Numerous reports have presented evidence that soil aggregates are formed and/or stabilized by microorganisms. In most instances, the aggregates were not studied in respect to size distribution. The present report is concerned with the size distribution of aggregates and particles in a soil material after inoculation with three different inocula and incubation for varying periods of time

Haskins, Francis A.

McCalla, T. M.

Weakly, H. E.

English

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Agronomy & Horticulture -- Faculty Publications Agronomy and Horticulture Department 1967 Size Distribution of Soil Aggregates as Influenced by Microorganisms H. E. Weakly University of Nebraska-Lincoln T. M. McCalla University of Nebraska-Lincoln Francis A. Haskins University of Nebraska-Lincoln, fhaskins@neb.rr.com Follow this and additional works at: https://digitalcommons.unl.edu/agronomyfacpub  Part of the Plant Sciences Commons Weakly, H. E.; McCalla, T. M.; and Haskins, Francis A., "Size Distribution of Soil Aggregates as Influenced by Microorganisms" (1967). Agronomy & Horticulture -- Faculty Publications. 209. https://digitalcommons.unl.edu/agronomyfacpub/209 This Article is brought to you for free and open access by the Agronomy and Horticulture Department at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Agronomy & Horticulture -- Faculty Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. SCiENCE© 1967 by The Williams & Wilkins Co.NOTEVol. 103, No.1Printed in U.S.A.Size Distribution of Soil Aggregates as In-fluenced by Microorqosuems? Numerous re-ports (for example 2-7) have presented evi-dence that soil aggregates are formed and/orstabilized by microorganisms. In most in-stances, the aggregates were not studied in re-spect to size distribution. The present reportis concerned with the size distribution of ag-gregates and particles in a soil material afterinoculation with three different inocula andincubation for varying periods of time.Peorian loess" subsoil; a material virtuallydevoid of water-stable aggregates, was used.Prior to use, the Peorian loess was passedthrough a 2-mm. sieve.Fifty-four petri plates, each containing theequivalent of 35 g. of oven-dry soil, were pre-pared. These plates of soil were autoclavedfor 1Yz hours, after which 0.35 g. of auto-claved, ground wheat straw was added toeach. The plates and contents were then auto-olaved for an additional 1Y2 hours.Stachybotrys atra (2), a formaldehyde-resist-ant Fusarium, and a suspension of topsoilwere used asinocula. All inocula were preparedin a sterile solution containing the followingsalts in 1 liter of water: NH;NO., 8 g;K,HPO,, 4 s: MgSO"7 H20, 2 g; and FeSO.,less than 0.1 g. At the time of inoculation,each plate received the equivalent of 3.5 ml.of this salt solution and sufficient water tobring the moisture level to approximately fieldcapacity. Eighteen plates of each soil wereinoculated with each of the three inocula.Inoculated plates were weighed and then in-cubated at 25° C. At weekly intervals eachplate was brought to its original weight bythe addition of sterile water.At the end of 1, 2, 4, 8, 12, and 16 weeks,tlll'ee Plates representing each of the threemacula were removed from the incubator andair-dried. Prior to drying, a small sample of. 1 Contribution from the Northern Plains~:a~~h, Soil and. Water Conservation ResearchAivislon, Agricultural Research Service, U. S. Dep.Stgr.: and the Nebraska Agricultural Experimentla~lon. Journal Series Paper No. 1692, Nebraskabgrlcultural Experiment Station Lincoln Ne-J:aska 68503. "75soil from each aggregation plate was platedon soil extract and rose bengal agars for quali-tative examination of the microflora present.After all plates had been dried, determina-tions of size distribution were made for each.Wet-sieving was used to separate water-stableaggregates and particles larger than 50 p.. byYoder's method (9) as modified by Swansonet al. (8). Ultimate particle-size analysis wasused for particles of less than 50 p.. by thestandard procedure of Bouyoucos (1).As shown in figure 1,the aggregation ofPeorian loess was modified considerably. ThePeorian loess in the field contains 14 per centsand (>50 p..), 66 per cent silt (2 to 50 p..),and 20 per cent clay «2 p..) with virtuallyno natural aggregation.The uninoculated control, autoclaved and withnutrient solution, resembled the O-time samplefor all treatments until it became contaminatedafter 2 