EXTENSIOIST COURSE IN SOILS. 41 



LESSON VI. THE NITROGEN SUPPLY OF THE SOIL. 



(Ref. No. 2, pp. 110-119.) 



As stated in Lesson II, nitrogen is one of the most important 

 elements oi plant growth. Nearly four-fifths of the atmosphere, or 

 over 70,000,000 pounds over each acre of land, is nitrogen. While 

 this is sufficient to support plant growth for thousands of years, yet 

 atmospheric nitrogen can not be utilized directly in plant growth but 

 must first be combined in the soil with other elements before plants 

 can absorb it. It will be the purpose of this lesson to explain how the 

 nitrogen of the air becomes transformed so as to be used by plants 

 and to discuss briefly the practical means of maintaining the soil- 

 nitrogen supply. 



Comhined nitrogen in the atmosphere (Ref. No. 1, p. 22). — From the 

 decay of vegetable and animal materials, burning, electrical dis- 

 charges, and other causes the atmosphere derives certain substances, 

 among which are ammonia and nitric acid, both compounds of 

 nitrogen. These gases are readily absorbed by the moisture of the 

 atmosphere, and when this moisture condenses and falls as rain or 

 snow it carries with it into the soil the nitrogen compounds which 

 it contains. While the available nitrogen thus added to the soil is 

 not large, yet it is an appreciable quantity and contributes in a 

 small way to the soil's fertility. 



The -fixation of atmospheric nitrogen in the soil (Ref. No. 7, pp. 213- 

 223) . — The nitrogen of the soil which plants require comes ultimately 

 from the atmosphere. A large supply of this nitrogen is collected 

 from the atmosphere in the soil through the action of microorganisms 

 called bacteria. The nitrogen-fixing bacteria of the soil may be 

 divided into two classes. On-e class lives independently in the soil 

 and secures nitrogen direct from the air for its growth. After these 

 bacteria perform their life's work their bodies decompose and the 

 combined nitrogen which they contain becomes available for the 

 growth of plants. The amount of nitrogen fixed by this class of 

 bacteria in ordinary cultivated soils has been estimated by different 

 investigators at from 15 to 40 pounds per acre. Probably the latter 

 figure, is much above the general average, even under favorable con- 

 ditions. The other class of nitrogen-fixing bacteria lives in connec- 

 tion with the roots of certain plants, viz, of the family of legumes, 

 including clovers, alfalfa, beans, peas, and others. These bacteria 

 form nodules or tubercles in which the chemical combination of nitro- 

 gen with other elements takes place and from which the host plant 

 obtains much of its nitrogen for growth. 



Inoculation (Ref. No. 7, pp. 223-228). — The bacteria which form 

 tubercles on the roots of leguminous plants are generally different for 

 different species of plants. Those which live on alfalfa, however, are 



