Bacteria as Competitors with Crops 161 



not destroyers only. To some extent, they are also 

 builders, for any change caused by them in the. soil- 

 humus, whether it be ammonification, nitrification, or 

 denitrification, involves necessarily their multiplication. 

 Since, however, the bacterial bodies are complex in 

 their structure, they add to the soil considerable quan- 

 tities of highly organized materials derived from the 

 simple decomposition products of humus. Soil bacteria, 

 like higher plants, utilize for their growth the soluble 

 mineral salts in the soil, particularly the soluble phos- 

 phates and sulfates. They also make use of the am- 

 monia, and, particularly, the nitrates derived from the 

 humus for the building of their bodies. Indeed, certain 

 bacterial species, which can rapidly transform large 

 quantities of nitrate nitrogen into organic combinations, 

 have been isolated from the soil. 



It may thus happen that the soil bacteria actually 

 compete with the' crop for the available nitrogen, and 

 conditions probably exist in which the crop-yields are 

 considerably reduced on account of the transformation 

 of the soluble nitrogen compounds in the soil into the 

 insoluble portions of the bacterial bodies. Soil bacteria 

 are, then, the indispensable agents in the breaking down 

 of the soil-humus, and in the supplying of simple com- 

 pounds of nitrogen to higher plants. They may per- 

 form this work economically or wastefully, depending 

 upon soil and climatic conditions whereby their physio- 

 logical efficiency becomes greater or less. By proper 

 methods of cultivation, crop-rotation, or manuring, 

 the bacteriological efficiency of these organisms can be 

 controlled to advantage. The withdrawal of soluble 



