August, 19J0.J 



131 



Scientific Agriculture. 



of production than is attained on the 

 unmanured plot, being an average of 

 38*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 Hie down 

 and fall back on the land. At Rotbam- 

 sted portions of the Broadbalk 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 vegeta- 

 tion is neither cut nor grazed by stock, 

 and analyses of the soil after about 

 twenty-three years had elapsed since the 

 land had been under the plough show 

 the following changes : — 



Nitrogen, lb. per acre. 



\ liilnil Gai " 



In soil to 27 inches. h „ " ;„ soil per 

 irri.r:}. mm. oy ram. annum- 



1881-83. 1901. 



Broadbalk,1881... 5,910 8,110 

 Geescroft, 1883.. .6,040 6,980 



80 



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 associated the nodule 

 bacteria, Pneudomonas 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 ot the Geescroft field,, which is 

 almost devoid of leguminous plants. 

 The main factor has been the Azotobac- 

 ter, 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 

 plant from the carbon dioxide of the 

 atmosphere, and by the oxidation of 

 this carbonaceous material the Azoto- 

 bacter organism derives the energy neces- 

 sary to bring the free nitrogen gas into 

 combination. In the laboratory Azoto- 

 bacter must be supplied with sugar or 

 similar carbohydrates, and fixation of 

 nitrogen will then take place to an 

 extent that is proportional to the amount 

 of sugar oxidised ; in nature the requi- 

 site oxidisable carbohydrates is supplied 

 by the debris of previous vegetation. 

 We have seen that on the adjoining un- 

 manured plot of Broadbalk from which 

 the wheat is removed every year, fixation 

 is so small that it only just balances the 

 yearly loss of nitrogen due to drainage, 



&c; fixation is kept down at this low 

 level because, beyond the small root aud 

 stubble residue of the wheat plant, there 

 is no carbonaceous material supplied for 

 the Azotobacter. The much greater 

 nitrogen fixation in the Broadbalk than 

 in the Geescroft soil may be set down to 

 the presence of a fair amount, 2-3 per 

 cent, of calcium carbonate, a substance 

 which is almost absent from the Grees- 

 croft soil, yet without it the Azotobacter 

 cannot function properly. 



It is to the activity of Azotobacter 

 when thus supplied with carbohydrate 

 by the annual fall of vegetation that we 

 may attribute the accumulation of 

 nitrogen in virgin soils. The higher 

 plants alone, however long they might 

 have occupied the land, could only 

 restore what they had previously taken 

 from the soil, and thus could originate 

 no such vast stores of nitrogen as are 

 found in the virgin soils like the black 

 steppe soils of Manitoba and the North- 

 west. This conclusion is strengthened 

 by the fact that such steppe soil? are 

 always well supplied with calcium car- 

 bonate, a necessary factor in the artion 

 of Azotobacter. The organism itself has 

 also been isolated from all such soils 



We are now in a position to see Losv 

 far these various examples can be made 

 to interpret the conditions which pre- 

 vail in practice. 



In the first place, it is clear that the 

 growth of successive cereal crops which 

 are wholly removed from the land will 

 rapidly reduce the stock of nitrogen 

 originally in the soil, not only by the 

 amounts withdrawn in the crop, but also 

 because of the oxidising actions which 

 the cultivation sets up in the land. 

 Moreover, the richer the land to begin 

 witb, the greater will be the annual 

 losses. When the laud gets anywhere 

 near the pitch of impoverishment re- 

 presented by the Broadbalk unmanured 

 plot, not only is the annual con- 

 version from dormant into available 

 plant food small, but the wasteful 

 oxidation is similarly reduced, and the 

 stock of nitrogen is only slowly 

 depleted. If instead ot cropping con- 

 tinuously with cereals a more conserva- 

 tive .system of farming is introduced, in 

 which leguminous crops become a regular 

 feature in the rotation, and a certain 

 amount of carbonaceous matter is re- 

 turned to the laud as by the folding off 

 of green crops by sheep, the recuperative 

 agencies fixing nitrogen become suffi- 

 cient to repair the losses due to the 

 crops and the waste by drainage and 

 oxidation, and a moderate level of 

 fertility may be maintained indefinitely 



