INFLUENCE OF MAN ON SOIL FERTILITY— JACKS 335 



No other well-tried system of settled agriculture except paddy-rice 

 growing is known that will at least maintain, if not increase, the fer- 

 tility of tropical soil. Rice is the almost universal basis of settled 

 tropical agriculture, as wheat is of temperate agriculture. The min- 

 erals in the floodwaters together, perhaps, with nitrogen fixed by algae 

 often found on paddy fields usually suffice to maintain soil fertility 

 under continuous cultivation for hundreds or even thousands of years 

 without needing a very complex social organization to operate the 

 system. Kice growing, with a little pasturage and livestock, can pro- 

 vide the minimum necessities of a settled tropical society. Otherwise, 

 tropical agriculture is mainly of the shifting-cultivation type which 

 precludes permanent settlement. A patch of land will be cleared and 

 cultivated for two or three years, after which the available plant nu- 

 trients in the soil will have been used up, and crops will fail. The 

 land is then abandoned for, say, 10 to 20 years, during which a secon- 

 dary growth of vegetation will invade the soil, restore its fertility 

 and make possible another short period of cultivation. Shifting agri- 

 culture is essentially exhaustive, the purpose of the abandonment of 

 cultivation being to rest the soil and restore its fertility. Such a 

 system can only work with a very low population density. 



The impact of European civilization on the Tropics has greatly 

 accelerated, but does not seem to have altered, the normal course of 

 soil evolution under Man. European colonists cannot live by shift- 

 ing cultivation, and they have tried with some success to introduce 

 peace and better health into their colonies. Consequently, colonial 

 populations have recently tended to exceed the limits at which the 

 land can be rested long enough to restore its fertility. In every 

 tropical colony (using the term in its widest sense) shifting cultiva- 

 tion is breaking down, and invariably and inevitably soil-exhausting 

 settled agriculture is taking its place. Social and soil evolution is 

 going through the normal stage of soil-exhausting agriculture, often 

 accompanied by catastrophic soil erosion. The wealth required to 

 create soil fertility and, still more, the demand for a high standard of 

 nutrition from a large, well-to-do urban proletariat are absent. Until 

 this demand appears there will be no incentive to bury money in the 

 soil. To the few Europeans who operate highly capitalized plan- 

 tations in the Tropics, however, the incentive of supplying their own 

 urban markets, at home and abroad, is making itself felt. 



We already have examples of intensive, fertility-producing agri- 

 culture in the Tropics that is basically similar to intensive European 

 agriculture. In Southern Khodesia a system of ley farming with 

 large applications of nitrogen has given consistently high yields of 

 maize, meat, and milk, and has improved the condition of the soil. 

 But it has not been operated long enough to merit the term "per- 



