THE FERTILITY OF SOILS IN THE TROPICS 363 



activity. On the whole it may be said that though the 

 alternations of wet and dry seasons probably affect the 

 biological activities of soils in the tropics to quite an 

 appreciable extent, such variations will depend for their 

 magnitude on local conditions, and in any case will con- 

 trast markedly with the regular rise and fall in activity 

 characteristic of soils in temperate regions. 



In relation to the rate at which organic matter tends 

 to decay as the result of bacterial activity, the result of 

 our investigations may be summarized 'by saying that as 

 the outcome of thirteen separate sets of measurements 

 on soils from different localities, both in situ and also on 

 samples under laboratory conditions, it was found that 

 in periods varying between six and twelve months the 

 content of organic matter became reduced by amounts 

 ranging between 12 and 30 per cent, of the total origin- 

 ally present. The effect of organic matter on the 

 physical and mechanical characters and properties of soils 

 has for long been understood and appreciated; of more 

 recent date is the apprehension of the functions exercised 

 by it in relation to the supply of energy for the mainten- 

 ance of various important bacterial reactions; chief among 

 these may be cited the putrefactive processes resulting 

 under suitable conditions in the liberation of a constant 

 supply of carbon dioxide which assists in rendering avail- 

 able the locked up stores of plant food in the soil. 



Of importance also appear to be those organisms which 

 are responsible for the fixation of atmospheric nitrogen 

 without the intervention of leguminous host plants, 

 notably those comprised in the Azotobacter group. The 

 existence of organisms of this type has been demon- 

 strated in soils throughout the Leeward Islands and their 

 activity in fixing nitrogen measured approximately in a 

 number of instances. 



A study of the soil conditions obtaining on plots of 

 cacao in the Dominica Botanic Gardens, comprising a set 

 of manurial experiments in which each unit has received 

 the same manurial treatment for the past twelve years 

 continuously, has shown that on those plots which have 

 received moderately heavy dressings of organic manure 

 considerable accretions of nitrogen have taken place in 