weeks of incubation, as indicated byplate counts of fungi and bacteria. It then re-sembled the plates inoculated with soil flora.Plating of each treatment at each interval ofincubation showed the S. atra and Fusariumsp. present in their respective treatments in ap-parently pure culture. The plates inoculatedwith soil showed a variety of bacteria and fungipresent.After inoculation and incubation of thePeorian loess, there was a significant shift, asdetermined by analysis of variance and Dun-can's Multiple Range test, in the size distribu-tion of aggregates in the direction of largersizes. With time there was a significantchange with the three inocula in all size classes.Within one week all the clay-size particles(all three inocula) became aggregated intounits greater than clay size, and none of theclay-size particles «2 p..) reappeared evenafter 16 weeks of incubation. Likewise, withonly two exceptions, none of the small silt-size fractions (2 to 5 p..) reappeared.Within one week after inoculation with thesoil flora, there was a significant increase inthe per cent of the soil material present as sand-size aggregates (50 to 1000 p..). Even after 16weeks of incubation, this general increase wasstill evident, although the coarse sand-sizesTIME (weeks)FIG. 1. Effect of duration of incubation upon the size distribution of soil aggregates andparticles in Peorian loess inoculated with various microorganisms: A = > 2000",; B2000-1000",; C = 1000-500",; D = 500-250",; E = 250-100",; F = 100-50",; G = 50-20",; H =20-5",; I 5-2",; and J = < 2",.REFERENCES77(7) Martin, J. P., and Waksman, S. A. 1941 In-fluence of microorganisms on soil aggrega-tion and erosion: II. Soil Sci. 52: 381-394.(8) Swanson, N. P., Dedrick, A. R, and Weakly,H. E. 1965 Soil particles and aggregatestransported in runoff from simulated rain-fall. Trans. Am. Soc. Agr. Engrs. 8: 437-440.(9 ) Yoder, R E. 1936 A direct method of ag-gregate analysis of soils and a study ofthe physical nature of erosion losses. J. Am.Soc. Agron, 28: 337-351.H.E. WEAKLY, T. M. MCCALLA, AND F. A.HASKINSUniversity of Nebraska, LincolnReceived for publication March 11, 1966NOTEF. A. 1955 Stachybotrys atra, an effectiveaggregator of Peorian loess. Soil Sci. Soc.Am. Proc. 19: 179-181.(3) McCrtlla, T. M. 1946 Influence of somemicrobial groups on stabilizing soil struc-ture against falling water drops. Soil Sci.Soc. Am. Proc. 11: 260-263.(4) McCalla, T. M., Haskins, F. A., and Curley,It D. 1958 Soil aggregation by micro-organisms following soil fumigation. SoilSci. Soc. Am. Proc. 22: 311-314.(5) McHenry, J. R, and Russell, M. B. 1944Microbial activity and aggregation of mix-tures of bentonite and sand. Soil Sci. 57:351-357.(6) Martin, J. P. 1945 Microorganisms and soilaggregation. Soil Sci. 59: 163-174.16128SOIL FLORA5TACHYBOTRYS ATRA4(1) Bouyoucos, G. J. 1951 A recalibrationthe hydrometer for making mechanianalysis of soils. J. Am. Soc. Agron. 43: 43438.(2) Downs, S. C., McCalla, T. M., and Haskinorganisms causes appreciable shifts in tamounts of soil found in the various size frations, In the aggregate size range of greatethan 2000 flo, the stability was temporary, persisting for 8 to 12 weeks for Fusarium anS. atm. In the smaller-sized classes, the sbility was more permanent, existing beyothe 16-week incubation period. Visual eamination of the plates showed that consierable amounts of fungal mycelia were presetfollowing inoculation with the S. atra 0Fusarium sp. This mycelial growth may haycontributed to the observed aggregation.NOTE2were diminished after 4 weeks. Inoculation ofthe soil material with the Fusarium micro-organism also caused a significant shift in theper cent aggregates toward the sand-size range.This shift persisted throughout the 16-weekincubation period. There was a significant in-crease in aggregates in the size greater than2000 flo, which reached its maximum after 1or 2 weeks incubation. A similar pattern existedfor the Peorian loess inoculated with S. atm.The most striking effects, however, were pro-duced by S. aira in aggregates greater than2000 flo. About 52 per cent of the total soilmaterial at the 2-week incubation intervalwas made up of aggregates of this size range.The data indicate that these aggregates werequite stable for at least 12 weeks, but betweenthe 12th and 16th weeks of incubation theywere degraded extensively.I t is evident that the activity of soil micro-76403020100ABCDEfGHIJ40302010060504030201000

