334 SPECIAL ACTION UPON GRASSES AND CLOVERS, 



a still greater degree the nature and extent of their special action. If 

 the soil already abound in potash, in soda, in lime, or in magnesia, then 

 the influenee of these compounds may depend entirely upon the sul- 

 phuric acid they contain. But suppose the land to be deficient in lime, 

 then the gypsum we add will act not only in virtue of the sulphuric acid, 

 but of the lime also which it contains, and thus its apparent effect will 

 be much more striking than when the land is naturally calcareous, or 

 has been previously dressed with hnie. So if it be deficient in potash, 

 the sulphate of potash will be more efficient than it could be expected to 

 prove upon a soil in which sulphuric acid alone is wanting. And so 

 also, if lime and potash abound, and soda or magnesia be deficient, the 

 sulphates of these latter bases will exercise a special action upon the 

 soil, by supplying it at the same time with sulphuric acid and with soda 

 or magnesia also. Thus on land to which lime has been abundantly 

 added, according to the ordinary practice of husbandry, the sulphate of 

 soda has the best chance of proving useful to vegetation, not only because 

 it is more soluble, and is, therefore, more independent of the seasons, 

 but because it is capable of supplying two different substances — sulphuric 

 acid and soda — neither of which are directly added in the ordinary 

 manuring of the land, but both of which the plants may find difficulty 

 in obtaining. 



d. Another consideration will indicate further special applications of 

 these several sulphates, independent of the sulphuric acid which they 

 in common contain. If we refer to the table (p. 220, ) in which is exhibit- 

 ed the constitution of the ash of the several clovers and grasses, we find 

 the constituents of our sulphates to be present in 3 00 parts of the ash in 

 the following proportions : — 



^^'n/yf' Red Clover. ^hUe Lucerne. Sainfoin. 



Potash 8-81 19-95 31-05 13-40 20-57 



Soda 3-94 5-29 5-79 6-15 4-37 



Lime 7-34 27-80 23-48 48-31 21-95 



Magnesia .... 0-90 3-33 3-05 3-48 2-88 



Sulphuric Acid . . 3-53 4-47 3-53 4-04 3-41 



Of the two clovers the red contains more lime and much less potash, 

 therefore the sulphate of lime is more likely to benefit the red clover, 

 and the sulphate of potash the white, which is consistent with the results 

 of experiment. A similar difference exists between lucerne and sainfoin, 

 to the former of which lime and soda are more necessary than the latter. 

 The first column under rye grass shows, on the other hand, how very 

 much smaller a proportion of all the four — potash, soda, lime, and mag- 

 nesia — is required by this green crop than by the others; and therefore 

 that the same weight of any one of these sulphates, which, when applied 

 as a top dressing to one crop (rye grass), would cause it to thrive luxuri- 

 antly, may be insufficient to supply the most necessary wants of another 

 crop (clover or sainfoin.) Not only the kind of mineral manure, there- 

 fore, which we mix with the soil, but the quantity also, must be deter- 

 mined by the kind of crop we intend to raise. (For the theoretical opinions 

 of other authors in regard to the action of gypsum, see Appendix, No. VI.) 



6°. Nitrates of Potash and Soda. — The efficacy of these ttvo substan- 

 ces as manures in certain circumstances is now generally acknowledged, 



