190 AGRICULTURE 



and mangolds three times as much of this 

 element. Similarly as regards potash, wheat 

 and barley removing only about 30 Ib. per 

 acre, whereas red clover and swedes to give 

 a full yield must have two or three times 

 as much, and turnips about five times as 

 much of this substance. The variations in 

 the requirements of crops for phosphoric 

 acid are not quite so wide as in the case of 

 the two nutritive materials already referred 

 to, but even here we find variations between 

 a minimum of some 20 Ib. per acre in the 

 case of potatoes, swedes, red clover, and 

 the cereals generally, and of 50 Ib. in the 

 case of the mangold crop. If, therefore, 

 one were to grow only a single crop on any 

 field, one would have to manure very liberally 

 with the particular ingredient that the crop 

 specially affected, and at the same time one 

 would know that materials for which the 

 crop had little use were likely to be going 

 to waste. 



It is found that the residues of roots, 

 stubble, and leaves, left in and on a field by 

 the growth of a particular crop, prove ex- 

 cellent plant food for another crop of a 

 different species, whereas one would not 

 often find that these residues sufficed to 

 grow a correspondingly large crop of the 



