JOURNAL OF ACRtQILniRAL RESEARCH 
Vol. XXXI Washington, D. C., September 15, 1925 No. 6 
PLATE COUNTS OF SOIL MICROORGANISMS 1 
By N. R. Smith, Associate Bacteriologist , and S. Worden, formerly Junior Scien¬ 
tific Aid , Bureau of Plant Industry , United States Department of Agriculture 2 
INTRODUCTION 
Although plate counts of the microorganisms in soil samples have 
only restricted value, nevertheless, improvements in the method 
for making such determinations are desirable. Recently recom¬ 
mendations have been made on three points, (1) procedure in prepar¬ 
ing the suspension of the soil organisms, (2) the use of special media 
for plating, and (3) frequent repetitions of the platings. These 
recommendations have been tested experimentally, and the results 
are presented in this paper. 
METHODS FOR PREPARING SUSPENSIONS OF SOIL 
MICROORGANISMS 
The intensity with which soil samples suspended in water should 
be shaken has been tested repeatedly, and it was the general belief 
that thorough shaking by hand for about five minutes gives a satis¬ 
factory suspension. The efficiency of this method, however, has 
been questioned by Whittles (14)? who subjected soil suspended in 
water to a high frequency vibration with an electric vibrator. On 
plating out from these vibrated suspensions, he obtained counts 
widely surpassing those obtained with the hand-shaking method. 
In his first experiments with a soil which gave about 10 million 
organisms per gram by the usual plating procedure, he counted 250 
millions per gram soil from the vibrated suspension. Later platings 
with a modified technique gave even counts of 100 and 270 colonies 
per plate from the 1-10 11 dilution equal to 10 and 27 billion 4 organ¬ 
isms per gram soil. 
The colonies in this case attained their maximum growth within 
4 days, and there were no “slow growers.” Whittles suggested that 
this was due to the freeing of the bacteria from the enveloping col¬ 
loidal gel which contained their metabolic products. The dispersion 
of the gel by the vibration was declared to nave removed the inhibit¬ 
ing influence and to have caused the rapid growth. An “alkaline 
shake medium” containing a protective colloid (gelatin) was found 
to be helpful in bringing about a good dispersion. For comparison, 
direct microscopic counts were made in the soil samples. The num¬ 
bers recorded (6,300 millions per gram) are much higher than those 
usually found in soil tests and agree well with the lower vibrator counts. 
1 Received for publication September 18,1924; issued October, 1925. 
2 The writers are especially indebted to F. Lohnis for many helpful suggestions and for critical reading 
of the manuscript, and to F. L. Goll for the drawing of the curves. 
3 Reference is made by number (italic) to “Literature cited,” pp. 516. 
4 In England and Germany the term billion is used for a million millions and not for a thousand millions 
as in America. Billion in this paper is a million millions. 
Journal of Agricultural Research, Vol. XXXI, No. 6 
Washington, D. C. Sept. 15,1925 
Key No. G-492 
66850—25f-1 
( 501 ) 
CER £AL INVESTIGATIONS. 