Size Distribution of Soil Aggregates as Influenced by Microorganisms

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University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
Agronomy & Horticulture -- Faculty Publications Agronomy and Horticulture Department 
1967 
Size Distribution of Soil Aggregates as Influenced by 
Microorganisms 
H. E. Weakly 
University of Nebraska-Lincoln 
T. M. McCalla 
University of Nebraska-Lincoln 
Francis A. Haskins 
University of Nebraska-Lincoln, fhaskins@neb.rr.com 
Follow this and additional works at: https://digitalcommons.unl.edu/agronomyfacpub 
 Part of the Plant Sciences Commons 
Weakly, H. E.; McCalla, T. M.; and Haskins, Francis A., "Size Distribution of Soil Aggregates as Influenced 
by Microorganisms" (1967). Agronomy & Horticulture -- Faculty Publications. 209. 
https://digitalcommons.unl.edu/agronomyfacpub/209 
This Article is brought to you for free and open access by the Agronomy and Horticulture Department at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Agronomy & Horticulture -- 
Faculty Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
SCiENCE
© 1967 by The Williams & Wilkins Co.
NOTE
Vol. 103, No.1
Printed in U.S.A.
Size Distribution of Soil Aggregates as In-
fluenced by Microorqosuems? Numerous re-
ports (for example 2-7) have presented evi-
dence that soil aggregates are formed and/or
stabilized by microorganisms. In most in-
stances, the aggregates were not studied in re-
spect to size distribution. The present report
is concerned with the size distribution of ag-
gregates and particles in a soil material after
inoculation with three different inocula and
incubation for varying periods of time.
Peorian loess" subsoil; a material virtually
devoid of water-stable aggregates, was used.
Prior to use, the Peorian loess was passed
through a 2-mm. sieve.
Fifty-four petri plates, each containing the
equivalent of 35 g. of oven-dry soil, were pre-
pared. These plates of soil were autoclaved
for 1Yz hours, after which 0.35 g. of auto-
claved, ground wheat straw was added to
each. The plates and contents were then auto-
olaved for an additional 1Y2 hours.
Stachybotrys atra (2), a formaldehyde-resist-
ant Fusarium, and a suspension of topsoil
were used asinocula. All inocula were prepared
in a sterile solution containing the following
salts in 1 liter of water: NH;NO., 8 g;
K,HPO,, 4 s: MgSO"7 H20, 2 g; and FeSO.,
less than 0.1 g. At the time of inoculation,
each plate received the equivalent of 3.5 ml.
of this salt solution and sufficient water to
bring the moisture level to approximately field
capacity. Eighteen plates of each soil were
inoculated with each of the three inocula.
Inoculated plates were weighed and then in-
cubated at 25° C. At weekly intervals each
plate was brought to its original weight by
the addition of sterile water.
At the end of 1, 2, 4, 8, 12, and 16 weeks,
tlll'ee Plates representing each of the three
macula were removed from the incubator and
air-dried. Prior to drying, a small sample of
. 1 Contribution from the Northern Plains~:a~~h, Soil and. Water Conservation Research
Aivislon, Agricultural Research Service, U. S. Dep.
Stgr.: and the Nebraska Agricultural Experiment
la~lon. Journal Series Paper No. 1692, Nebraska
bgrlcultural Experiment Station Lincoln Ne-
J:aska 68503. "
75
soil from each aggregation plate was plated
on soil extract and rose bengal agars for quali-
tative examination of the microflora present.
After all plates had been dried, determina-
tions of size distribution were made for each.
Wet-sieving was used to separate water-stable
aggregates and particles larger than 50 p.. by
Yoder's method (9) as modified by Swanson
et al. (8). Ultimate particle-size analysis was
used for particles of less than 50 p.. by the
standard procedure of Bouyoucos (1).
As shown in figure 1,the aggregation of
Peorian loess was modified considerably. The
Peorian loess in the field contains 14 per cent
sand (>50 p..), 66 per cent silt (2 to 50 p..),
and 20 per cent clay «2 p..) with virtually
no natural aggregation.
The uninoculated control, autoclaved and with
nutrient solution, resembled the O-time sample
for all treatments until it became contaminated
after 2 weeks of incubation, as indicated by
plate counts of fungi and bacteria. It then re-
sembled the plates inoculated with soil flora.
Plating of each treatment at each interval of
incubation showed the S. atra and Fusarium
