n6 Conservation of the Fertility of the Soil, [may, 



shown by analysis, together with that which has been brought 

 down by the rain, we find that the soil contains about 36 lb. 

 per acre more nitrogen at the end of the period than we should 

 have expected. This quantity is too small to be significant^ 

 it would be more than covered by the experimental errors in 

 the determinations ; we may, therefore, conclude that the 

 nitrogen required by the crop has just about been supplied 

 by the soil and the rain. There must, however, have been 

 other losses from the soil ; a good many weeds are annually 

 removed, and they contain some nitrogen ; drainage water 

 flows away containing, as we know by trial, some nitrates;, 

 there must also have been some bacterial liberation of nitrogen 

 gas. These losses of nitrogen may not be large in the aggre- 

 gate, but as there is no such falling off in the stock of nitrogen 

 in the soil as would balance them, it follows that some re- 

 cuperative agencies must have been at work in the soil 

 restoring the stock of nitrogen. We know that Azotobacier 

 and similar bacteria are present in this soil ; we know also 

 that there is a certain amount of weed of a leguminous nature 

 growing every year among the wheat; these are the two 

 sources of combined nitrogen which we may credit with 

 the restoration of the stock of nitrogen in the soil. 



However, their action is not sufficient to maintain the stock 

 unimpaired, even in this impoverished soil, when the crops 

 are wholly taken away. 



B. We may now take another case, that of land very rich 

 in organic matter and under arable cultivation, the crop 

 as before being wholly removed. An example of this kind 

 is afforded by the farmyard manure plot on Broadbalk, where 

 14 tons per acre of farmyard manure containing about 200 lb. 

 of nitrogen are applied every year. 



Broadbalk, Plot 2. Nitrogen, lb. per acre. 



In soil, In soil, Gain in Added in Added ' Removed Unaccounted 



1865. 1893. 28 years. manure. by rain. in crop. for. 



4343 4976 633 5600 107 1361 -3713 



Under these conditions the losses of nitrogen are enormous; 

 of the yearly supply of nitrogen not quite a quarter has been 

 recovered in the crop, and less than a quarter remains behind 

 a9 an enrichment of the soil ; more than half has been per- 

 manently lost through the destructive agencies enumerated 

 above (3 and 4). 



The production of this plot of land with its annual applica- 



