An aggregate is a group of primary particles that cohere to each other
more strongly than to other surrounding soil particles. Most adjacent
particles adhere to some degree. Therefore, disintegration of the soil mass
into aggregates requires imposition of a disrupting force. Stability of aggregates
is a function of whether the cohesive forces between particles
withstand the applied disruptive force.

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Aggregate Stability and Size Distribution

The origin of cation exchange capacity (CEC) lies in the negative charges carried by soil particles, usually clay, organic matter and sesquioxides. A full discussion of the origin and nature of these charges is presented in Chapter 41 (Zelazny et aI., 1996). Basically these charges fall into two distinct categories, being either permanent or variable (i.e., pH dependent) depending on whether or not ambient conditions (pH or salts) in the soil solution affect their magnitude. Much confusion in the literature concerning the measurement and interpretation of CEC has stemmed from the lack of recognition that these charges fall into two distinct categories exhibiting different behavior. These problems will be addressed in the discussion of the methods for the determination of CEC.

Cation exchange capacity and exchange coefficients.

Salinization is the accumulation of water-soluble salts in the soil solum or regolith to a level that impacts on agricultural production, environmental health, and economic welfare. Salt-affected soils occur in more than 100 countries of the world with a variety of extents, nature, and properties. No climatic zone in the world is free from salinization, although the general perception is focused on arid and semi-arid regions. Salinization is a complex process involving the movement of salts and water in soils during seasonal cycles and interactions with groundwater. While rainfall, aeolian deposits, mineral weathering, and stored salts are the sources of salts, surface and groundwaters can redistribute the accumulated salts and may also provide additional sources. Sodium salts dominate in many saline soils of the world, but salts of other cations such as calcium, magnesium, and iron are also found in specific locations. Different types of salinization with a prevalence of sodium salts affect about 30% of the land area in Australia. While more attention is given to groundwater-associated salinity and irrigation salinity, which affects about 16% of the agricultural area, recent investigations suggest that 67% of the agricultural area has a potential for "transient salinity", a type of non-groundwater-associated salinity. Agricultural soils in Australia, being predominantly sodic, accumulate salts under seasonal fluctuations and have multiple subsoil constraints such as alkalinity, acidity, sodicity, and toxic ions. This paper examines soil processes that dictate the exact edaphic environment upon which root functions depend and can help in research on plant improvement.

World salinization with emphasis on Australia

Summary
Crusting and erosion of cultivated soils result from aggregate breakdown and the detachment of soil fragments by rain, and the susceptibility of soil to these processes is often inferred from measurements of aggregate stability. Here, theories of aggregate breakdown are reviewed and four main mechanisms (i.e. slaking, breakdown by differential swelling, mechanical breakdown by raindrop impact and physico–chemical dispersion) are defined. Their relative importance depends on the nature of the rain, as well as on the soil's physical and chemical properties. The relations between aggregate breakdown, crusting and water erosion are analysed, and existing methods for the assessment of aggregate stability are reviewed. A unified framework for the measurement of aggregate stability is proposed to assess a soil's susceptibility to crusting and erosion. It combines three treatments having various wetting conditions and energies (fast wetting, slow wetting, and stirring after pre-wetting) and measures the resulting fragment size distribution after each treatment. It is designed to compare different soils, or different climatic conditions for a given soil, not to compare time-dependent changes in that soil.

Aggregate stability and assessment of soil crustability and erodibility: I. Theory and methodology

https://pubag.nal.usda.gov/download/25475/PDF

Salinity–mineral nutrient relations in horticultural crops

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.

/pdf/response-of-soils-to-sodic-and-saline-conditions-6ozdz3gvds.pdf

Response of soils to sodic and saline conditions

The effects of electrolyte concentration and soil sodicity on the infiltration rate and extent of crust formation of a calcareous and a noncalcareous soil were studied using a rain simulator. The infiltration rate was more sensitive to the sodicity of the soil and to the electrolyte concentration of the applied water than was the permeability of the underlying soil. The mechanical impact of the raindrops and the relative freedom for particle movement at the soil surface may account for the greater sensitivity of the infiltration rate. These observations suggest that crust formation is due to two mechanisms: (i) a physical dispersion of soil aggregates caused by the impact action of the raindrops, and (ii) a chemical dispersion which depends on the soil exchangeable sodium percentage (ESP) and the electrolyte concentration of the applied water.

