304 SYSTEMS OF MANURING CROPS [CHAP. 



from the atmosphere ; indeed, the Rothamsted experi- 

 ments would show that the land is left richer in nitrogen 

 after a big clover crop has been grown and taken away. 

 A further consideration of the rotation field at 

 Rothamsted shows that the clover crop alone would be 

 able to maintain the fertility of the land at about the 

 condition which would produce such yields as are shown 

 in the table. For instance, the Agdell field in the 47th 

 to the 50th years gave the following crops on the portion 

 which had received no nitrogen throughout the whole 

 period, though phosphates and potash are supplied to 

 the Swede crop. 



Table LXXXIX.— Produce of Agdell Field under Rotation. 

 No Nitrogen supplied in Manure. (Rothamsted.) 



1894 



1895 

 1896 

 1897 



Clover Hay 

 Wheat 

 Swedes 

 Barley 



Carted away , 

 Carted away 

 Consumed on the land 

 Carted away 



64-7 cwts. 



r39-6 bushels, and 25-3 cwts. 



\^ Straw. 



I2-0 tons. 



{37'7 bushels, and 24-9 cwts. 

 Straw. 



If, then, in this case the Swede turnips had also 

 received whatever manure would have been made from 

 the clover hay and the wheat and barley straw, it is 

 evident that the production would have been little short 

 of the average indicated in Table LXXXVIIL, and 

 that the nitrogen necessary to maintain the fertility of 

 the land at such a level would be supplied indefinitely 

 by the recurring clover crop. In the Agdell example 

 phosphatic and potassic fertilisers were however freely 

 employed, and it is obvious that the soil possesses no 

 power of increasing its stock of these constituents in the 

 same way as it can obtain nitrogen from the atmosphere. 

 Three hundred and fifty pounds of superphosphate per 

 acre during the four-year period of rotation would, 



