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VIII. Influence of Exchangeable Cations on Aggregation

VIII. Influence of Exchangeable Cations on Aggregation

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S O I L AGGREGATION



31



creased. Uronic carbon analyses showed reduced amounts at the low

pH levels. It was suggested that decreased aggregation at the high

acidity level was associated with a decrease in microbial activity with

a corresponding decrease in the production of soil-binding substances.

2. Exchangeable Cations in General



In a recent study (Aldrich and Martin, 1954) the influence on soil

aggregation of a wide variety of exchangeable cation ratios with and

without the addition of fresh organic residues was determined. Exchangeable magnesium was varied from 2 to 80 per cent, potassium

from 2 to 60 per cent, sodium from 0 to 60 per cent, and calcium-hydrogen from 70 per cent hydrogen saturated through base saturation to 5

per cent excess lime. The following results were noted: Increasing

magnesium exerted no effect an aggregation; high exchangeable potassium only slightly reduced aggregation in one soil but appreciably

reduced it in another; increasing sodium greatly reduced aggregation,

whereas increasing hydrogen had no influence; an excess of free lime

reduced aggregation; and the organic matter additions increased aggregation regardless of the exchangeable cation status of the soil. Increased aggregation, following organic matter additions, was least in

the high-sodium soils.

Microbial activity was high in all the soils receiving organic residues.

I n the most acid soils, bacteria and actinomycetes were greatly reduced

in numbers but fungus numbers were greatly increased. This further

indicates that the fungi are as effective as the bacteria and actinomycetes in producing soil-binding substances during the decomposition

of complex organic residues in the soil. Sodium increased microbial

numbers but decreased aggregation. This cation disperses certain organic binding substances as well as the soil particles. The dispersion of

organic substances could make them more susceptible to attack by soil

organisms, thus explaining the increase in microbial numbers in the

high-sodium soils.

The detrimental effect of sodium on soil structure is well known

(Richards et al., 1954). Soils containing a high percentage of sodium are

usually referred to as alkali soils. De Sigmond (1928) and Magistad

(1945) have suggested that exchangeable sodium and potassium be

considered as additive in defining alkali soils, but there is evidence that

potassium does not adversely affect soil physical properties as does

sodium or to the same degree as sodium (Richards et al., 1954). In the

study by Aldrich and Martin (1954), aggregate analysis of the <50-p

units in Hanford soil indicated that potassium exerted a marked dispersing action on this soil. Casual examination of the high-potassium and



32



JAMES P. MARTIN



et al.



high-sodium soils, however, indicated different physical characteristics.

The sodium soils were exceedingly difficult to crumble when dry,

whereas the potassium soils crumbled with relative ease. Measurement

of the 2-p particles in the two soils revealed that the clay particles of the

potassium soils were essentially completely aggregated, whereas those

in the sodium soils were 30 per cent dispersed.



3 . Effect on Natural and Synthetic Soil Conditioner Substances

Geoghegan (1950) reported that two polyuronides, pectin and

alginic acid, exerted little effect in sodium and calcium soils but had a

marked aggregating effect in hydrogen soils. This observation agrees

with the results of Martin and Aldrich (1955). In this study, the effect

of exchangeable cations on the binding action of a variety of natural

polysaccharides and of carboxymethyl cellulose appeared to be directly

related to the concentration of carboxyl groups in the material. Taking

a neutral soil saturated largely with calcium as a starting point, as the

concentration of uronic units in the polysaccharide increased, the binding action increased with increasing exchangeable hydrogen and decreased with increasing sodium or potassium. Straight sugar group

polysaccharides tended to produce greater aggregation than the polyuronides and were not as readily influenced by changes in exchangeable

cations. Typical results of this study are illustrated in Table V.

The influence of exchangeable hydrogen on the binding action of

polyuronides may be explained on the basis of its effect on hydrogen

bonding. Under neutral or alkaline conditions, an alcoholic hydrogen

atom could be attracted by a carboxylic acid oxygen in the same molecule or in another molecule of the same material. This would reduce its

attraction to the clay particles. Under acid conditions, on the other

hand, hydrogen would be strongly held by the carboxylic acid oxygen.

This would reduce its coordinate bond attraction for the alcoholic hydrogen atom, and the attractive force of the latter for the clay particle

would therefore be increased.

