212 CROPS GKOWN IN EOTATION 



when farmed on the four-course system will rise rather than 

 fall in fertility if during the rotation it receives manure supplying 

 150 II3. of nitrogen j^er acre, even though the roots are wholly 

 removed from the land. Such a quantity of nitrogen would be 

 supphed by 15 tons of fair ordinary dung. It should be, how- 

 ever, remembered that the state of equilibrium thus attained is. 

 not a very high one, and that if the land is to ])e kept in higher 

 "condition," with a generally larger production throughout^ 

 then the losses of nitrogen by drainage and denitrification will 

 also be greatly increased. Hence if the higher level of produc- 

 tion is to be maintained it will require an additional expendi- 

 ture of nitrogen as manure, not merely enough to make up for 

 the larger amount removed in the greater crops, but a consider- 

 able surplus in order to compensate for the increased wastage. 



When the mineral constituents of plant food are considered 

 — the phosphoric acid and potash — there is no difficulty in 

 estimating the annual loss or gain to the soil, because we know 

 that there are no recuperative agencies at work to increase the 

 original stock of such mineral substances in the soil, nor, on 

 the other hand, are the only possible losses, those by drainage,, 

 of any moment. The annual draft on the soil can then be 

 estimated with accuracy if we know the amounts of the con- 

 stituents in question which are contained in the manure supplied 

 and in the crops removed. 



On the unman m^ed plot, from which everything is removed, 

 the loss of phosphoric acid is about 7*5 lb. per acre per annum 

 under the rotation, a figure Avhich is very close to the annual 

 withdrawal of phosphoric acid from the unmanured plots 

 where wheat and barley are respectively grown year after year. 

 On the continuous wheat plot the amount removed in the crop 

 is 8 "9 lb. per acre per annum, on the barley it becomes 7 '8 lb. 

 per acre joer annum. Again, as regards the potash, the average 

 removal from the unmanured plot under rotation is 13 '2 lb.,, 

 whereas the continuous wheat plot similarly unmanured loses 

 14*3 lb., and the continuous barley plot 11 '6 lb. per acre per 

 annum. 



