168 COCOA 



CHAP. 



the analysis conveys no clear impression as to the fertility 

 of the soil. For instance, certain soils which, according 

 to the analysis, contained up to 3 per cent of potassium 

 (a very high amount), turned out to be of very little 

 fertility ; but their productiveness could be greatly 

 increased by manuring with sulphate of potassium, it 

 being therefore clear that the soil contained very little 

 available potassium. The very fertile mud of the Nile, 

 on the other hand, contains only 0'5 per cent of potas- 

 sium, but the yield of the crops cannot be increased by 

 potassium manuring ; the potassium in the soil is, 

 therefore, almost wholly present in an available 

 form. 



Many analysts have tried to find a method by 

 means of which the available quantity of the different 

 elements can be ascertained, but they have met with 

 little success. It is true that in some cases the analysis 

 of an extraction of the soil with 1 per cent citric acid 

 gave figures which seemed to agree fairly well with the 

 results of the manuring experiments, and to indicate 

 approximately how much available food was present ; 

 but for general use this method also is unreliable. It 

 may therefore be said that the chemical analysis of a 

 soil is as yet unable to give us reliable data as to the 

 quantities of the different elements which are available 

 for a definite species of plant for another circumstance 

 which complicates the problem is the fact that a 

 compound which is to a certain extent available for 

 one species of plant may not be available for another 

 species. 



Up till now we have been dealing only with one 

 side of the question of manuring viz. the supplying 

 of the soil with elements which are used by the plant 

 as food but, as has already been mentioned, manuring 

 is in many cases also applied to improve the structure 

 of the soil. For this purpose only the humus-producing 

 manures are of importance : farmyard manure, green- 

 dressing, compost and mulch of leaves and grass. These 

 manures are of the greatest value in the tropics, where 



