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III. Effect of Organic Residues on Aggregation

III. Effect of Organic Residues on Aggregation

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SOIL AGGREGATION



13



stances (Martin, 1942). The concentration or quality of the watersoluble materials increased during the early and intermediate stages

of composting, and decreased during the rater stages. When the materials were mixed with the soil and allowed to decompose, however,

much greater aggregation occurred than that produced by the water

extracts of the fresh and composted materials.

The importance to soil aggregation of organic substances which are

apparently produced largely through microbial activity has been

stressed by numerous investigators. Robinson and Page ( 1951) tested

artificial aggregates of Brookston clay loam for resistance to slaking by

the wet sieving procedure before and after oxidation of the organic

fraction with hydrogen peroxide. The stability of the aggregates of the

oxidized soil was very poor in comparison with the unoxidized soil. It

was concluded that the organic matter associated with the clay was

largely responsible for aggregate stability. Studies by Metzger and Hide

(1938) and Weldon and Hide (1942) indicated that the organic matter

content of severaI soils was much higher in the well-aggregated fractions than in the poorly aggregated fractions. Baver (1935) suggested

that certain organic materials bind soil particles together through

physicochemical processes. Several investigators have demonstrated the

existence of organo-clay complexes (Ensminger and Gieseking, 1942;

Springer, 1940; Myers, 1937). Ensminger and Gieseking (1939, 1942)

found that certain proteins were adsorbed within the crystal lattice

structure of montmorillonite type clays and that adsorption made them

more resistant to enzymatic action. Bartholomew and Goring (1948)

and Goring and Bartholomew (1950, 1951) worked with certain organic phosphorus compounds and found that decomposition was retarded by clay which adsorbed the phosphorus compounds. Fixation by

the clays varied greatly depending on the nature of the organic compound, the type of clay, and the pH of the systems.

Kroth and Page (1947) made a study of natural and synthetic soil

aggregates in which the electron microscope was utilized as one approach to the problem. All parts of the investigation indicated that the

aggregating substances were uniformly distributed throughout the

aggregates and in contact with each soil particle. No evidence of aggregate capsules or coatings was found. In a later study, Robinson and

Page (1951) stated that the basis of aggregate stabilization by organic

matter is a modification of the properties of clay. It was concluded from

their work and that of others that the organic matter promotes aggregate stability by reducing swelling of montmorillonite-type clays and

by reducing the destructive forces of entrapped air during wetting of



14



JAMES P. MARTIN



et al.



the soil; by decreasing wettability; and by strengthening the aggregate

through colloidal organic depositions and development of fibrous material around and through the aggregate.

It appears evident that clay and organic complexes do undergo

physicochemical reactions (Broadbent, 1953) These reactions probably

influence soil aggregate stability.

During periods of intense microbial activity the soil organisms

themselves may mechanically bind soil particles together (Waksman,

1916; Waksman and Martin, 1939; Martin and Waksman, 1941;

McCalla, 1942). Under such conditions the binding action of fungus

filaments, for example, can be seen with the eye or better with the

microscope. Hubbel and Chapman (1946) concluded that living organisms were the most important factor in binding soil particles together and that organic substances produced by microbial activity were

not important. This view is not supported by most investigators (Myers

and McCalla, 1941; Pohlman and Nottingham, 1941) . Strong evidence

against it was obtained by Martin and Aldrich (1952) in a study of the

influence of soil fumigation on aggregation. The numbers of organisms

in treated soils, following an initial decrease, attained levels as high as

14 or more times those in the untreated soils. The kinds of organisms

also varied. In an acid soil, fungi predominated, whereas in neutral and

alkaline soils bacteria and actinomycetes predominated. There was no

correlation between types or numbers of organisms and aggregation,

indicating that it is not numbers of microbes which are most important

in soil aggregation, but more likely products of their activity during

decomposition of added organic materials.

The increased soil aggregation which takes place during the

microbial decomposition of organic residues in the soil probably results

from chemical and physical interactions among certain products of decomposition of the organic matter, substances synthesized by the soil

microbes, and the soil particles. In addition, a mechanical binding of

the soil particles by microbial cells and filaments during periods of intense microbial activity in the soil may be involved to a limited extent.

~



2. Influence of Kind and Amount of Organic Material

Investigations by Browning and Milam (1944), G o t h and Page

(1947), Martin and Waksman (1940, 1941), and Martin (1942) have

shown that, in general, materials containing relatively large amounts of

readily decomposable constituents exert the greatest and quickest aggregating effect; somewhat more resistant materials require a longer time

to exert their maximum aggregating influence but continue to be effective over a longer period of time; and extremely resistant or relatively



SOIL AGGREGATION



15



inert substances, such as well-composted materials, certain lignified

wood by-products, and some peats, have little or no influence on aggregation. I n the study by Martin (1942), the aggregating effect of various

organic residues or mixtures of residues was tested before, during, and

after cornposting for 200 days. After the first period of composting the

aggregating effect was less than that of the original residues, and after

the 200-day period the binding action was still less. This and the other

studies emphasize the importance of the decomposition process in soil

aggregate stabilization.

