VI 



THE CULTIVATION OF COCOA 155 



air. These " organic constituents," which form by far 

 the greatest portion of the plant, form humus when 

 decaying in the soil. After weeding, the "inorganic 

 constituents" as well as the "organic constituents" 

 are returned to the soil. 



As regards solid constituents, we may therefore 

 say that the weeds do not in fact rob the soil, and that, 

 when not removed from the field or destroyed by fire, 

 they even return more to the soil than they took from 

 it. In this respect, accordingly, a soil, on which weeds 

 are allowed to grow and are regularly cut, has a great 

 advantage over a soil which is wholly free from weeds. 



We now have to deal with the water which the 

 weeds take from the soil. Every plant takes a large 

 amount of water from the soil by its roots ; this water 

 spreads through the whole plant and is again evaporated, 

 principally by the leaves. There is thus a continuous 

 movement of water, which goes from the soil into the 

 plant and, being evaporated again, is lost into the air 

 as invisible vapour. The amount of water lost in this 

 way by the soil through the agency of plants is very 

 important. It has, for instance, been calculated that 

 one plant of the sun-flower (Helianthus annuus] 

 evaporates during one summer 66 kilograms of water ; 

 an old beech evaporates 9000 kilograms during summer 

 time, and one hectare, planted with 400 of such trees, 

 loses during this time 3,600,000 kilograms of water, 

 simply through the evaporation of the trees. On the 

 other hand, the amount of water which a soil evaporates 

 directly under the influence of the sun is of course 

 smaller when a soil is covered with plants and conse- 

 quently shaded, than when no plants are growing on it. 



It is clear, then, that in the case of a soil on which 

 plants are growing, the direct evaporation is small, but 

 the evaporation through the plants is important ; while 

 in the case of a soil without plant growth, the direct 

 evaporation is important, and indirect evaporation 

 through plants is of course not present. There remains 

 the question on which of these two soils is the total 



