448 On the Continuous Growth of Wheat, ^-c. 
Unexhausted Manures. 
18. In the absence of vegetation, or when applied to crops in 
excess of their requirement, both potash and phosphoric acid 
form insoluble compounds w ith the soil and become available 
for future crops. 
19. In the absence of vegetation, or when the amount supplied 
is in excess of the requirements of the crop, nitrates and salts of 
ammonia do not appear to form permanent compounds with the 
soil, but, on the contrary, are liable to be washed out by rain, or 
to be otherwise lost. 
20. The application of a larger amount of nitrogen, as nitrates 
or salts of ammonia, than the crop could utilize owing to a 
want of minerals, does not appear to prevent the nitrification of 
the organic nitrogen of the soil. 
21. The stock of nitrogen of the soil itself, therefore, may 
be reduced, although the annual application of nitrogen may be 
much in excess of the amount of that substance removed in the 
crop. 
22. When large crops of wheat have been grown by the 
application of nitrates, or salts of ammonia with mineral manures, 
the soil does not appear to have gained or lost fertility. Nitri- 
fication of the organic matter in the soil may have gone on as 
usual, but the loss has been made good by the amount of 
nitrogen stored up in the stubble and underground roots of the 
large crops previously grown. 
23. When dung is applied continuously to land, the accu- 
mulation of unexhausted fertility becomes very large, and the 
removal by crops of the substance accumulated would extend 
over a long series of years. 
24. Dung applied to land in the ordinary processes of agri- 
culture will not be entirely exhausted until a considerable 
number of years have elapsed from the time of its first 
application. 
