- i6 7 - 



However, their action is not sufficient to maintain the stock 

 unimpaired, even in this impoverished soil, when the crops are 

 wholly taken away. 



B. We may now take another case, that of land very rich in 

 organic matter and under arable cultivation, the crop as before 

 being wholly removed. An example of this- kind is afforded by 

 the farmyard manure plot on Broadbalk, where 14 tons per acre 

 of farmyard manure, containing about 200 Ib. of nitrogen, are applied 

 every year. 



BROADBALK, PLOT 2. NITROGEN, LB. PER ACRE. 



Under these conditions the losses of nitrogen are enormous; 

 of the yearly supply of nitrogen not quite a quarter has been re- 

 covered in the crop, and less than a quarter remains behind as 

 an enrichment of the soil; more than half has been permanently 

 lost through the destructive agencies enumerated above (3 and 4). 



The production of this plot of land with its annual application 

 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 equilibrium thus attained is at a much 

 higher level 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 die down and fall 

 back on the land. At Rothamsted 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 Broad- 

 balk 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 



