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PRINCIPLES OF SOIL MICROBIOLOGY 



function for measuring the bacteriological condition of the soil and 

 potential crop production, or soil fertility. The relative numbers of 

 bacteria, actinomyces and fungi can further indicate the chemical 

 condition of the soil, such as soil reaction, degree of decomposition of 

 organic matter, etc. A comparison between the crop yields of a series 

 of plots, fertilized in the same manner for a number of years, in which 

 definite differences in fertility have been established, with bacterial 

 numbers, is given in figure 62. 



The plots recorded in figure 62 have been fertilized as follows: 



The A plots were unlimed, the B plots limed, receiving two tons of ground lime- 

 stone per acre every five years; plots 4 and 19 received minerals only (640 pounds 

 acid phosphate and 320 pounds potassium chloride per acre); plot 5 received 

 manure (32 tons per acre) and minerals; plot 7, no fertilizer; plot 9, sodium 

 nitrate (320 pounds per acre) and minerals; plot 18, manure, minerals and sodium 

 nitrate. 



The addition of lime was found to have a more stimulating effect 

 upon the numbers of microorganisms than upon crop yield. It 

 brought about, in the particular plots, a change in reaction from one 

 below the acid limit for the development of Azotobacter and not very 

 favorable for the development of actinomyces to a reaction very favor- 

 able for the development of both groups of organisms. This, com- 

 bined with the redistribution of the various groups of soil organisms, 

 as a result of liming, may account for the lack of perfect parallelism 

 between the unlimed and limed soils in regard to numbers and crop 

 yields. 



Let us compare two plots, each ^ of an acre in size. Plot 9A re- 

 ceived yearly applications of 320 pounds of sodium nitrate per acre, 

 640 pounds of acid phosphate and 320 lbs. of potassium chloride. Plot 

 11B received an equivalent amount of ammonium sulfate and the 

 same minerals; two tons of lime were also applied every five years. 

 The following results were obtained: 



Total bacteria and actinomyces 



Actinomyces, per cent 



Fungi (on acid medium) 



Reaction of soil, pH 



Total crop yield, pounds 



