ROTATION OF CROPS. 247 



the lime and carbonic acid predominate in the continuously 

 vegetating and much more largely nitrogen -accumulating 

 clover. 



Eeferring to the probable or possible significance of these 

 facts, it is obvious that, so far as the nitrogen of the plant is 

 taken up as nitrate of a fixed base, that base, so far as it does 

 not pass back into the roots, will remain in the above-ground 

 parts of the plant, most probably in combination with an 

 organic acid, which will be converted into carbonic acid in 

 the incineration, and be found as such in the ash, if not ex- 

 pelled by an excess of fixed acid, or by silica. 



In the case of the cereals of the rotation, it is probable that Nitrogen 

 most, if not all, of their comparatively small amount of nitro- ^^f s 

 gen is taken up as nitrate. Potash is by far the predominat- as nitrate. 

 ing base in the ash of the grain, straw, and total produce ; 

 lime is in much less amount, both actually and in equival- 

 ency; and magnesia is in less amount still, though it is a 

 characteristic constituent of the grain-ashes. There is prac- 

 tically no carbonic acid in either wheat or barley grain-ash, 

 and but little in the straw-ash ; and if there have been organic 

 acid salts formed with the base of the nitrate, the carbonic 

 acid may have been expelled in the incineration, by the 

 excess of fixed acid in the grain-ash, or by silica in the straw- 

 ash. 



Taking the produce by the mixed manure as the most 

 normal, the root-crops of the rotation come next in amount of 

 nitrogen assimilated over a given area. Potash and lime are 

 the predominating bases. There is much more potash than 

 lime in the more definite product — the root; but the pro- 

 portion of lime to potash is much greater in the leaf-ash, 

 as would be expected if the nitrogen had been taken up 

 chiefly as calcium nitrate, and the nitric acid subjected to 

 decomposition in the leaves. 



Lastly come the Leguminosse, with their much higher 

 amounts of nitrogen assimilated. These plants also doubtless 

 derive at any rate much nitrogen from nitrates in the soil and 

 subsoil ; and it has been shown that their great assimilation 

 of nitrogen is associated with very large amounts of lime and 

 carbonic acid in their ashes. 



Referring to the results with the rotation beans grown by 

 the mixed manure, calculation shows that, taking the total 

 crop, corn and straw together, it contained very much less 

 lime than would be required if the whole of its nitrogen had 

 been taken up as calcium nitrate ; so that either part of the 

 nitrogen must have been taken up as nitrate of some other 

 base, or in some quite different state of combination, or 

 as free nitrogen ; or some of the lime must have been elimin- 



