AGRONOMY 33 



the rate of oxidation is closely parallel to the produc- 

 tiveness of the soil. It is essential that the soil con- 

 ditions should be aerobic, as in arable soils ; pasture 

 (lawn) soils, where the conditions are anaerobic, do 

 not show the same relationship. It appears that the 

 rate of oxidation affords a measure of bacterial activ- 

 ity which is closely connected with productiveness. 



Nitrogen. — Nitrogen is an essential element in 

 the growth of trees, and it is present in the soil as 

 nitrogen gas, nitrates, nitrites, ammoniacal salts 

 and organic matter. 



The air contains four-fifths of its volume of 

 nitrogen gas, and this gas enters the pores of the 

 soil, and by means of bacteria (microbes) present 

 in the soil the atmospheric nitrogen is introduced 

 into the roots of leguminous trees, shrubs and other 

 plants. This power of obtaining free nitrogen is 

 due to the roots becoming inoculated with the 

 microbes present in the soil. The microbes, which 

 give rise to tubercles on the roots, enter into a kind 

 of partnership (the symbiosis of De Bary) with the 

 host plant for mutual advantage. The microbes 

 have the power of bringing the nitrogen of the air 

 into organic combination within the plant ; and the 

 soil is also enriched in assimilable nitrogen.'^ 



' Jamieson (Report of the British Association, 1906) states that 

 he has discoyered an organ specially fitted to absorb nitrogen from 



D 