Effect of Electrolyte Concentration and Soil Sodicity on Infiltration Rate and Crust Formation

The changes in hydraulic conductivity and clay dispersivity of a Fallbrook soil (fine-loamy, mixed, thermic Typic Haploxeralfs) as a function of concentration (^ 3 meq/liter) and Na adsorption ratio (€ 30) of the percolating solutions were measured. Both phenomena were very sensitive to the level ot exchangeable Na and to salt concentration of the soil solution. When the salt concentration is 3.0 meq/liter, the hydraulic conductivity and clay dispersion decrease if the exchangeable sodium percentage (ESP) values exceed 12. Conversely, in distilled water, clay dispersion and hydraulic conductivity decrease at ESP values as low as 1 to 2%. We hypothesize that mineral soils, which readily release salt during leaching with distilled water, will not be susceptible to exchangeable Na < 10%. Conversely, mineral soils with low rates of mineral dissolution, like the Fallbrook soil, should be susceptible even at lower ESP values. Also, surface soils, when leached with rainwater, will be especially susceptible at such low levels of exchangeable Na. Additional Index Words: clay swelling. Shainberg, I., J. D. Rhoades, and R. J. Prather. 1980. Effect of low electrolyte concentration on clay dispersion and hydraulic conductivity of a sodic soil. Soil Sci. Soc. Am. J. 45:273-277. O OF THE MAJOR FACTORS affecting the suitability of a water for irrigation is its sodicity hazard. We have useful criteria to predict the steady state exchangeable sodium percentage (ESP) of a soil that will result from irrigating with the water. However, we do not have criteria as reliable for predicting how the water and the resulting ESP of the soil will affect soil structure and soil hydraulic conductivity (HC) (Rhoades, 1972). For example, Felhendler et al. (1974) measured the HC of two montmorillonitic soils (a sandy loam and a silty loam) as a function of the sodium adsorption ratio (SAR) and salt concentration of the percolating solution and found that both soils were only slightly affected by the SAR of the percolating solution up to SAR 20 as long as the concentration of the percolating solution exceeded 10 meq/liter. However, when the percolating salt solution was replaced by distilled water, simulating rainfall, the response of the two soils differed drastically. The HC of the silty soil dropped to 42 and 18% of the initial value for soils with ESP values of 10 and 20, respectively. The HC of the sandy loam soil dropped to 5 and 0% of the initial value for the same conditions, respectively. To explain these results, Felhendler et al. (1974) proposed that the variable HC response of soils to sodic conditions depends on the Contribution from USDA-SEA-AR, U.S. Salinity Laboratory, Riverside, CA 92501. Received 22 May 1980. Approved 20 Oct. 1980. 2 Visiting Soil Scientist and Soil Scientists, respectively. The permanent address of I. Shainberg is Institute of Soils & Water, ARO, The Volcani Center, Bet-Dagan, Israel. 3 SAR = Na/[Ca + Mg)/2], where all concentrations are in meq/liter. SAR and ESP are approximately numerically equivalent for most arid land soils at values of less than 30 (U.S. Salinity Laboratory Staff, 1954). potential of their clays to disperse. The clay in the sandy loam soil, which is very sensitive to sodic conditions, dispersed and appeared in the leachate of the column. Converely, the clay in the silty loam soil did not disperse, nor did its HC change so drastically upon leaching with distilled water. The importance of dispersion in affecting soil permeability has also been observed by Rhoades and Ingvalson (1969), who concluded that dispersion rather than swelling was the operative process that decreases permeability in vermiculitic soils. Similarly, Frenkel et al. (1978) concluded that plugging of the soil pores by dispersed clay particles is the major cause of reduced HC in montmorillonitic, vermiculitic, and kaolinitic soils in the range of

Effect of Low Electrolyte Concentration on Clay Dispersion and Hydraulic Conductivity of a Sodic Soil1

