166 COCOA 



CHAP. 



increase the bearing -power of the cocoa tree. Un- 

 fortunately, in the case of cocoa systematic experiments 

 of this sort are as yet rare, and in several countries no 

 manuring experiments at all have been made. We are, 

 therefore, still very ignorant as to the requirements of 

 cocoa on different soils as regards manuring. 



Before describing the results which have been 

 obtained, a few words may be said in regard to the 

 general significance of manuring. Broadly speaking, 

 the aim of manuring is twofold : 



(1) To supply the soil with elements which are 

 necessary as plant-food. These may be given to the 

 soil in the form of artificial manures (basic slag, sulphate 

 of ammonia, etc.) or as organic matter, whether of animal 

 origin (pen manure, guano, dried blood, bone-dust, etc.) 

 or vegetable origin (green-dressing, mulch, compost, 

 ground-nut cake, etc.). 



(2) To improve the structure of the soil. For this 

 purpose only the organic or humus-affording manures 

 are of importance ; the artificial or, as they are some- 

 times called, the " chemical " manures do not play this 

 part. Among the organic manures the most important 

 improvers of soil structure are farmyard or pen-manure, 

 green-dressing, mulch and compost. 



The elements which a plant requires are taken either 

 from the air (oxygen, carbon) or from the soil. Among 

 the latter there are some which are always present in a 

 greater quantity than the plant ever needs (e.g. sodium, 

 magnesium, iron), 1 and others which are not always 

 present in the quantity necessary to give the highest 

 yield (e.g. nitrogen, potassium, phosphorus and lime or 

 calcium). Now it is necessary not only that the soil 



1 The very limited space which can be devoted in this book to these 

 principles of agricultural chemistry makes it necessary for the rules to be stated 

 in very general terms. It is well known, for instance, that cases exist in which 

 iron is present in the soil in too small a quantity, the plant showing the defici- 

 ency by a yellowish colour of the leaves. This happens so seldom, however, 

 that it need not be discussed here. The student who is acquainted Avith the 

 subject will find in this chapter many other rules stated which are not without 

 exception, and which are to be taken only as representing the general idea. 

 Too many details would, it was feared, simply confuse the general reader. 



