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VIII. Influence of Exchangeable Cations on Aggregation
S O I L AGGREGATION
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
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
JAMES P. MARTIN
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
Effect of Different Exchangeable Cation Percentages on Aggregating Influence of
Various Soil Conditioner Substances on Yo10 Loam'
Percentage aggregation of
< 50-p particles
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).
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
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.
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