624 
Rotation of Crops. 
direct attention to some of the points of chief interest brought 
to view, on the consideration of the amount, and of the distribu- 
tion, of some of the more important individual mineral con- 
stituents in the respective crops ; and for the purposes of such 
an illustration reference will chiefly be made to the amounts of 
phosphoric acid, and of potash, but in some cases to that of lime 
also, in the crops. 
The Amounts of Phosphoric Acid in the Rotation , and in the 
Continuous Crops. 
Table VIII. records the results relating to the amounts of 
phosphoric acid in the different crops or parts of crops : — 
The Phosphoric Acid in the Root-crops. — The figures show 
that, without manure, the rotation turnip crops took up an 
extremely small amount of phosphoric acid, reaching in only 
one case to an average of lb. per acre per annum. By super- 
phosphate alone the amount was increased to an average of 
about 10 lb. ; and although this increase only represents about 
one-tenth of the phosphoric acid applied in manure it is very 
important, as it is directly connected with the greatly increased 
development of fibrous feeding root within the soil, which is a 
special effect of phosphatic manures when applied to turnips ; 
and it is by virtue of this development that these crops so 
markedly exhaust the available nitrogen within the soil, and 
especially the surface soil. As has been shown, there is abun- 
dant evidence that the increased amount of nitrogen taken up 
under the influence of phosphates unaccompanied by any 
supply of nitrogen itself, is at the expense of the stores of the 
soil ; and that it is not due to a capacity to take up either 
combined or free nitrogen from the atmosphere, by virtue of an 
increased development of leaf-surface, under the influence of the 
phosphatic manure. 
With the mixed manure, supplying, besides superphos- 
phate, salts of potash, soda, and magnesia, and a liberal amount 
of nitrogen as well, there was, although the supply of phosphoric 
acid by manure was exactly the same, now about twice as much 
of it taken up, as a coincident of the greatly increased growth, 
due partly to the other mineral constituents at the same time 
added, but especially to the influence of the increased available 
supply of nitrogen. Still, only a small proportion of the phos- 
phoric acid applied was taken up, considering the recognised 
importance of its application for turnips, and its undoubted 
specific effects on their growth as above described. 
Comparing the amounts of phosphoric acid in the rotation 
