1910.] Conservation of the Fertility of the Soil. 119 



the greater will be the annual losses ; when the land gets 

 anywhere near the pitch of impoverishment represented 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 of cropping con- 

 tinuously with cereals a more conservative system of farming- 

 is introduced, in which leguminous crops become a regular 

 feature in the rotation, and a certain amount of carbonaceous 

 matter is returned to the land, as by the folding off of green 

 crops by sheep, then the recuperative agencies fixing nitrogen 

 become sufficient 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 without the introduc- 

 tion of any extraneous source of nitrogen. 



Such, indeed, was the state of affairs in Europe prior to 

 the discovery of artificial manures and foods ; the farm had 

 to be self-supporting, the nitrogen that came back to the land 

 in the farmyard manure had all been taken from the land 

 previously; it was less than that which left the land by the 

 amounts in the corn, meat, milk ; and wool sold off the farm, 

 and by all that was lost and wasted in making the farmyard 

 manure. These losses were, however, so far balanced by the 

 gains of nitrogen due to bacterial agencies that the fertility 

 of the soil at its low level remained unimpaired; e.g., there 

 is evidence that the average production of wheat in the south 

 and east midlands of England had remained at about 20 

 bushels per acre for a long period up to the early years of 

 the nineteenth century. That the land can attain such an 

 equilibrium of production and fertility is indicated by some 

 of the results obtained on the Agdell Field at Rothamsted, 

 where a four-course rotation of swedes, barley, clover or bare 

 fallow, and wheat is followed. The experiment started in 

 1848, and since that time the soil has been analysed in 1S67, 

 1874, 1883, and 1909. For our purpose the instructive plot 

 is that which receives no nitrogen as manure, but minerals, 

 i.e., phosphoric acid and potash, once in each rotation ; it 

 is divided into four sub-plots, two on which clover (or beans) 

 is grown before the wheat, two on which there is a bare 

 fallow; one each of these two again has the swede crop 

 returned to the land, whereas on the other it is carted away. 



