Soil aggregate stability is a crucial soil property affecting soil sustainability and crop production A broad outline of the processes and agents of aggregate formation and aggregate stabilization are presented and discussed in this review Aggregate stability is difficult to quantify and interpret The aim of aggregate stability tests is to give a reliable description and ranking of the behavior of soils under the effect of water, wind and management Numerous methods have been used to determine aggregate stability with varying success The different methodologies complicate the comparison among aggregate stability data It is also difficult to obtain a consistent correlation between aggregate stability and other important soil properties such as soil erodibility or crusting potential This paper reviews the different methods of measurement of soil aggregate stability used in the literature, paying attention to the conditions of sample collection in the field and sample preparation and treatmen

Soil aggregate stability: a review

The interaction between soil particles and the soil solution depends on the types
 and amounts of soil clays. Montmorillonite is the most reactive clay. Ca-clay forms
 tactoids (quasi crystals) and has limited swelling and dispersion. Na-clay forms single
 platelets and disperses freely. In a mixed Na/Ca system, “demixing” of the cations
 occurs as the Na ions concentrate on the external and the Ca ions on the internal
 surfaces of the tactoids. The demixing explains why a small percentage of exchangeable
 Na sharply increases the zeta potential and dispersion of montmorillonite clay.
 The hydraulic conductivity of a soil depends on both Na and the total salt concentration
 of the percolating solution. High hydraulic conductivity may be maintained, even at
 high exchangeable sodium percentage (ESP) values, if the solution concentration is
 above a critical (threshold) level. When waters of very low salt content are used,
 decreases in hydraulic conductivity and clay dispersion occur even in soils with low
 ESP values (<10).
 The main mechanism for hydraulic conductivity reduction in waters of medium to high
 salinity (replacecodegt0.5 dS/m) is clay swelling. High content of expansible clays
 increases the susceptibility of soils to intermediate (10<ESP<20) sodicity. The effect
 of low sodicity (ESP < 10) on clay swelling is minimal and the hydraulic conductivity
 of these soils is maintained at high values.
 When waters having low salt concentrations (salt concentration below the flocculation
 value of the clay) are used, clay dispersion, movement, and lodgement in the conducting
 pores comprise the main mechanism responsible for hydraulic conductivity reduction.
 These results occur even at low ESP. The most important soil property which determines
 its susceptibility to sodic conditions, when the soil is leached with rain or snow
 water, is the potential of the soil to release electrolytes from primary minerals
 and/or CaCO3 dissolution. Soils which release sufficient electrolytes to maintain the concentration
 of the soil solution above the flocculation value of the clay will not disperse, and
 thus will not be sensitive to low sodicity.

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Response of soils to sodic and saline conditions

Agricultural production in the arid and semiarid regions of the world is limited by poor water resources, limited rainfall, and the detrimental effects associated with an excess of soluble salts, constrained to a localized area or sometimes extending over the whole of the basin. In order to minimize vagaries of arid weather, bring more land under irrigation, and produce and stabilize greater yields per unit area, numerous water development projects have been commissioned all over the world. Extension of irrigation to the arid regions, however, usually had led to an increase in the area affected by shallow water tables and to intensifying and expanding the hazards of salinity. This is because irrigation water brings in additional salts and releases immobilized salts in the soil through mineral dissolution and weathering, and losing water volumes through evapotranspiration and concentrating the dissolved salts in soil solution. Fertilizers and decaying organic matter also serve as additional salt sources. Atmospheric salt depositions, though varying with location, may be an important source along the coasts. The relative significance of each source in contributing soluble salts depends on the natural drainage conditions, soil properties, water quality, soil water, and agronomic management practices followed for crop production.

Salt-Affected Soils: Their Reclamation and Management for Crop Production

Hardsetting soils are soils that set to a hard, structureless mass during drying and are thereafter difficult or impossible to cultivate until the profile is re wetted. The term hardsetting, originally used when the concept was first defined, will be used here rather than the hyphenated spelling that has sometimes been used. Only in Australia is this form of behavior specifically identified, mapped, and given the name “hardsetting” (Northcote et al., 1975).

Hardsetting Soils: Behavior, Occurrence, and Management

Sodium salts tend to dominate salt-affected soils and groundwater in Australia; therefore, sodium adsorption ratio (SAR) is used to parameterise soil sodicity and the effects of sodium on soil structure. However, some natural soils in Australia, and others irrigated with recycled water, have elevated concentrations of potassium and/or magnesium. Therefore, there is a need to derive and define a new ratio including these cations in place of SAR, which will indicate the dispersive effects of Na and K on clay dispersion, and Ca and Mg on flocculation. Based on the differential dispersive effects Na and K and the differential flocculation powers of Ca and Mg, we propose the concept of 'cation ratio of soil structural stability' (CROSS), analogous to SAR. This paper also gives the results of a preliminary experiment conducted on three soils varying in soil texture on hydraulic conductivity using percolating waters containing different proportions of the cations Ca, Mg, K, and Na. The relative changes in hydraulic conductivity of these soils, compared with the control treatment using CaCl2 solution, was highly correlated with CROSS. Clay dispersion in 29 soils treated with irrigation waters of varying cationic composition was highly correlated with CROSS rather than SAR. It was also found that CROSS measured in 1:5 soil/water extracts was strongly related to the ratio of exchangeable cations. These results encourage further study to investigate the use of CROSS as an index of soil structural stability in soils with different electrolytes, organic matter, mineralogy, and pH.

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Cation ratio of soil structural stability (CROSS)

Summary

Na-Mg and Na-Ca exchange isotherms were determined at electrolyte concentrations of 500, 50, 5 and 1 meq per litre for illite, vermiculite and montmorillonite and for three soils containing illite and montmorillonite. This enabled comparisons to be made of clay swelling, dispersion and soil hydraulic conductivity changes between the Na-Mg and Na-Ca systems at known SAR, ESP and electrolyte concentration.



Na-Mg montmoriUonite and a montmorillonitic soil behaved identically to the Na-Ca systems at the same ESP and electrolyte concentration: there was no specific effect. At the same SAR, the higher ESP in the Na-Mg system caused greater changes than in the Na-Ca systems. The Na-Mg vermiculite, illite, illitic soil and mixed illite-montmorillonitic soil showed greater changes than the Na-Ca systems at the same ESP, and there was a specific effect. At the same SAR, the higher ESP in all the Na-Mg systems apart from vermiculite increased the differences, but for vermiculite with a lower ESP, the differences were reduced. The lack of a specific effect for montmorillonite is probably related to the demixing of cations with the divalent ions concentrated on the non-swelling (internal) surfaces of the crystals.

W. A. Rahman

Papers

The influence of magnesium in saline and sodic soils: a specific effect or a problem of cation exchange?