240 



Transformation of Mineral Substances 



the surfaces of which are smoothed out. The amount of available 

 phosphorus in the soil is measured by the growth of Azotobacter on 

 the surface of the soil, as determined by the appearance of slimy 

 colonies. The available phosphorus in a given soil is measured from 

 the actual amount of growth of the Azotobacter colony. 



Some of the methods are based upon the use of fungi. These 

 methods are similar to the above, except that available nitrogen is 

 also added to the solution or to the soil. The fungus growth is used 

 as a measure of the available phosphorus. 



Transformation of Potassium, Calcium, Magnesium, and Iron 



In addition to the elements already discussed, others that are 

 required for plant and animal nutrition and that are present in or 

 introduced into the soil are also subject, directly or indirectly, to 

 activities of microorganisms. Some of these elements, including po- 



1 3 



6 9 12 

 Weeks 



15 18 



13 



9 o 

 5 ^ 

 1 



Fig. 96. Relation lictwueii sultur oxidation and water-soluble potassium in 

 composts containing sulfur and greensand marl (from Rudolfs). 



tassium, calcium, magnesium, and iron, are utilized for the metabolism 

 of the soil organisms. They are used either as nutrients or as cata- 

 lysts and are, therefore, required in very small amounts. Their trans- 

 formation by microorganisms and their role in microbial metabolism 

 depend on the nature of the element, on the nature of the organism, 

 and on soil conditions. 



Potu'isiuni is found in soil both in organic forms and in the form 

 of zeolitic and nonzeolitic silicates. It is added to the soil in soluble 



