THE CULTIVATION OF COCOA 157 



It is therefore necessary, in order to supply oxygen to 

 the roots, that the soil should be sufficiently aerated. 



It may be taken for granted that in a well-aerated 

 soil the amount of air, and the circulation of that air, 

 is sufficient to supply the roots of the cocoa, of the 

 shade trees, and of the weeds with oxygen ; and 

 accordingly weeds do no harm of any importance by 

 taking a portion of the air in the soil. But they are 

 harmful to the free respiration of the cocoa roots in 

 another way, for they cover the soil with a layer, which 

 is more or less close according to the sort of weed. 

 Grasses, for instance, make a hard and compact layer 

 of stems and roots, while other weeds (e.g. those of 

 the family of the black pepper or Piper aceae, of the 

 Compositae, of the banana family or Musaceae, of the 

 Papilionaceae, of the ginger family or Zingiberaceae, 

 etc.) have only a slightly developed root-system and do 

 not appropriate so great a part of the surface. Planters 

 distinguish these two kinds of weeds very clearly : they 

 know that the former the " noxious weeds," as they 

 call them are harmful, as they keep the soil compact 

 and prevent the free access of air, while the latter are 

 much less noxious and are called " harmless weeds." 



Summarising this discussion, we may say that weeds 

 are partly noxious, partly harmless, partly even useful. 

 They are noxious : (l) by preventing the free access of 

 air to the soil ; (2) by absorbing and evaporating water 

 from the soil, thus making the soil still drier in times 

 of drought. 



They are harmless as regards the absorption of 

 plant-food from the soil. 



They are useful by giving humus to the soil. 



There are, however, other influences of the weeds 

 which must be considered. One is a useful influence 

 which hardly needs explanation : this lies in preventing 

 the washing away of the soil on sloping land. Cocoa 

 plantations are often established on the slopes of 

 mountains, and the steeper the slope the more care 

 must be taken to prevent the soil being washed away. 



