282 Bacteria in Relation to Country Life 



but scant amounts of it to the crop during the growing 

 season. Others contain much smaller quantities, yet 

 supply the needs of large harvests. There is scarcely 

 a doubt that bacteria play an important part in deter- 

 mining these differences. For one thing, the carbon 

 dioxid evolved in the decay of organic matter hastens 

 the solution of soil phosphates, just as it hastens the 

 solution of the lime carbonate. 



In fertile soils well supplied with humus the bacterial 

 activities are rather vigorous. Large amounts of carbon 

 dioxid are produced and the solution of the soil-phos- 

 phates proceeds at a comparatively rapid rate. Higher 

 plants, therefore, find in the soil bacteria powerful 

 allies in securing an abundance of available phosphoric 

 acid. The bacteria, in their turn, are affected not only 

 by the amount of humus present in the soil, but, also, 

 by its quality. A greater or slighter proportion of nitro- 

 gen in the humus influences directly not only the num- 

 bers, but, also, the kinds of bacteria that become most 

 prominent. Our knowledge of the relations in this case 

 is extremely limited. Future investigation will undoubt- 

 edly find this an interesting as well as a profitable field 

 to develop. 



The influence of soil bacteria in the supply of phos- 

 phoric acid to crops may be best understood by remem- 

 bering that the element phosphorus exists in the soil 

 in two general classes of compounds, — inorganic and 

 organic. The inorganic or mineral combinations of 

 phosphoric acid exist in the m.ore or less weathered rock 

 particles as phosphates of lime and magnesia; to a slight 

 extent, also, as phosphates of iron and alumina. These 



