1 82 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



conditions so as to facilitate absorption by the plant. It has been 

 shown in agricultural practice that absorption of nitrate is greatly 

 facilitated by properly balancing the manures, and we may hope for a 

 good deal by adopting the same plan in horticulture. At present 

 horticultural practice is, to say the least, indiscriminate, and there is 

 considerable scope for improvement. 



Again, however, a cycle is operating : opposed to all this loss of 

 nitrogen are two sets of processes by which the losses are made good 

 and nitrogen is fixed in the soil. 



The Nitrogen Cycle in the Soil. 

 Complex Nitrogen Compounds. 



Nitrogen 



The growth of leguminous crops, as is well known, adds to the 

 stores of nitrogen in the soil through the combined operations of the 

 plant and the organisms present in the root nodules. Leguminous 

 plants occur in nearly all natural vegetation and their activity is 

 very wide-spread, constituting by far the most considerable source of 

 added nitrogen. 



The second source of increased nitrogen is to be found in the 

 activities of certain free-living bacteria which can fix nitrogen on 

 their own account and do not require the co-operation of a living 

 plant. At least two of some importance are known : Clostridium, 

 which was the first to be discovered, is an anaerobic organism, i.e. 

 it works in absence of air, but it can form a close association with 

 aerobic organisms so that it can work in presence of air, and certainly 

 its action is not precluded in the soil. The second, Azotobacter, is 

 more potent and has proved far more attractive to investigators. 

 It works very vigorously in crude cultures, but is less active in pure 

 cultures ; it used to be supposed to lose its potency on cultivation, but 

 recent work has shown that it remains active in a suitable medium. 



Both Clostridium and Azotobacter build up complex nitrogen com 

 pounds from gaseous nitrogen, a process that requires energy ; and 

 both draw the necessary energy from the oxidation of the plant 

 residues — presumably the carbohydrates — in the soil. It is reasonable 

 to suppose that both organisms are active in the soil, but no rigid 



