446 
On the Continuous Growth of Wheat on the 
of farmyard dung were applied to the barley crop, after which 
period the plot was divided into two; on one half the dung 
was continued as before, and upon the other half it was stopped. 
Up to the present time 12 unmanured crops have been taken, 
which yielded an average of 34^ bushels per acre, and, as the last 
crop in a rather favourable season exceeded 35 bushels, there is 
evidence of a long future before the fertility of the twenty years' 
dung will be exhausted. 
In the same field all the plots which have been manured with 
rape-cake showed by analyses that the soil contained a con- 
siderably larger amount of nitrogen than any of the plots where 
minerals, or minerals with salts of ammonia or nitrate, had been 
used. While therefore fertility may be stored up in the soil in 
the form of such mineral substances as potash or phosphate, it 
does not appear that the more valuable substance, nitrogen, can 
be stored up unless as united with carbon. Or in other words, 
while the soil fixes potash and phosphoric acid independent of 
vegetation, nitric acid is only fixed by the agency of vegetation. 
SUMMARY AXD GENERAL CONCLUSIONS. 
The Soil. 
1. A soil which in the ordinary course of agriculture would 
have received an application of manure before another crop was 
sown, has produced forty crops of wheat in succession, averaging 
14 bushels per acre, solely by means of its existing fertility. 
2. At the commencement of the experiment the soil contained 
a large amount of organic nitrogen, derived from the debris of 
pre-existing vegetation. It also contained a large amount of the 
mineral food of plants. 
3. Every year a certain proportion of the organic nitrogen 
has been nitrified by organisms existing in the soil. 
4. Part of the nitrates formed has been employed in the 
growth of the wheat crop ; part has been washed out of the soil 
or otherwise lost. 
5. The loss of nitric acid is greater in wet seasons, and the 
amount taken up by the wheat crop is in consequence smaller. 
Dry seasons should therefore be favourable for the production 
of large crops of wheat. 
6. The stock of soil fertility in the form of organic nitrogen 
has been considerably reduced during the forty years that the ex- 
periments have been carried on ; and the amount of such reduc- 
