In a district of heavy rainfall, the moment bush is cleared and land is left bare fertility is lost at a surprisingly rapid rate. Washing and leaching are the principal factors which contribute to this loss, and in any system of permanent cultivation the necessity for keeping the ground covered with vegetation, and taking every possible step to prevent wash, is of paramount importance. Europeans have been slow to realize the importance of this point, and there is no doubt that the planters of Ceylon and Malaya are suffering for it to-day. The native farmer, by his method of mixed cropping and by leaving the stumps of the bush in the ground, effectively minimizes the loss by leaching, but his system of growing crops on hills instead of ridges, as is common in many parts of West Africa, allows much wash to occur, with the natural consequence of a loss of fine soil.

Growing plants have a remarkable influence in causing rain to enter the soil, instead of allowing it to run off from the surface, as any one can see for himself by merely comparing a lawn and a piece of bare ground during a heavy shower. But this is by no means all. Except when a soil is exceedingly dry, natural processes are always going on by which plant food is converted into the soluble state. Especially the nitrogen contained in the insoluble decayed organic matter is always rapidly changing into a soluble form. Unless there are plants growing on the land to take it up as fast as it becomes soluble, every rain leaches out much nitrogen. Experiments prove that the weight of crop which can be grown in West Africa, as commonly elsewhere, depends primarily on the supply of nitrogen. Bush fallows restore fertility by the accumulation of nitrogen in insoluble organic matter, which, when the land is again cultivated. will decompose and provide soluble nitrogen again There are also recuperative bacterial processes, by which enough nitrogen from the air is accumulated to provide for the growth of bush or grasses; but under cultivation the rate of loss by leaching, let alone the absorption by the crops, is much more rapid than the recuperative processes.