Kellogg and Orvedal 13 



is almost a necessity. Under such cover, the tendency of rapid leaching 

 of nutrients will be offset greatly by the equally rapid uptake of these 

 nutrients by plants. Thus, the absorbed nutrients will again be brought 

 to the surface and subsequently released upon the death and decom- 

 position of the vegetative material. Generally, the common practice in 

 the temperate regions of growing a single crop in a field, with periods 

 without plant growth between harvest and sowing, is unsuited to the 

 humid tropics. We need there to think of mixed cultures rather than 

 monocultures, even rather than rotations of crops grown in mono- 

 culture. 



Exposure of the soil to direct sunlight has been observed to be harm- 

 ful to many tropical soils. Nearly continuous shade is essential for the 

 maintenance of productivity. This is partly a matter of fertility and 

 partly a matter of soil structure. The removal of all vegetation in some 

 places even brings about severe drying at and near the surface, resulting 

 in formation of a hard laterite crust or pan in soils such as Ground- 

 Water Laterites. 



The corridor system of crop rotation, perhaps best developed in the 

 Belgian Congo, shows great promise as a good system for many tropi- 

 cal soils, especially where commercial fertilizers cannot be had eco- 

 nomically. This system is simply a modification of the old practice of 

 shifting cultivation. Under the corridor system, a given tract of arable 

 land is divided into long strips, say 1,000 yards long and 100 to 300 

 yards wide. A corridor is provided for each year of cropping and for 

 each year under forest fallow. These are arranged so that some stage 

 of forest borders each corridor in crops. Besides, a few specimen trees 

 may be left for partial shade and regeneration. On a comparatively good 

 soil, one may have a six-year rotation of crops with twelve years of 

 forest fallow. Such an area needs eighteen corridors. Each year an old 

 corridor is returned to forest fallow and a new one cut and planted to 

 crops (9). 



Good results have been obtained from organic manures and com- 

 posts in India (7) and elsewhere. Although the use of manures has 

 increased yields in the temperate regions too, they are of special im- 

 portance in the tropics because of the gradual release of nutrients in 

 the organic matter as contrasted to mineral fertilizers. Where leaching 



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