11 



the only typical muck soils available. It will be observed that 

 these show a fixation of 4.38 and 5.02 mg respectively. One 

 mg of nitrogen at 0C and 760 mm pressure represents ap- 

 proximately 0.80 cubic centimeters. The per cent of nitrogen 

 determined for these soils dried to constant weight, at 100- 

 110C, were .4331 and .5481, which is very greatly in excess of 

 any of the remaining soils. The average of sixteen soils of 

 which the nitrogen content ranged uniformly above .2024 per 

 cent, with a limit of .5481 was 4.91 mg, while the average of 

 sixteen soils which fixed more than 4.91 mg and which had a 

 nitrogen content of uniformly less than .2024 per cent, was 

 6.72 mg. Soil number 1 which had a nitrogen content of 

 .0926 per cent, fixed 10.74 mg which was the highest value of 

 any. It is reasonable, therefore, to conclude that a soil which 

 contains much above .1000 per cent of nitrogen, other things 

 being favorable, may equal or surpass any other soil in nitrogen 

 fixing possibilities. Probably the number of azotobacter 

 present in the soil determined the speed of the reaction. 



Azotobacter chroococcum was found to be universally 

 distributed over the state. Many of the cultures which evinced 

 strong nitrogen-fixing properties were covered with an imperfect 

 floating membrane of brownish color shading off to almost 

 black. 



A. chroococcum was definitely isolated from an alfalfa 

 field at a depth of three feet, to which particular reference will 

 be made later. 



Fungus growth developed on the surface of the medium in 

 some instances. It is significant that in those overrun with 

 molds and similar vegetation, the liquid frequently exhibited 

 decided colors, usually yellow, though in one instance pink. 



The following table shows the amount of nitrogen fixed in 

 milligrams and the percentage of moisture and nitrogen in 

 each of the seventy samples of soil investigated: 



