126 THE DEVELOPMENT OF BRITISH FORESTRY 



forming materials, which is not the case with land under 

 trees. To make a proper comparison between the drain 

 on the soil in either case the analysis should be confined 

 to the annual growth of the same or analogous plant struc- 

 tures of crops grown under similar conditions, such as the 

 straw or haulm of agricultural crops, and the timber of forest 

 crops on manured and cultivated ground. If oats or barley 

 were dibbled into ground covered with weeds or rubbish, 

 the ash constituents of the resulting crops Avould probably 

 be as small, weight for weight of dry substance, as those 

 of forest trees. The fact, again, appears to have been 

 overlooked that the comparison is being made between a 

 crop using the most fertile portion of the surface soil, and 

 another partly obtaining its nourishment from imperfectly 

 weathered subsoils. 



It is a well-known fact that cultivation, in the form of 

 trenching to a depth of two or three feet, stimulates the 

 growth of forest trees for the first few years enormously, but 

 that this increased rate of growth is not maintained beyond 

 a certain period. On two soils of equal quality it is probable 

 that trees will attain the same size ultimately whether the 

 ground at the time of planting is trenched or not. The 

 Rothamstead experiments have shown that the fertility of 

 the soil cannot be reduced beyond a certain point under 

 cereals, and it is equally probable that a permanent 

 increase in fertility cannot be produced by forest crops. 

 It is more probable that the im^^rovement in growth 

 noticed in trees following previous crops is due to the 

 physical change which has taken place by the accumula- 

 tion of humus on the surface, and the killing out of 

 surface growth, rather than to any increase of natural 

 fertility, other than that due to the temporary nitrogen 

 supply from the humus. Soil improvement in this Avay 

 has its limits, and there is probably no good reason for 

 the assumption that afforestation can do more than 



