THE FERTILITY OF SOILS IN THE TROPICS 



I 



413 



Coarse sand 

 Fine sand 

 Silt ... 

 Fine silt 

 Clay ... 

 Humus 



No. 9 is an instance in which the closeness of texture 

 is due to the large preponderance of silt, the growth 

 here being exceptionally poor. In the other cases the 

 predominance of the clay and fine silt fractions is such 

 as in Europe would probably be associated with extreme 

 infertility. 



Stones and gravel are very rarely found in Malayan 

 soils; in fact, in most cases no material coarser than 

 0*5 mm. is met with. 



Chemical Composition. It seems characteristic of 

 tropical soils that the analytical results yielded by even 

 the most fertile of them, are much poorer than those of 

 temperate climates. 



This is particularly noticeable when comparing the 

 "available" amounts of potash and phosphate; but no 

 doubt the higher temperature and greater rainfall cause 

 more rapid decomposition of the complex soil consti- 

 tuents, and also perhaps produce conditions more 

 favourable for bacterial growth, so that the plant has 

 always a sufficiency to draw upon. 



Nitrogen. The percentages of nitrogen in the ten soils 

 whose mechanical analyses have been given are as fol- 

 lows : 



No. 



2 



3 

 4 

 5 



Percentage 

 , 0*091 

 . 0-090 

 0-177 

 . 0-II4 

 , 0-127 



No. 

 6 



8 



9 

 10 



Percentage 



0'20 

 . 0-19 

 . 0-248 

 . 0-078 

 , O'2O 



Nos. i and 2 being the best yielders, it is seen how 

 little nitrogen the rubber tree requires; in fact, manurial 

 experiments on No. i have shown further addition of 

 this element to have only a very slight effect, small 

 though the quantity present is. 



As analyses have indicated that in producing a crop 

 of 400 Ib. of dry rubber only 3^ Ib. of nitrogen are taken 



