238 MODIFICATION OF THE SOIL POPULATION 



soil organisms, several methods have been developed which may be 

 used to obtain information concerning the relative amounts of 

 available phosphorus, potassium, and calcium in soils. Cultures 

 of Azotobacter have been quite generally used in such studies. 

 The following procedures are typical of these methods. 



For determining deficiencies of phosphate in soil, a solution 

 medium is prepared to contain all nutrients required by Azotobac- 

 ter, with the exception of phosphorus. To this medium is added a 

 certain amount of the soil to be tested. The material is sterilized 

 and then inoculated with Azotobacter. Since the only source of 

 phosphorus is the soil which is added, and since considerable phos- 

 phorus is required by the bacterium to grow, the relative growth 

 of Azotobacter or the amount of nitrogen fixed is an index of the 

 amount of available phosphorus contained in the soil. By adding 

 graded quantities of available phosphate to a series of flasks con- 

 taining the same medium treated in the manner mentioned above, 

 still further information can be obtained regrading the response 

 of the soil to phosphate treatment. By following a similar tech- 

 nique but with regard to calcium carbonate instead of phosphate, 

 the studies yield information concerning reaction and lime require- 

 ment of soil. 



Such methods have been modified so that the soil itself is used 

 in place of solution media. A series of samples of soil are mixed 

 with starch and with graded quantities of phosphate, calcium car- 

 bonate, or potassium salt depending upon whether phosphate, 

 lime, or potash deficiencies are to be studied. The soils are then 

 made pasty with water and put into dishes for incubation. If 

 conditions are favorable, Azotobacter grows well and forms raised 

 colonies on the surface which have the appearance of tiny pearls. 

 If conditions are unfavorable, no colony development occurs or 

 there is very hmited growth (Fig. 85). 



Growth of certain fungi has also been utilized to give informa- 

 tion of the same nature. For phosphorus determinations, nutrient 

 solutions are prepared containing an abundance of all nutrients 

 with the exception of phosphate, which is added in graded amounts 

 to a series of flasks containing the medium. A certain amount of 

 the soil under investigation is added to each flask, and the media 

 are all inoculated with a fungus such as Aspergillus niger. After 

 the incubation period, the fungus growth is removed from the 

 flasks, washed, dried, and weighed. The extent of growth of 



