XI.] MANURES FOR TROPICAL CROPS 335 



receive any fertiliser, but should be manured by keep- 

 ing the land closely grazed with sheep receiving hay 

 roots, cake, and corn, etc. 



Tropical and sub-tropical crops. — It is very difficult to 

 lay down any general rules for the manuring of tropical 

 and sub-tropical crops, because the conditions of soil 

 and climate are subject to such extreme variations that 

 entirely different methods of treatment have to be 

 pursued in different countries. Certain general prin- 

 ciples may, however, be indicated, to be taken into 

 account whenever any scheme of manuring has to be 

 tentatively adopted in practice. All the processes by 

 which the insoluble constituents of plant food in the soil 

 are rendered available for the plant are greatly acceler- 

 ated in tropical soils, always provided they contain a 

 sufficiency of water. The decay of organic matter takes 

 place with extreme rapidity, so that the humus content 

 of cultivated soils will be quickly reduced unless means 

 are found of repairing the losses ; for the same reason 

 all organic manures containing nitrogen are both more 

 quickly and more completely utilised by the plant than 

 they are in temperate soils. The higher temperature of 

 the soil water, the greater production of carbon dioxide 

 in the soil, also result in a more rapid weathering of the 

 mineral constituents of the soil, so that the reserves of 

 phosphoric acid and nitrogen present in the soil are 

 more available in tropical countries. 



It also follows that smaller amounts of manure in 

 proportion to the plant food withdrawn by the crop are 

 effective under tropical conditions ; whereas, in England, 

 one cannot hope to recover more than one-half of the 

 nitrogen applied as farmyard manure, in a hot soil with 

 an abundant rainfall nearly the whole will be available, 

 and a correspondingly smaller application will be 

 required. It is always the crops of short duration on 



