24 



Before entering upon the main teaching of this rotation, I 

 would call your attention to one very curious result, namely, 

 the action of residues of superphosphate upon the clover crop. 

 I have already mentioned that one division of the field has been 

 cultivated without the use of any manure, while the middle 

 division, during the first nine rotations, received 3J cwts. 

 of mineral superphosphate with the turnips. The power 

 of producing clover was originally equal on both divisions, 

 for the first clover crop in 1850 amounted to 6,200 Ibs. 

 of hay per acre on the unmanured portion, and to 6,362 Ibs. 

 where the turnips had received superphosphate. The subse- 

 quent clover crops are, however, very different in the two 

 divisions, the land receiving superphosphate yielding generally 

 two or three times as much clover as the land entirely 

 unmanured. The crops of clover-hay per acre were as 

 follows : 



In the last two years mentioned above the swedes received 

 potash salts as well as superphosphate, but it is evident that 

 the difference we refer to occurred when only superphosphate 

 was applied. 



This great effect, produced by the residue from a moderate 

 application of superphosphate, is probably due to a consider- 

 able extent to the gypsum which the superphosphate supplied. 

 The sulphuric acid of the gypsum is very probably the active 

 constituent. We sometimes forget in our manuring experiments, 

 that albuminoids, in which the clover crop is so rich, cannot 

 be produced without sulphur. 



It is clear that in the experiment just quoted the money 

 value of the residue of the superphosphate far exceeded the 

 original cost of the manure. It is equally clear, I think, to 



