44 



PRINCIPLES OF SOIL MICROBIOLOGY 



to 131,000 fungi per gram of soil when plated out on three different 

 media, with a ratio between the fungi and bacteria developing on the 

 plate as 1:40 or 1:50. However, whenever possible, these results 

 should be checked up by microscopic observations. 



An acid medium similar to the one described above, combined with a 

 low dilution allows the determination, with a fair degree of accuracy, of 



7A 7B 



19A 4A 



14 000 



12.000 



10.000 



8.000 



.000 



2.000 



Bacteria, thousands per gram 

 Fungi, hundreds per gram 



-, — —_ — — — - -. Average yield, 1908-1921 



Yield for 1921 



Fig. 5. Influence of soil treatment upon the numbers of bacteria and fungi 

 in the soil: 7A and 7B, no fertilizer or manure; 4A, 19A, 19B, receiving only 

 mineral fertilizers (phosphate and KC1); 11A and 11B, ammonium sulfate and 

 minerals; 9A, sodium nitrate and minerals; 5A, stable manure and minerals; 

 the B plots limed in 1908, 1913, 1918 (after Waksman and de Rossi). 



the relative abundance of fungi in the soil. The more acid the soil is, 

 the greater is the number of fungi relative to the number of the other 

 soil microorganisms. Figure 4 gives the changes in the number of 

 fungi during the year and at different depths and fig. 5 shows the in- 

 fluence of soil treatment upon the numbers of bacteria and fungi in 

 the soil. 



