154 Bacteria in Relation to Country Life 



and in the third, still slighter. This diminishing effect 

 from the application of the manures is due partly to 

 the reduction of their plant-food by the first crop and 

 partly to the gradually decreasing availability of the 

 remaining portion. 



It seems that the bacteria first attack the substances 

 they can destroy most readily. After these are used up, 

 they cannot grow very fast on account of the greater 

 difficulty in securing their food from the residues. In 

 the course of time, only the most resistant portions of 

 the soil-humus remain, portions that change so slowly 

 as to prohibit normal crop growth. Chemical analysis 

 shows that the carbon disappears from the humus at 

 a more rapid rate than does the nitrogen. Hence, in 

 old humus, a smaller proportion of carbon is found, and 

 a larger proportion of nitrogen. In regions of scant 

 rainfall, the proportion of humus in the soil is ftiuch 

 smaller than that in humid regions. But the humus of 

 arid soi s is richer in nitrogen to such an extent as to 

 be able to supply a sufficient amount of it to the growing 

 crops in the irrigated districts. 



