98 



estimated that the plants growing wild contain about 30,000 tons 

 of this rubber, which will supply the factories for some years. 

 Some 7 to 12 per cent, of impure rubber can be obtained from the 

 plant. 



EXTRACTS FROM A PAPER "ON THE ACCUMU- 

 LATION OF FERTILITY BY LAND ALLOWED 

 TO RUN WILD."* 



By A. D. HALL, M.A., Director of the Rothamsted Experimental 

 Station (Laws Agricultural Trust.) 

 • It is well known that the fertility of " virgin" soils is due to 

 the accumulation of debris of a natural vegetation which has 

 been in occupation of the soil for a long epoch previously. Only 

 when the climate and rainfall are suitable to the growth of the 

 plants and the partial preservation of their residues, does a virgin 

 soil of any richness arise : on the one hand, virgin soil may be as 

 poverty stricken as the most worn-out European field because it 

 has never carried any vegetation ; on the other hand, as in the 

 tropics, the debris of an extensive vegetation may decay with such 

 rapidity that no reserve of fertility accumulates. In temperate 

 climates, and with a particular distribution of the annual rainfall, 

 occur the grassy treeless prairies and steppes which provide the 

 ideal conditions for the accumulation of fertility. But that 

 fertility does increase when land is in the state of permanent 

 grass has long been an axiom in our farming : the results set out 

 below will serve to show at what rate the increase takes place 

 under prairie conditions in England, i. e. when the land is left ab- 

 solutely to itself and not even grazed by stock. 



In 1882, about an acre of the upper end of the Broadbalk 

 field at Rothampsted, which had then been carrying wheat for 

 forty years in succession, was not harvested, the crop was allowed 

 to stand and shed its seed without cultivation of any kind. In the 

 following season a fair quantity of wheat came up on this part of 

 the field, but gradually got weaker as the season advanced and 

 the weeds increased their hold on the land. The wheat was still 

 left to struggle on without cultivation, and by the fourth season 

 only three or four stunted plants could be found, each carrying but 

 one or two grains in the ear. With these the wheat disappeared 

 and has never been seen again in that part of the field. This 

 illustrates the fact that our farm crops have become so specialised 

 that they are unable to exist in competition with weeds and other 

 natural vegetation and are entirely dependent on cultivation to re- 

 lieve them from that competition. The piece of land in question 

 has been left untouched since that time, and has covered itself 

 with a coarse grassy herbage interspersed with thorn bushes and 

 briars, young oaks, and other shrubs of the district. Before, how- 

 ever, these shrubs could meet and establish a continuous covert 

 they were stubbed from one portion so as to leave the herba- 

 ceous and grassy vegetation only in possession. This piece of 



* From The Journal of Agricultural Science, Vol. I, Pt. 2, May, 1905. 