The level of aggregation attained following organic matter applications is also dependent upon the amount of residue applied and the state

of aggregation of the soil to which it is applied (Browning and Milam,

1941, 1944). In general, large applications are more effective than

small, and aggregation is increased more in a soil which is poorly

aggregated owing to lack of organic matter than in one which is well

aggregated.

Growth of various crops in the soil affects aggregation. Johnston

et al. (1942) studied the influence of various cropping systems on soil

aggregation. Bluegrass sod was most effective in maintaining stable

soil granulation. Other crops in order of decreasing effectiveness were

clover, oats, rotation corn, and continuous corn. Metzger and Hide

(1938) reported that alfalfa and sweet clover leave the soil in a better

state of aggregation than do several other nonsod crops. All evidence

indicates that sod crops increase or maintain soil aggregation better

than most or all other crops (Strickling, 1951). This is no doubt associated with the amount of root residue left in the soil which can be

utilized as microbial energy material, with the good distribution of the

root residues throughout the soil mass, and with the action of the root

system in breaking up soil lumps into smaller units.

After a single application of organic material is applied to the soil,

aggregation reaches a maximum and then declines. Although the effects

of a single large application may last for periods of a year or longer, in

order to maintain good aggregation, periodic addition of organic

materials is necessary.

The work and views of F. Y. Geltzer with respect to organic matter,

soil microbe, and soil structure relationships have been quite widely

quoted (Stallings, 1952; Russell and Russell, 1950; Bremmer, 1954). It

is Geltzer’s opinion (1944) that the best structure is produced in a soil

during the decomposition of fungal hyphae by soil bacteria. According

to her theory, organic materials are first attacked by soil fungi which

produce substances with little aggregating power. The fungal hyphae

are then attacked by soil bacteria which produce gummy substances



16



JAMES P. MARTIN



et al.



which combine with the clay particles upon their release by autolysis of

the bacteria. I t is these substances that supposedly form the stable

aggregate structure. It is probable that bacteria decomposing fungus

cell material do produce soil-binding substances, but it is unlikely that

they would produce o r synthesize certain kinds of organic substances

while decomposing fungus cell material and different substances while

decomposing plant or other types of complex organic residues. In addition, as will be pointed out in a later section, the fungi as a group are

just as effective or more effective than the bacteria in binding soil

particles into water-stable aggregates. It is probable that certain

microbes from all groups are important in the soil aggregation process

and that a complex energy material is, in general, more important than

the source of the energy material.

Alderfer and Merkle (1944) found that mulches increased soil

aggregation in the field when decomposition of the mulch material occurred. The fact that decomposing plant residues contain water-soluble

aggregating materials suggests that improved soil structure following

mulching may be due to the leaching into the soil of water-soluble

binding substances from the surface residues. In the absence of leaching

which would occur during rains, sprinkler irrigation, or irrigation, aggregation of the surface soil could result from the decomposition of the

mulch material in contact with the surface.



3 . Influence of Environmental Conditions

The effect of factors such as temperature and moisture on the

aggregating action of organic residues has received little attention. A

study by Martin and Craggs (1946), however, clearly indicated that

the beneficial action of organic residues on soil structure is markedly

influenced by environmental conditions. Typical results are presented

in Table I. I n general, as the temperature of incubation increased, the

aggregating action of the residue decreased. At low temperatures a

longer incubation period was required for maximum aggregation to

take place. The temperature effect may be explained in two ways. In

the first place temperature will affect the nature of the microbial population involved in the decomposition processes. It is possible that the

microbial population active at low temperatures produces a greater

quantity of, or better quality, aggregating substances than those active

at high temperatures. On the other hand, it is well known that high

temperatures favor the rapid decomposition of organic substances

(Waksman, 1938; Waksman and Gerretsen, 1931). It is possible that

at elevated temperatures effective organic aggregating substances are

produced through the activities of the microbes, but that these sub-



SOIL AGGREGATION



17



TABLE I

Influence of Temperature and Moisture on Soil-Aggregating Effect of Organic

Residues in Declo Loam'

Percentage aggregation of < 5 0 - p particles n t

various incubation periods, days

___

-~

~



RfTect of teinperature2

-~ __

10

30

50

100

__

10"



Control

Alfalfa

Cow ma iiure

Alfalfa-grass Iiay

Sucrose



30

60

37

56

71



31

71

41

ti7

73



Control

.UEalfa

Cow manure

Alfalfa-grass h a ?

Sucrose



31

69

46

66

70



31

66

53

67

68



22"



40"



Control

Alfalfa

Cow manure

Alfalfa-grass hay

Sucrose



32

67

54



59

58



31

58

55

57

54



c.

32

73

44

70

66



59



3!)



53

56

51



36

70

52

68

7.2



31

58

54

57

50



3.2

69

51

66

73



30

49

48

52

48



Martin and Crangs (1046).