Quality of Irrigation Water

The flocculation value, the minimum salt concentration which causes flocculation within 24 hours, was determined for mixtures of Naand Ca-montmorillonite (API-25) and Fithian illite (API-35). The equivalent fraction of exchangeable Na, £Na, varied from 0 to 1. We prepared Na/Ca clay suspensions by adding the appropriate amounts of homoionic dry clays to a series of Na/Ca chloride solutions (0.2 to 80 molc nr; 0.2 to 80 meq liter') with appropriate sodium adsorption ratio and shaking the mixture for 1 hour. The flocculation value of Camontmorillonite (0.25 mol,, nr) increased rapidly with small amounts of exchangeable Na: the flocculation value for an JENa of 0, 0.2, 0.4, and 1 was 0.25, 6, 9, and 12 molc m-", respectively. The flocculation values for Caand Na-illite were 0.25 and 55 molc nr, respectively, and increased linearly with £Na. Van der Waals attraction forces are mainly responsible for flocculation of Ca-montmorillonite and illite systems. Edgeto-face attraction is dominant in flocculating Na-montmorillonite. The large effect of small amounts of exchangeable Na in montmorillonite suspensions results from demising of exchangeable ions whereby Na* is the predominant cation on the external surfaces when CHa < 0.20. Additional Index Words: Wyoming montmorillonite, Fithian illite, van der Waals forces, double-layer theory, (NaPOaJu, electrophoretic mobility, swelling pressure, sodium adsorption ratio, clay gel water content. Oster, J. D., I. Shainberg, and J. D. Wood. 1980. Flocculation value and gel structure of sodium/calcium montmorillonite and illite suspensions. Soil Sci. Soc. Am. J. 44:955-959. 1 Contribution from U. S. Salinity Laboratory, AR-SEA, USDA, 4500 Glenwood Drive, Riverside, CA 92501. Received 26 Nov. 1979. Approved 18 June 1980. "Soil Scientists. Dr. Shainberg's permanent address is Institute of Soils and Water, ARO, The Volcani Center, Bet-Dagan, Israel. R PUBLICATIONS report unusually large effects of low exchangeable Na fractions, ENa. on the hydraulic conductivity (Frenkel et al., 1978; Pupisky and Shainberg, 1979) and on infiltration rate (Oster and Schroer, 1979) of soils leached with dilute solutions where the salt concentration was < 18 molc m3~ (18 meq liter"). Clays moved within the soil pores when £Na ^0.15 and hydraulic conductivity decreased drastically within the soil layers where clay particles lodged. Infiltration rates increased, however, with irrigation water concentration at an E^& as low as 0.02. These results suggested that a small amount of adsorbed Na markedly increase the dispersivity and mobility of the clay fraction in dilute solutions. Little information exists on clay dispersivity, or its counterpart flocculation, of mixed Na/Ca clay systems (Na/Ca indicates both Na and Ca are present in the exchanger phase). The minimum NaCl concentration that causes flocculation, referred to here as flocculation value, of Na montmorillonite is 12 to 16 molc m~; the flocculation value for Ca montmorillonite in a CaCl2 solution is 0.2 molc m~ (van Olphen, 1977). The flocculation values of Naand Ca-illite suspensions are 40 and 0.25 molc m~, respectively (ElSwaify, 1976; Greene et al., 1978, respectively). Sodium montmorillonite in dilute suspensions exists as single platelets (van Olphen, 1977). Calcium montmorillonite exists in packets (tactoids or quasicrystals), each consisting of several (4-9) clay platelets with a film of water 0.45 nm (4.5A) on each internal surface 3 Concentrations such as cation exchange capacity, exchangeable cations, and solute concentrations are expressed in moles of (either negative or positive) charge, with the symbol mole. Published September, 1980

I. Shainberg

Papers

Response of soils to sodic and saline conditions

Effect of Electrolyte Concentration and Soil Sodicity on Infiltration Rate and Crust Formation

Effect of Low Electrolyte Concentration on Clay Dispersion and Hydraulic Conductivity of a Sodic Soil1

Quality of Irrigation Water

Flocculation Value and Gel Structure of Sodium/Calcium Montmorillonite and Illite Suspensions