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VII. Water Stability of Soil Aggregates

VII. Water Stability of Soil Aggregates

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iron oxides by sodium dithionite treatment did not result in any changes in the

permeability of a bed of aggregates as compared with the control treatment,

sodium sulfate. The changes in specific area following the removal of iron oxides

indicated that, in all but one instance, the iron oxides are principally present as

discrete particles with relatively large surface areas. They qualified their conclusion by suggesting that it was possible that minor amounts of iron oxide may be

present as active-binding agents. However, Desphande et al. (1968) also reported that acid treatment (0.1 M HCI), which removed silicon, aluminium, and

minor amounts of iron, as compared with the 2-15% by weight removed by

dithonite treatment from the soils studied, mostly produced larger changes in

physical properties.

Because of the effects of the removal of relatively small amounts of aluminium

on the results for permeability, wet sieving, mechanical analysis, and swelling

determinations, Desphande et al. (1968) considered that reactive aluminium in

the form of interlayers or islands between contiguous crystals or clay domains

could confer stability. The authors observed that ferric oxides are not known to

form chloritic type interlayers, although chlorites containing high proportions of

Fez+ in the brucite layer are well known. The small quantity of silicon and

aluminium may arise from very small particles with a composition similar to the

clay minerals.

These considerations suggest that the various empirical approaches to assessing aggregate stability are limited and are not capable of unequivocal interpretation with respect to the basis of stability. Virtually no attention has been given to

the disposition of stabilizing agents in the porous matrix of the soil or indeed to

the nature of the porous matrix itself.

The actual forces involved in cementation need definition. It should be emphasized again that a soil is a condensed particle system to which the excellent

studies of heterocoagulation between clays and oxides (Tama and El-Swaify,

1978) may have limited applicability in considering the role of oxides or their

precursors in stabilizing soil aggregates. The basic questions which need attention are: What is the nature of the forces between differently charged particles at

close distances of approach? What is the relative role of van der Waals forces in

such an interaction? It seems timely to address this difficult problem since considerable information on the surface chemistry of oxides has accumulated in

recent decades (Bowden et al., 1980; Goldberg, 1992).






The outstanding feature of soil structural behavior is the profound influence

that organic matter, or rather some moiety of it, can have on the water stability of



soil aggregates. If variations in stability can be attributed to different levels of

organic matter in soils, then the questions which need to be addressed are: What

part of the organic matter is operative? What is its disposition in the soil? What is

the mechanism of stabilization?

Using sintered glass funnels Quirk and Panabokke (1962) studied the rate of

wetting, under suction, of aggregates ( 1 cm') of a red-brown earth soil taken

from an area with no previous history of cultivation (virgin) and from an adjacent

area of continuous wheat-fallow (cultivated). At suctions of 0.2 and 1 kPa the

cultivated aggregates took up water much faster and attained a larger water

content than the virgin aggregates. The mechanical compositions of the two soils

(20% clay, 28% silt) were almost the same but the levels of organic matter were

sufficiently different (virgin 2.7%, cultivated 1.3% organic carbon) so that the

variations in the rate of wetting could reasonably be attributed to organic matter.

This effect does not seem to be due to a larger finite contact angle for the virgin

soil because when the cultivated aggregates are wet slowly in stages

(10+3+ 1+0.2 kPa) the water content of the aggregates is virtually identical to

that of the virgin aggregates. Furthermore, the total porosity of the virgin and

cultivated aggregates was 25.9 and 26.7 cm3/ 100 g, respectively, and the rate of

uptake for a nonpolar liquid was the same for each soil so that the different

wetting behavior does not appear to result from different porosities, pore size

distributions, or pore continuity.

In a period of 5 min at a suction of 0.2 kPa the virgin and cultivated aggregates

reached water contents of 18 and 33 cm3/ 100 g; thus the original porosity of the

cultivated aggregates was exceeded, and with a Ca soil of this texture the additional uptake could not be attributed to swelling so the additional water uptake

must be due to failure within the wetting aggregate. The cultivated aggregate,

wet rapidly at 0.2 kPa suction, slaked when placed in water whereas when the

aggregates were wet in stages to 0.2 kPa they did not slake. This observation

suggests that the aggregate failure is caused by rapid wetting and Quirk and

Panabokke (1962) used the term incipient failure to describe the phenomenon.

