[ 43 ] 



43- The decay of organic matter helped by the various 

 processes described, results in a suppjy of Phosphoric acid, and 

 Nitrogen in an available form is also supplied for the nourish- 

 ment of plants. Generally speaking the more organic-matter 

 there is in a soil the more nitrogen does it contain, and the 

 proportion of organic matter may be roughly considered as a 

 direct measure of the fertility of a soil. Generally speaking 

 also the more mixed a soil is (i.e. the larger the number of 

 rocks and minerals out of which it is formed) the more fertile 

 it is. Hence alluvial soils, and soils formed at the junction 

 of two geological formations, are more fertile than soils 

 resting on single formations. 



