Experimental Plots at Rothamsted, from 1864 to 1883. 447 
tion has been ascertained by analyses of the soil made at different 
periods. The stock of both potash and phosphoric acid has also 
been largely reduced. 
7. Although so much soil fertility has been removed, the 
stock that remains would appear to be sufficient to grow crops of 
wheat for a very long period ; the produce, however, must in 
process of time necessarily be lower than it has hitherto been. 
Manures. 
8. Mineral manures alone have added very slightly to the 
produce grown upon the unmanured land. 
9. Manures containing nitric acid alone, or some compound 
of nitrogen which is easily nitrified, have considerably increased 
the crop. 
10. The soil therefore contained a stock of minerals which the 
wheat crop was unable to make use of, owing to the insufficient 
supply of nitrogen in some available form. 
11. Manures consisting of potash, phosphoric acid, and 
ammonia or nitrates, appear competent to grow large crops of 
wheat continuously. 
12. A given weight of nitrogen as nitric acid, has produced 
more growth in the wheat crop than the same weight of nitrogen 
in salts of ammonia. 
13. The amount of nitrogen supplied in manures is very 
much in excess of the amount recovered in the increase of the 
crop. 
14. After a certain amount of growth has been reached, each 
increase of crop requires a proportionately larger application 
of manure. When the price of grain is high, larger crops can be 
grown more profitably than when the price is low. 
15. When farmyard dung is employed to grow wheat, a con- 
siderably larger amount of nitrogen must be applied to produce 
a given increase in the crop, as much of the nitrogen contained 
in the dung is not in an active form. 
16. A given weight of nitrogen, in the form of nitric acid, 
will produce more growth in the crop to which it is applied 
than the same weight of nitrogen in dung ; but the influence of 
the nitrate upon succeeding crops will be very much less. 
17. There is no evidence to show whether the whole avail- 
able effect of the nitrogen in one manure is greater than it is 
in the other. 