The points of weakness in a porous material would be expected to be the

coarse pores so Quirk and Panabokke (1962) measured the strength of the virgin

and cultivated aggregates shown in Table X . It can be seen that, for a wide range

of water content values, the virgin aggregates were appreciably stronger than the

cultivated aggregates. It is also notable that the remolded cores of each material

had almost identical strengths under a range of conditions, suggesting that the

presence of organic matter itself did not impart increased strength and that the

disposition of the organic matter was important. Quirk and Panabokke (1962)

advanced the hypothesis that the strengthening of coarse pores by an organic

matter moiety is the basis for aggregate stabilization by organic matter. By

introducing polyvinyl alcohol [(CH,CHOH). MW 70,000] into different pore

classes of the cultivated aggregates, Quirk and Williams (1974) showed that the

pores which drain between 3 and 10 kPa were significant in this respect. This



Table X

Mechanical Strengtha (Load in Kilograms) of Virgin

and Cultivated Aggregates and Cores










1 .o




I .88




0.3 1






'' Determined using an Atterberg balance.

same group of pores is responsible for the rapid attainment of field capacity and

hence must have a high probability of continuity within the aggregate. These

pores thus have a dual function as they are also the sites where organic matter

strengthens the aggregates so that they are stable to rapid wetting.

The different suctions used in this study are significant in that they determine

the rate at which water is transferred to the aggregates. The amount of water

taken up by the cultivated aggregates in 5 and 10 min at 0.2, 1, and 3 kPa suction

corresponds to rainfall rates of 5 cm/hr for 5 min, 2.2 cm/hr for 10 min, and 1

cm/hr for 10 min. They concluded that for rainfall rates significantly greater than

1 cm/hr for 10 min that appreciable structural damage, due to incipient failure,

would occur for red-brown earth soils in rotations in which the cultivation phase

exceeds about three years. The soil samples were obtained from plots at the

Waite Institute where the probability of a shower at 3 cm/hr for 15 min is once a

year and for a shower of 2 cm/hr for 30 min is also once a year. These results

have implications with respect to the general concept of water stability which

needs to be defined and understood in more precise terms than those afforded by

observing the degree of slaking of soil aggregates on immersion in water; an

assessment of water stability would, of course, need to include the effect of

raindrop action which would be enhanced by incipient failure.






The behavior of saline and sodic soils has been reviewed from different points

of view by Richards (1954), Rhoades (1982), Bresler et al. (1982), Shainberg

and Letey (1984), and Oster el al. (1994); specific attention will be given here to

the threshold concentration concept.



The Gapon equation, which has been widely used in research concerning

saline and sodic soils (Richards 1954), has been discussed by Sposito (1981) in

relation to the more conventional cation exchange equations. In the Gapon equation the ions are shown as reacting in equivalents:

NaX (s)

+ 0.5 CaC1,




+ NaCl(aq)


and the Gapon equilibrium constant is:

which may be rearranged to give

where E N , and Ec, are the equivalent fractions of sodium and calcium ions on the

exchange complex and the square brackets refer to concentrations in solution

(mmol liter-1) rather than activities. The ratio of ion concentrations is of a

similar magnitude to the corresponding ratio of ion activities over the concentration range common to salt-affected soils, even though the activities themselves

vary considerably. In considering K , values for individual soils it is realistic to

recognize that soil clays are frequently a mixture of clay mineral types and

contain four principal cationic species: sodium, potassium, magnesium, and

calcium. The ratio EN,/Ec, is referred to as the exchangeable sodium ratio

(ESR), and the ratio, if only sodium and calcium ions are present, E,,IEN, +

Eca, expressed as a percentage is the exchangeable sodium percentage (ESP);

more generally the ESP is the percentage of exchangeable sodium relative to the

exchange capacity. The ratio (Na+)/(Ca2+)0.5is referred to as the sodium adsorption ratio (SAR) of the equilibrium solution, and in practice Mg2+ is coupled

with Ca2+ in the denominator.

The Gapon equation has two interesting features. First, if a soil is equilibrated

with a solution having a given SAR value, and it is desired to percolate the soil

with a more dilute solution and at the same time maintain the ratio of sodium to

calcium on the soil colloid surfaces constant, then if the sodium ion concentration in the solution is reduced by a factor then the calcium ion concentration must

be reduced by the square of that factor. Second, if water being used to reclaim a

sodic soil is simply diluted, this dilution decreases the SAR value and thus favors

the adsorption of calcium on the soil surfaces, so the exchangeable sodium

percentage is reduced and reclamation is assisted.

Difficulties encountered in the irrigation of sodium-affected soils or in the

reclamation of saline-sodic soils can be circumvented by controlling the electrolyte level in the irrigation water; this prevents the deterioration of soil structure

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VII. Water Stability of Soil Aggregates

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