458 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



Carbon substrates 



Partial oxidation 

 Red 



Cytochromes 

 No 



Membrane envelope 



Figure 6. — A diagram of a hypothesis for nodule nitrogen fixation. One nitrogen-fixing 

 unit is shown. Reducing power, generated in the bacteroids by partial oxidation of 

 carbon substrates supplied by the host plant, is passed along an electron transport 

 chain involving haemoglobin and is used for the ultimate reduction to NH3 of N2 

 activated in the membrane envelopes. X= unknown steps between the bacteroid 

 metaboHsm and haemoglobin. Y= unknown steps between haemoglobin and the 

 production of NH3. (F. J. Bergersen. Bacterial. Rev., vol. 24, pt. 1, pp. 246-250. 

 1960.) 



though the synthetic nitrogenous fertilizers are making a valuable 

 contribution in various farming systems, the amounts employed are 

 still small in comparison with the total amounts of nitrogen concerned 

 in the world's crop production. Cereal crops in Britain remove about 

 50 pounds of nitrogen from an acre of soil, and a similar amount is 

 present in about 1,000 gallons of milk; yet only 22 pounds of fertilizer 

 nitrogen is returned to the soil, and a substantial amount of this is lost 

 by bacterial denitrification or washed away by rain. As long as the 

 world's population increases and arable land remains even at its pres- 

 ent acreage there will be rising demand for nitrogenous fertilizers; 

 unf ortmiately it is not economical to use them on a large scale in under- 

 developed countries where, of course, the need for protein for animal 

 and human consumption is greatest. 



