166 



on the Broadbalk wheatfield at Rothamsted affords a useful example, 

 and we have figures which show the change in its fertility be- 

 tween 1865 and 1893, a period of 28 years. 



BROADBALK FIELD, ROTHAMSTED, PLOT 3. NITROGEN, LB. PER ACRE. 



In the first place it will be seen that the nitrogen in the soil 

 declines when the crop is wholly removed and no manure is added, 

 though the decline becomes slow after the first large loss of con- 

 dition has taken place. It will be remembered that the yield of 

 the unmanured wheat plot at Rothamsted fell off pretty quickly 

 for a few years, but for the last fifty years has remained almost 

 constant at an average of about 12.5 bushels per acre. 



If we make out a balance-sheet and set off the nitrogen re- 

 moved in the crop against that which the soil has lost, as shown 

 by analysis, together with that which has been brought down by 

 the rain, we find that the soil contains about 36 Ib. 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 aggregate, but 

 as there is no such falling off in the 'Stock of nitrogen in the soil 

 as would balance them, it follows that some recuperative agencies 

 must have been at work in the soil restoring the stock of nitrogen. 

 We know that Azotobacter and similar bacteria are present in this 

 soil; we know also that there is a certain amount ot 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. 



