AVAILABILITY OF SPECIFIC NUTRIENTS IN SOILS 237 



the abundance of certain specific groups of soil microorganisms. 

 Some methods give information concerning the bacteria as a 

 whole; others deal with individual physiological groups of organ- 

 isms such as those that decompose cellulose, fix nitrogen, or oxidize 

 ammonia to nitrite or nitrite to nitrate. Actinomyces, fungi, 

 algae, and protozoa are somewhat less frequently the object of 

 such investigations (see Table 11). 



There are many striking correlations between the abundance 

 of organisms, as determined by these methods, and the pro- 

 ductive capacity of soils (Fig. 84). The relationships are not 

 absolute, however, since not all microbial cells even of a single 

 species work with the same speed in the soil. A small number of 

 organisms in one soil may be more active than a larger number in 

 another soil, because of the fact that many individuals of the soil 

 population are resting forms during a large portion of the time. 



Biological Activity of the Soil Population as a Whole. — 

 A large number of the soil microbes are concerned with the decom- 

 position of the organic materials in the soil. Consequently, 

 knowledge of their activity is of considerable value in estimating 

 the biological condition of the soil. In order to obtain such infor- 

 mation, measurements have been made of the rate of formation of 

 carbon dioxide in the soil. Since this gas is one of the principal 

 products resulting from the decomposition of all organic sub- 

 stances, and since most of the gas is of biological origin, informa- 

 tion concerning its formation is of exceptional value as an index 

 of microbial activity in soils (Fig. 84). 



One of the most accurate measurements of biological activity is 

 the determination of the amount of heat produced during the 

 development of the organisms. The potential energy contained 

 in the various substances which are used by microbes as foods 

 undergoes a change, a part becoming transformed to other forms of 

 energy as heat and a part remaining in the new compounds which 

 have been synthesized by the cells of the organisms that grow at 

 the expense of the food material. Microorganisms are frequently 

 very inefficient in utilizing the energy contained in food materials, 

 and a very large portion of the energy is eliminated as heat. Such 

 heat is formed irrespective of the source of energy that furnishes 

 the organisms with the power to develop. 



Determination of the Availability of Specific Nutrients 

 IN Soils. — Utilizing the available knowledge of the nutrition of 



