REMOVAL OF MINERAL CONSTITUENTS 213 



It thus appears that when land is continuously cropped 

 without manure and without the restoration of any parts of 

 the crops grown to the soil, the annual withdrawal of the chief 



TABLE LXXV. Phosphoric Acid and Potash removed ly Crops 

 grown in Rotation. Agdell Field. 



mammal constituents will be about the same whether the land 

 is put under a rotation or grows a cereal crop every year. Of 

 course the rotation plot in this case is practically growing two 

 cereal crops only in the four years, with a fallow between each, 

 so small is the production of roots in the first year of the 

 course. 



Table LXXVI. (p. 214) brings together for comparison 

 the annual losses on the three unmanured plots in question. 



As regards the manured plots growing clover or beans, we 

 find that a little more than 80 Ib. of phosphoric acid is 

 removed during the four years of the rotation and must be 

 replaced by manure if the fertility of the land is to be main- 

 tained. If 15 tons per acre of dung be given during the 

 rotation, more phosphoric acid will be returned than is 

 withdrawn by the crop ; but, as the phosphoric acid in dung is 



