382 SCIENCE IN AFRICA 



are almost useless in the east, where also the yields from food crops, 

 such as maize and yams, are only about half that in the west. The 

 oil-palm, which will suffer poor soil but must have a high rainfall, 

 grows better, however, on the alluvium of the Niger delta than on 

 the richer soils of the western provinces where rainfall is less. 



In the southern provinces generally, the pressure of population 

 is sometimes so great that permanent cultivation is forced upon the 

 people. The time actually allowed for fallowing varies from a pro- 

 portion of about seven years' rest to one of cultivation where the 

 population is fairly thin, to one year's rest to two of cultivation in 

 the dense areas. In a few places the land has reached such a state 

 of infertility that woody plants are incapable of regeneration, and 

 even weeds take a long time to become established. Except in 

 the densest areas there is not much erosion, but when the vegeta- 

 tion is removed, considerable loss of fertility results. 



GREEN MANURING 



To improve the conditions in Southern Nigeria, described 

 above, the method of approach has been to protect the soil with 

 cover crops, which, when dug in as green manure, extend the 

 period of cultivation and reduce the period of lying fallow or rever- 

 sion to bush. The combination of making mounds and contour 

 ridges with cover crops and green manuring is generally considered 

 to be the most practical way of combating loss of fertility. The 

 process of green manuring is based on the double rainy season in 

 Southern Nigeria, which enables two crops to be produced during 

 the year, one of them being green manure. At the central research 

 station at Ibadan experimental plots have been kept in permanent 

 fertility with green manures for many years. It is realized that a 

 new method will not be generally adopted unless its results are 

 demonstrably more profitable, and do not involve a heavy burden 

 of additional work, and all experiments are conducted with this 

 consideration in view. In essence the system worked out at Ibadan 

 consists of one green manuring with a leguminous crop, usually 

 Mucuna aterrima or Calopogonium mucunoides, and a deep cultivation 

 every three years. During the intervening period one or two cover 

 crops are interspersed between the normal food crops, arranged at 

 times when the soil is otherwise bare of vegetation. By such 



