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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 (18 Hectol. per hectare) 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 1867, 

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

 which receives no nitrogen as manure, but minerals, i. e., phos- 

 phoric 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 ; on each of these two 

 again has the swede crop returned to the land, whereas on the other 

 it is carted away. The following table shows the percentage of 

 nitrogen in the surface soil (9 inches) at the respective dates, together 

 with the average crops on each plot over the period 1852-1903: 



NITROGEN PER CENT IN SOIL OF AGDELL FIELD, ROTHAMSTED. 

 The plots all receive mineral manures, but no nitrogen. 