Moisture content maintained a t 5 5 % of

'Temperature of incubation 25' C.



32

50

47

53

47



30

43

47

50



41



50



36

70

55

64

73



36

69

53

64

71



33

67

53

71

70



3t'

6.2

56

65

67



75% of saturdion

30

54

46

51



46



30

61

49

60

67



55" c.



Control

Alfalfa

Cow manure

Alfalfa-grass hay

Sucrose



.2U



50% of saturation



c.

57



10



25% of saturation

33

68

45

66



c.

32

66

56

64

58



Effect of moisture3



32

63

53

65

57



32

59

55



64

53



Saturated

31

41

43

41

34



99

48

38

46

53



28

38

38

45

45



31

36

40

43

38



1



capncitg.



stances are in turn quickly destroyed by further microbial activity. The

shorter period of incubation required for maximum aggregation at the

higher temperatures, followed by more or less rapid decline (Table I),

tends to support this view.

Several workers have indicated that aggregate stability of field soils

may be subject to seasonal variation (Alderfer, 1946; Wilson and



18



JAMES



P. M A R T I N



et



Ul.



Browning, 1946; Wilson et ul., 1948). Strickling (1951) reported large

seasonal variation in 1949 but not in 1947 and 1948. It was observed

that 1949 was one of the hottest and most humid years on record, and it

was suggested that the climate may have stimulated the decomposition

of organic aggregate-stabilizing substances by the soil organisms.

In soil saturated with water (Martin and Craggs, 1946). the beneficial action of organic residues in aggregating the soil was greatly reduced. I n a waterlogged soil the activities of the fungi and strictly

aerobic bacteria are greatly retarded. Decomposition is carried on primarily by anaerobic bacteria. It appears that the latter population does

not produce the quantity or quality of soil-binding substances produced

by the population of a well-drained soil.



SPECIESO N AGGREGATION

IV. EFFECTOF MICROBIAL

Pure culture studies have demonstrated that microbial species vary

widely in their ability to bind soil particles. In one study, using sucrose

as an energy source, Aspergillus niger and Azotobacter indicum were

much more effective than Rhizopus nigricans or Pseudomonas fZuorescens in binding the soil (Waksman and Martin, 1939). Cunninghamella

blakesleeana proved to be more effective than a bacterial culture in a

study by Peele (1940). McCalla (1946) studied the effect of different

microbial groups in increasing the stability of soil lumps against falling

water drops. The order of decreasing effectiveness was fungi, actinomycetes, certain bacteria, yeasts, and the majority of bacteria tested. The

presence of organisms of low stabilizing power reduced the effectiveness of organisms which produced high stabilization. In another study,

a Cladosporiurn sp. was much more effective than Mucor or Rhizopus

species in binding the soil particles (Martin and Anderson, 1942).

Gilmour et al. (1948) reported that the binding ability of various

fungus species depended to some extent on the soil and on the source of

energy material used.

Inasmuch as soil organisms vary in growth habits, structural makeup, decomposition products formed, and substances synthesized, it

would be expected that their effect on soil granulation would vary.



V. NATUREOF ORGANICSOIL-BINDING

SUBSTANCES

Increased soil aggregation following organic matter application

could be brought about by one or more of the following:

1. Mechanical binding of the soil particles by microbial filaments

or cells during periods of intense microbial activity.

2. Presence of binding substances in the organic residues.

3. Organic waste products formed during the decomposition of the



19



SOIL A G G K E G x l l O N



original material, dead microbial cells, or secondary decomposition

products.

4. Organic binding substances synthesized by the soil organisms.

Which of these are the most important is a matter of conjecture,

but the last two are probably very important if not the most important.

Geltzer (1937) came to the conclusion that during the decomposition

of organic matter in the soil there is an accumulation of synthetic

microbial substances which bring about the binding of soil particles

into aggregates. Peele (1940) demonstrated that bacterial mucus from

several species produced water-stable aggregates when incorporated

with the soil.

I . Pol ysacchurides



In a study designed to determine the nature of soil-binding substances synthesized by soil organisms, a polysaccharide of the levaii

type produced by Bacillus subtilis was found to be effective binding

material (Martin, 1945). I n a continuation of this study (Martin,

1946), several bacterial polysaccharides were found to be very effective

binding agents (see Table 11). As little as 0.1 g. of one material in 100

TABLE IT



Effect of Bacterial Polysaccharides on Aggregation of Declo Loam’



I’olysaccliaride



Cotrcctitra tioii,



,\ggregation

of ( 5 0 - r

p:i rticlrs,



%



%



-



None

Fructosan



froiii



Harillus subtilts



0.1

0 .3



45

51



Fructosan froiii Amtobarter intlic.tirrr



0 .1

0.3



59

65



Dextran froill a soil bacteriuiii No. 1



0.1

0.3



60

63



L)extmn from a soil bacteriutn No. 2



0 1

0.3



70

I5



Dextran froiu Leuconostor de.rtranicz~rrr



0.1

0.3



56



66



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