Sept. 15, 1925 
Plate Counts of Soil Microorganisms 
509 
Table V .—Influence of various soil-extract agars on the number of microorganisms 
growing on the plates 
Millions per gram soil 
Soil-extract agar made from— 
Muck_ 
Peat loam__ 
Greenhouse soil_ 
Rich lawn soil__ 
Granby loam_ 
Genesee silty clay loam__. 
Silty clay loam..... 
Hollis loam... 
Dunkirk fine sandy loam. 
Coloma loamy sand_ 
Egg-albumen agar_ 
Mannite-salts agar_ 
1 
2 
3 
4 
5 
Average 
30.5 
43.5 
18.6 
128 
197 
83.5 
31.2 
37.8 
14.7 
137 
252 
94.5 
30.4 
33.9 
14.6 
152 
252 
96.6 
29.0 
39.4 
15.5 
135 
238 
91.3 
31.6 
34.3 
17.4 
116 
216 
83.0 
33.7 
42.8 
19.1 
140 
263 
99.7 
36.8 
44.3 
18.0 
149 
260 
101.6 
24.9 
33.5 
13.7 
“94 
207 
“74.5 
22.6 
31.1 
13.5 
143 
220 
86.0 
25.2 
32.1 
13.1 
122 
233 
85.0 
26.7 
34.1 
16.2 
66 
181 
65.0 
6.7 
16.6 
7.4 
65 
23.9 
« Spreaders. 
The data in Table V are very instructive. The fluctuation in the 
counts throughout the central part of the table is not greater than 
would be the case if several samples were plated on the same medium, 
but the appearance of the colonies on the different soil-extract agars 
varied considerably. Those agars containing larger amounts of 
organic matter were more favorable for the development of rapidly 
spreading colonies, especially of fungi. On the other hand, those 
made from light soils and containing a relatively small amount of 
soluble material, did not allow the development of the same number 
of colonies. Egg-albumen agar gave counts comparable to those on 
soil-extract agar when used for plating field soils, but in the case of 
the rich greenhouse soils its efficiency was only 50 per cent and 75 
per cent. Thornton’s mannite-salts agar again proved to be a rather 
poor substrate. 
OTHER MEDIA FOR PLATING 
Conn ( 1 ) made total counts of bacteria in various type soils by 
using as plating media Lipman and Brown’s “synthetic” agar, 
Brown’s albumen agar and Fischer’s soil-extract agar, in comparison 
with soil-extract gelatin and asparaginate agar. From his data he 
concluded that soil-extract gelatin and asparaginate agar 9 were the 
best for plating, but on account of the indefinite composition and the 
difficulty of handling gelatin, he declared asparaginate agar pref¬ 
erable. Accordingly, in plating out some samples of a field soil, 
the writers used soil-extract gelatin and asparaginate agar, together 
with a mannite-nitrate soil extract, and a “synthetic” agar. The 
mannite-nitrate agar was made by adding 1 per cent mannite and 
0.02 per cent KN0 3 to soil-extract agar. The synthetic agar consisted 
of 2 per cent agar dissolved in distilled water containing 0.05 per cent 
K 2 HP0 4 , 0.1 per cent sodium asparaginate, 0.25 per cent sodium 
citrate, and 0.1 per cent sodium albuminate. 
Table VI shows the value in these experiments, of these media for 
making total counts of microorganisms in the soil. Again, soil- 
extract agar proves superior. Although these tests are not numerous 
9 The composition of asparaginate agar was: 1,000 c.c. distilled water; 12 gms. agar; 1 gm. sodium aspar* 
aginate; 1 gm. dextrose; 0.2 gm. MgSOi; 1.5 gm. NH 4 II 2 PO 4 ; 0.1 gm. CaCh; 0.1 gm. KC1; trace FeCl3. 
66850—251-2 