Further, under alkaline conditions, the cations associated with the

carboxylic acid groups of the polysaccharides and with the clay particles

would tend to form hydroxides with the hydroxyl ions in solution. The

negatively charged clay and organic particles thus formed would tend

to repel each other. In acid systems, the polyuronide particles would

exist largely in the unionized form, and therefore the clay and polyuronide particles would not repel each other.

These observations suggest that hydrogen bonding through the

alcoholic group of polysaccharide materials is more important than are

uronic acid groups in their binding action on soil particles, although



33



SOIL AGGREGATION



TABLE V

Effect of Different Exchangeable Cation Percentages on Aggregating Influence of

Various Soil Conditioner Substances on Yo10 Loam'

Percentage aggregation of



Exchangeable

cation

percentages



075 Na

10% Na

30% Na



Con- VAMA

trol

0.2%



0.1%



34

33

33



70

75



66

FD

F%



34

34

34



70

71

75



6'2

49

41



42

23



34

32



70

74

75



62

52



42



39



4



68

71

70



44



83

66

42



19



34

33

34

29

1



IBMA



Carboxy- Dextran

methyl

from

cellulose

a soil

12OH bacterium

0.9%

0.3%



66



63



65

62

61



42



45

47



16



IF



43



< 50-p particles



Dextran from

Mesquite Leuconostoc

gum

dextranicum

0.3%

0.3%



52

65

61



52

53



54



73

77

75



66

62

58



51



75



44

42



76



66

66

59



51

42

25



75

83

86



66



54

52

51

45



75

75

75

65



84

66

56



79



Martin and Aldrich 11955).



the uronic acid group may be of greater importance with respect to

persistence in the soil.

The synthetic soil conditioners, VAMA and HPAN, are apparently

not as readily influenced by exchangeable sodium as are some of the

polyuronides and soil humus (Allison, 1952).



IX. WATERPENETRATION

UNDER PROLONGED

SUBMERGENCE

Water spreading for underground storage is practiced in certain

western areas (Michaelson and Muckel, 1937). For this purpose excess

water during periods of high runoff is turned into ponds in spreading

grounds. When the water is first turned into the ponds, percolation is

relatively rapid but soon drops off until the ponds virtually seal up.

Allison (1947) and McCalla (1951) have demonstrated that the sealing action is caused by microbial activity. I t is believed that the soil

pores become clogged with products of growth. If the soil is dried and

mixed, however, aggregation greatly increases. It appears that the same



34



JAMES



P. MARTIN



et al.



materials that exert a favorable effect on soil structure under one set of

conditions may exert an unfavorable effect under other conditions,

and emphasizes the importance of factors other than the presence of

binding substances in favorable structure formation in soils.



X. SUMMARY

AND CONCLUSIONS

Microorganisms influence the physical properties of the soil by aiding in the process of water-stable aggregate formation. The influence

may be direct or indirect, the latter acting through the compounds produced during decomposition. The favorable effect of microbes is contingent upon the decomposition of organic residues in the soil. During

the decomposition process, substances synthesized by the organisms

and products of decomposition undergo chemical and physical interactions with the soil particles which may increase aggregate stability.

During periods of intense microbial activity, the cells and filaments of

the organisms themselves may mechanically bind soil particles together.

The soil-binding substances produced through microbial activity are

slowly or quickly destroyed by subsequent microbial action. In order

to maintain aggregate stability at a high level, a continuous supply or

periodic additions of organic residues are necessary.

Microbial species and different types of organic residues vary in

their soil aggregate stability effects. Some fungi or bacteria are very

effective, whereas others have little influence. In general, complex

organic residues containing relatively large amounts of easily decomposable constituents bring about greater aggregation than substances

which are relatively resistant to decomposition. Low or moderate temperatures and moisture conditions are more conducive to stable aggregate formation than are higher temperatures and excessive moisture.

Exchangeable cations in the soil influence soil aggregation.

Polysaccharides synthesized by soil organisms are effective soilbinding substances but other organic compounds are undoubtedly active.

More work is needed to determine the nature of the active substances

produced through microbial decomposition of organic residues.

The effects of organic substances on soil binding appear to be associated with OH, NH,, and COOH groups, and with the length and

special characteristics of the molecules. Hydrogen bonding through the

alcoholic hydrogen may be the mechanism by which polysaccharides

bind soil particles. Different types of organic compounds may bind soil

particles through different mechanisms. More work is needed further

to elucidate this aspect of soil aggregate formation.

Some of the synthetic polymeric soil conditioners currently available for use are markedly resistant to decomposition and can, when



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