, to the plant. And as our cultivated crops usually 

 abound more in potash than in soda, the sulphate of potash ap- 

 pears likely to exercise a more decided and generally useful 

 action than the sulphate of soda. This forms a proper subject 

 for comparative experiments. 



It is to be observed, both of the acid and of the alkali con- 

 tained in these salts, that, supposing them to have reached its 

 sap, they not only contribute directly, and as so much necessary 

 matter, to build up the several parts of the plant, but they per- 

 form chemical functions besides, which, though not well under- 

 stood as yet, are believed to be necessary to healthy and rapid 

 growth. Whether to some plants (the leguminous plants, for 

 example) the sulphuric acid is more necessary than to others, or 

 more at some periods of their growth than at others, or whether 

 to some the special chemical action of potash is more useful than 

 that of soda these are points which remain to be cleared up by 

 future experiment and observation. 



Thirdly, As it is through the agency of alkaline matter that 

 the silica, so necessary to our straw and hay crops, is rendered 

 soluble, and capable of being conveyed into the sap of plants ; 

 and as the alkaline sulphates are capable of undergoing such 

 changes in the soil as may cause their alkalies to combine with 

 the silica in which the soil usually abounds it is not at all un- 

 likely that these salts may owe part of their action upon vege- 

 tation to the capacity of supplying potash or soda for the purpose 

 of conveying silica to the growing plant. 



Two things, besides, the reader will bear in mind that their 

 efficacy for any of the above purposes will depend very much 

 upon the proportion of potash, soda, and sulphuric acid already 

 present in the soil, and that the comparative action of the two 

 sulphates will be modified very much by the greater solubility 

 of the salt of soda. 



6. Suggestions for experiments with the sulphates of potash 

 and soda, applied alone. 



1. fo the practical man who carefully studies the preceding 

 section, it will appear that there is reasonable ground for 



