732 PRINCIPLES OF SOIL MICROBIOLOGY 



The available phosphorus in the soil may be calculated from the 

 amount of nitrogen fixed. To 100 grams of soil, 30 cc. of water con- 

 taining 2.5 grams of glucose, 0.2 gram K 2 S0 4 and 0.05 gram of MgCl 2 

 were added. The soil was then sterilized and inoculated with Azoto- 

 bacter. After 21 days incubation, the total nitrogen and phosphoric 

 acid were determined in the soil. The following process was used for 

 calculating the amount of available phosphorus. One hundred grams 

 of soil contained 0.164 gram nitrogen in the inoculated and 0.110 

 gram in the uninoculated soil. The amount of nitrogen fixed was, 

 therefore, 0.054 gram. Since Azotobacter cells contain 10 per cent 

 nitrogen and 5 per cent P 2 5 , 0.027 gram of the latter was made avail- 

 able in the given quantity of soil. The total P 2 5 in 100 grams of soil 

 was 0.103 gram, hence about 26 per cent of this phosphorus is readily 

 available. 42 A very fertile soil containing 0.084 per cent P 2 5 has shown 

 48.8 per cent of it utilizable; a soil of medium fertility contained 26.21 

 per cent utilizable P 2 5 , and a poor forest soil only 11.66 per cent 

 of the P 2 5 utilizable. 



There is no doubt that all agricultural soils can be made to fix nitro- 

 gen when an excess of an available source of energy is added. However, 

 the reaction of the soil, which favors the development of specific nitro- 

 gen fixing organisms, is of great importance in this respect, as pointed 

 out above. The presence of available phosphorus in the soil and the 

 soil reaction influencing the development of specific nitrogen-fixing 

 bacteria, are the two factors controlling the amount of nitrogen fixed 

 and mannite decomposed. 



Winogradsky 43 recently suggested several new methods for determin- 

 ing the nitrogen-fixing capacity of the soil, the results serving in a way 

 as a measure of the fertility of the soil. 



1. A silica gel plate, 9 cm. in diameter, is inoculated with a few grains of soil- 

 In the presence of Azotobacter, the soil will be surrounded, after 48 hours, with 

 the colonies of the organism. The relative abundance of the colonies will indi- 

 cate the biological activities of the soil. 



2. One-half gram of mannite is added to 50 grams of soil, which is incubated 

 for 48 hours. The soil is then examined microscopically and the abundance of 

 nitrogen-fixing bacteria serves as an index of the activity of the soil. 



3. Five parts of fresh soil are added to 100 parts of starch and the mixture 

 kneaded with sufficient water and placed in a Petri dish; an excess of water is 

 avoided. After 48 hours incubation, minute colonies will be formed on the 



42 Stoklasa, 1911 (p. 649). 



43 Winogradsky, 1925 (p. 11). 



