102 COMPARISON OF TRANSVAAL WITH ENGLISH SOILS 



per cent of nitrogen and rarely as much as O'l per cent of phosphorus 

 pentoxide, and generally much less. 



On the other hand, Transvaal soils are usually well supplied with 

 potash. In spite of their apparent poverty, however, soils of tropical 

 countries are often capable of yielding good crops. It is evident, there- 

 fore, that in judging soils from analytical results, consideration must 

 be given to other circumstances than actual composition. Doubtless, 

 important factors are the rates at which nitrification and other chemical 

 changes occur, the porosity and friability of the soil, as affecting root 

 development and the favouring influences of high temperature, 

 abundant moisture and sunlight upon plant growth. 



It is clearly difficult to establish any standard of composition as 

 indicating relative fertility and in any case, comparisons of the com- 

 positions of two soils can only be expected to afford indications of their 

 relative fertility when the soils are in the same locality or exposed to 

 the same climatic conditions. 



