SOIL EROSION: THE GROWTH OF THE DESERT IN AFRICA 

 AND ELSEWHERE " 



By Sir Daniel Hall, K. C. B., M. A., D. Sc, LL. D., F. R. S., M. R. I. 



Director of the John Innes Horticultural Institution; lately Chief Scientific Adviser 

 to the Minister of Agriculture and Fisheries 



[With two plates] 



It is a commonplace of geology that the surface of the earth is 

 constantly in motion — that our mountain ranges have taken shape and 

 our river courses and valleys have been excavated by the simple 

 agencies of rain, frost, and wind. But we are accustomed to think of 

 these agencies as operating over long periods, geological rather than 

 historic time. Even Darwin's discovery that earthworms can bury 

 stones on the surface of pasture land at the rate of an inch in every 

 4 or 5 years came as a surprise. But in many parts of the world a 

 much more rapid movement of the surface is going on, with serious 

 economic consequences to the agriculture of the country — and it is 

 this form of denudation, that we call soil erosion, with which I propose 

 to deal tonight. 



Usually we regard the soil as something particularly stable, but I 

 want you to realize that this stability is almost wholly due to its normal 

 covering of vegetation. By soil we understand the surface layer of 

 relatively fine-grained material — sand and silt and clay, generally 

 dark in color through the presence of humus — and traversed by the 

 roots of the surface vegetation. As a rule the soil proper is not more 

 than 6 inches to a foot deep; below this the humus becomes small in 

 amount, though there are a few "black soils," as in Russia or Manitoba, 

 which are rich in humus to a depth of 4 feet or so, but these are un- 

 common. But what I would remind you of, even at the risk of being 

 elementary, is that the fertility of land resides in this surface layer of 

 soil. The humus supplies the nitrogen, which is the chief element of 

 plant nutrition, and the phosphoric acid, of almost equal importance, 

 is chiefly accumulated in the upper soil. It is a fallacy of old standing 

 that the good earth lies below to be brought up by the man who will 

 put his plow in deep enough. Nevertheless, there is great virtue in 



1 A lecture delivered before the Royal Institution of Great Britain at the weekly evening meeting, Novem- 

 ber 12, 1937. Reprinted by permission from the pamphlet of the Royal Institution. 



303 



