386 SCIENCE IN AFRICA 



The native plants his crops at successive intervals, in accordance 

 with the length of the growing period of each, so that the crops 

 are not necessarily harvested in the rotation in which they were 

 planted. Also ripening periods vary, so that the labour of harvest- 

 ing is well distributed. Trials carried out at Tamale in 1929 showed 

 that one acre of land under mixed cropping yielded the same quan- 

 tity of produce as nearly two and a half acres cultivated on the 

 single crop system. It was therefore concluded provisionally that 

 mixed cropping was the superior of the two methods, but further 

 trials were considered necessary. The interplanting of groundnuts 

 with cotton, tried at Ngetta in Uganda since 1934, gave similar 

 results, provided the right proportions of groundnuts and seasons 

 for planting were adhered to (Uganda 1936, D.R.). In the Gam- 

 bia, however, trial interplantings of groundnuts and bulrush 

 millet {Pennisetum typhoides) and groundnuts and Guinea corn, 

 undertaken during 1934 at Wuli, did not give a sufficient yield of 

 groundnuts to warrant their cultivation by this method (Gambia 

 1934, D.R.,p. 10). Itis possible, however, that experiments on these 

 lines may yet prove successful, since in Madras where a short- 

 season bulrush millet is used, in which groundnuts are under- 

 planted, little interference is caused, provided the millet is har- 

 vested before the fruit has covered the ground. Mixed cropping is 

 usually looked upon as an insurance against adverse seasons; some 

 parts of the mixture may succeed or fail according to the vagaries 

 of the season. Difficulties may arise from lack of water when two 

 crops are competing on the same soil, so research is advisable to 

 find out to what extent and in what form mixed cropping should 

 be developed in any area. 



COMPOSTING 



Owing to climatic conditions, for example in Northern Nigeria, 

 the turning in of green manures is not always practicable, since 

 the ground requires to be moist and the crop succulent. If the 

 crop is too dry it does not decompose and fixed nitrogen in the 

 soil may be deficient; if too woody, the bacteria which break down 

 the cellulose exhaust the soil of nitrogen which they need for their 

 sustenance. For this reason the composting of vegetable waste 

 materials or even of crops, such as elephant grass, grown for this 



