204 



Nitrogen Fixation— Nonsymbiotic 



rest of the host plant. Therefore, symbiotic nitrogen fixation requires 

 a much smaller expenditure of energy. Azotohacter consumes at its 

 optimal rate of growth at least 40-50 units of carbohydrate, and 

 usually twice as much, per unit of nitrogen fixed, whereas Bond cal- 

 culated that the corresponding figure for the root-nodule bacteria in 



a bed 



Fig. 86. Growth of Azotohacter in soils treated with starch to test for deficiencies 

 in available nutrient elements. Upper row, soil deficient in phosphate; lower 

 row, soil not deficient in either potash or phosphate. Left to right, a, check, 

 nothing added; h, potasli added; c, phosphate added; d, phosphate and potash 



added (from Sackett). 



soybeans is only about 15 units, and that the total respiration of the 

 nodules consumes some 16 per cent of all the carbohydrate produced 

 in photosynthesis (Jensen). 



Effect of Humus on Nitrogen Fixation 



The beneficial action of humus on nitrogen fixation is frequently 

 ascribed to its inorganic constituents, particularly to its content of 

 aluminum and silicic acid. This assumption is confirmed by the 

 following two facts: («) artificial humus has no such effect; (h) the 

 source of the natural humus influences markedly the degree of its 

 beneficial action. The claim that the action of the humus is due to 

 its inorganic constituents has been further substantiated by the fact 

 that purified humates do not possess any stimulating effect. The 

 role of the colloid is probably due chiefly to its catalytic action and 

 its protective action against poisons; the protective action of the 



