AVAILABILITY OF SPECIFIC NUTRIENTS IN SOILS 239 



the fungus is limited by the amount of available phosphate in the 

 soil. For potash determinations the method is the same, with the 

 exception that the media contain ample phosphate but graded 

 amounts of some potassium salt (Table 56). 



There are thus numerous methods which have been used for 

 attempting to determine the productive capacity of soils. Some 

 of these methods are devised to give quite specific information, 

 while the object of others is to provide more general facts about the 

 soil. Use of several methods of study is preferred to use of indi- 



FiG. 85. — Growth of Azotobacter in soils treated with starch to test for defi- 

 ciencies in available nutrient elements. Upper row: soil deficient in phosphate. 

 Reading left to right: check, nothing added; potash added; phosphate added; 



phosphate and potash added. 



Lower row: soil not deficient in either potash or phosphate. Reading left 



to right: check, nothing added; potash added; phosphate added; phosphate 



and potash added (from Sackett). 



vidual procedures. Even such information is incomplete, and is 

 most valuable when it is used in conjunction with chemical and 

 physical determinations in estimating soil fertility. It is because 

 of the fact that the abundance of the soil population, the types of 

 organisms composing it and their activity are products of the 

 environment and are susceptible to change with modifications in 

 any of numerous conditions, that a certain amount of information 

 concerning the fertility of a soil can be obtained from a knowledge 

 of its microbiological condition. One of the principal diflSculties 

 is to obtain a true conception of the biological status of the soil. 



