214 CROPS GROWN IN ROTATION 



not in a very active form and as the growth of Swedes is very 

 specially dependent on an abundant and active supply of 

 phosphoric acid, it would probably be necessary to use 4 cwt. 



Table LXXVl 



or so of superphosphate per acre if a good average crop of 

 roots is to be obtained. The withdrawals of potash from the 

 soil during the rotation are more considerable, amounting to 

 nearly 200 lb. per acre, which in this experiment are only 

 partially replaced by the 150 lb. given in the manure for the 

 Swedes. We can assume, however, that even if this is not 

 supplied by the 15 tons of farmyard manure, which we have 

 l)een assuming as necessary to maintain both an average yield 

 and the fertility of the land, yet it will not be necessary to 

 afford any artificial supply of potash on a soil like that of 

 Rothamsted. The reserves of potash in such a strong soil are 

 enormous — at least 50,000 lb. per acre in the top 9 inches of 

 soil, of which 12,000 lb. is soluble in hydrochloric acid — and 

 a little of it becomes available every year under the action of the 

 weathering induced by cultivation. 



Practical Conclusions 



The following conclusions may be drawn from an examina- 

 tion of the results yielded by the Agdell Rotation Field :■ — 



1. On land continuously cropped without manure the Swede 

 crop is the first to feel the Avant of manure, the yield being 

 reduced to a minimum almost immediately. The leguminous 