sp. present in their respective treatments in ap-
parently pure culture. The plates inoculated
with soil showed a variety of bacteria and fungi
present.
After inoculation and incubation of the
Peorian loess, there was a significant shift, as
determined by analysis of variance and Dun-
can's Multiple Range test, in the size distribu-
tion of aggregates in the direction of larger
sizes. With time there was a significant
change with the three inocula in all size classes.
Within one week all the clay-size particles
(all three inocula) became aggregated into
units greater than clay size, and none of the
clay-size particles «2 p..) reappeared even
after 16 weeks of incubation. Likewise, with
only two exceptions, none of the small silt-
size fractions (2 to 5 p..) reappeared.
Within one week after inoculation with the
soil flora, there was a significant increase in
the per cent of the soil material present as sand-
size aggregates (50 to 1000 p..). Even after 16
weeks of incubation, this general increase was
still evident, although the coarse sand-sizes
TIME (weeks)
FIG. 1. Effect of duration of incubation upon the size distribution of soil aggregates and
particles in Peorian loess inoculated with various microorganisms: A = > 2000",; B
2000-1000",; C = 1000-500",; D = 500-250",; E = 250-100",; F = 100-50",; G = 50-20",; H =
20-5",; I 5-2",; and J = < 2",.
REFERENCES
77
(7) Martin, J. P., and Waksman, S. A. 1941 In-
fluence of microorganisms on soil aggrega-
tion and erosion: II. Soil Sci. 52: 381-394.
(8) Swanson, N. P., Dedrick, A. R, and Weakly,
H. E. 1965 Soil particles and aggregates
transported in runoff from simulated rain-
fall. Trans. Am. Soc. Agr. Engrs. 8: 437-
440.
(9 ) Yoder, R E. 1936 A direct method of ag-
gregate analysis of soils and a study of
the physical nature of erosion losses. J. Am.
Soc. Agron, 28: 337-351.
H.E. WEAKLY, T. M. MCCALLA, AND F. A.
HASKINS
University of Nebraska, Lincoln
Received for publication March 11, 1966
NOTE
F. A. 1955 Stachybotrys atra, an effective
aggregator of Peorian loess. Soil Sci. Soc.
Am. Proc. 19: 179-181.
(3) McCrtlla, T. M. 1946 Influence of some
microbial groups on stabilizing soil struc-
ture against falling water drops. Soil Sci.
Soc. Am. Proc. 11: 260-263.
(4) McCalla, T. M., Haskins, F. A., and Curley,
It D. 1958 Soil aggregation by micro-
organisms following soil fumigation. Soil
Sci. Soc. Am. Proc. 22: 311-314.
(5) McHenry, J. R, and Russell, M. B. 1944
Microbial activity and aggregation of mix-
tures of bentonite and sand. Soil Sci. 57:
351-357.
(6) Martin, J. P. 1945 Microorganisms and soil
aggregation. Soil Sci. 59: 163-174.
16128
SOIL FLORA
5TACHYBOTRYS ATRA
4
(1) Bouyoucos, G. J. 1951 A recalibration
the hydrometer for making mechani
analysis of soils. J. Am. Soc. Agron. 43: 43
438.
(2) Downs, S. C., McCalla, T. M., and Haskin
organisms causes appreciable shifts in t
amounts of soil found in the various size fra
tions, In the aggregate size range of greate
than 2000 flo, the stability was temporary, per
sisting for 8 to 12 weeks for Fusarium an
S. atm. In the smaller-sized classes, the s
bility was more permanent, existing beyo
the 16-week incubation period. Visual e
amination of the plates showed that consi
erable amounts of fungal mycelia were preset
following inoculation with the S. atra 0
Fusarium sp. This mycelial growth may hay
contributed to the observed aggregation.
NOTE
2
were diminished after 4 weeks. Inoculation of
the soil material with the Fusarium micro-
organism also caused a significant shift in the
per cent aggregates toward the sand-size range.
This shift persisted throughout the 16-week
incubation period. There was a significant in-
crease in aggregates in the size greater than
2000 flo, which reached its maximum after 1
or 2 weeks incubation. A similar pattern existed
for the Peorian loess inoculated with S. atm.
The most striking effects, however, were pro-
duced by S. aira in aggregates greater than
2000 flo. About 52 per cent of the total soil
material at the 2-week incubation interval
was made up of aggregates of this size range.
The data indicate that these aggregates were
quite stable for at least 12 weeks, but between
the 12th and 16th weeks of incubation they
were degraded extensively.
I t is evident that the activity of soil micro-
76
40
30
20
10
0
ABCDEfGHIJ
40
30
20
10
0
60
50
40
30
20
10
0
0


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