1910.] Conservation of the Fertility of the Soil. 117 



tion of manure greatly in excess of the requirements of the 

 crop still tends to reach an equilibrium ; after the first few 

 years the crop does not increase any further, nor does the soil 

 become any richer, because the agencies destructive of the 

 combined nitrogen increase at an accelerating pace until they 

 balance the greater intake of nitrogen. Of course, the equi- 

 librium thus attained is at a much higher level of production 

 than is attained on the unmanured plot, being an average of 

 58'6 against 12*5 bushels per acre for the last twenty years. 



C. We may now take a case where the crop is not removed, 

 but the whole of the vegetation is allowed to die down and 

 fall back on the land. At Rothamsted portions of the Broad- 

 balk and of the Geescroft fields have been allowed to run wild 

 since 1881 ; they are covered with a rough natural vegetation, 

 which on Broadbalk contains about 25 per cent, of leguminous 

 plants, but on Geescroft is almost exclusively grassy. The 

 vegetation is neither cut nor grazed by stock, and analyses 

 of the soil after about 23 years had elapsed since the land 

 had been under the plough show the following changes : — 



Nitrogen, lb. per aci^,. 





In soil to 27 inches. 



Added by rain. 



Gain in soil 

 per annum. 



1881-83. 



1904. 



Broadbalk, 1881 



Geescroft, 1883 



59IO 

 6040 



8jio 

 6980 



00 00 

 O 00 



92 

 41 



The very remarkable gain of nitrogen in the soil of these 

 two plots must be put down to the action of bacteria; on 

 Broadbalk there are leguminous plants with which are asso- 

 ciated the nodule bacteria, Pseudomonas radicicola, but that 

 these are not the only or even the main agents in fixing 

 nitrogen is seen from the gain of nitrogen in the soil of the 

 Geescroft field, which is almost devoid of leguminous plants. 

 The main factor has been the Asotobacter, the bacterium 

 which fixes nitrogen when free in the soil, and its presence has 

 been verified in the soil from both plots. The reason for its 

 activity on these pieces of land lies in the fact that the yearly 

 growth of vegetation is allowed to die back and fall on to 

 the land. Thus the soil receives an annual contribution of 

 purely carbonaceous material previously elaborated by the 



