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Soil structure changes: aggregate size and soil texture effects on hydraulic conductivity under different saline and sodic conditions.



Introduction

Soil hydraulic conductivity Hydraulic conductivity, symbolically represented as , is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures.  (K), which determines the ability of soil to conduct water, is an important parameter governing the soil phase of the hydrological hy·drol·o·gy  
n.
The scientific study of the properties, distribution, and effects of water on the earth's surface, in the soil and underlying rocks, and in the atmosphere.
 cycle, affecting solute solute /so·lute/ (sol´ut) the substance dissolved in solvent to form a solution.

sol·ute
n.
 transport and soil water availability for plants. Therefore, the K parameter is of hydrological, ecological, environmental, and agronomic a·gron·o·my  
n.
Application of the various soil and plant sciences to soil management and crop production; scientific agriculture.



ag
 significance. The value of K is strongly dependent on soil structure, when considering this structure in terms of 2 of its main components: (i) the geometry of pore spaces in the soil (the architecture of the pore spaces), and (ii) the stability of the structure. The geometry of the pore spaces includes pore-size distribution, pore tortuosity tortuosity
n.
1. The quality or condition of being tortuous; twistedness or crookedness.

2. A bent or twisted part, passage, or thing.
, and the connectivity and interactions among pores. Many studies have been designed to predict K by characterising the pore spaces in the soil (e.g. Assouline et al. 1998; Arya et al. 1999; Ghezzehei and Or 2000; Vogel and Roth 2001; Tuller and Or 2002; Hwang and Powers 2003). However, there are fundamental problems in characterising the pore spaces in the soil (Hunt and Gee 2002; Lebron and Robinson 2003), which becomes much more complicated when the soil structure is changed during its wetting and leaching (Ghezzehei and Or 2000; Dikinya et al. 2007). Moreover, Lebron et al. (2002) found that, in soils with aggregates, there is no significant relationship between soil texture Soil texture is a soil property used to describe the relative proportion of different grain sizes of mineral particles in a soil. Particles are grouped according to their size into what are called soil separates (clay, silt, and sand). The soil texture class (eg.  and pore size distribution, indicating that models predicting soil K using particle-size distribution to infer porosity may not be successful. Therefore, for soils with unstable structures, such as arid and semi-arid soils, it is important to gain an insight into the dynamic processes and mechanisms that cause the soil structure and K values to change.

Excessive levels of salts and Na occur in soils in large areas around the world, mainly in arid and semi-arid regions, and profoundly affect the structural stability and K of the soil (Bresler et al. 1982; Shainberg and Letey 1984; Sumner et al. 1998). In the absence of raindrop impact and external compacting pressures, there are 3 main mechanisms that could degrade the soil structure during wetting and leaching: (i) aggregate slaking, (ii) swelling, and (iii) clay dispersion. Most of the past studies on the effects of salinity and sodicity on the saturated hydraulic conductivity ([K.sub.s]) of soil (e.g. Frenkel et al. 1978; Pupisky and Shainberg 1979; Shainberg et al. 1981; Shainberg and Letey 1984; Alperovitch et al. 1985; Keren and Singer 1988; Frenkel et al. 1992; Levy et al. 1998; Keren and Ben-Hur 2003) focused on clay swelling and dispersion as the only 2 processes that cause [K.sub.s] reduction under soil leaching. Far fewer works (e.g. Abu-Sharar et al. 1987; Shainberg et al. 2001; Lado et al. 2004b; Levy et al. 2005) have evaluated the effects of soil slaking on [K.sub.s]. This is despite the fact that aggregate disintegration occurring as a result of slaking during wetting is well documented (e.g. Rengasamy and Olsson 1991; Loch 1994; Amezketa and Aragues 1995; Le Bissonnais and Arrouays 1997; Lado et al. 2004a; Mamedov et al. 2006; Ben-Hur and Lado 2008). Slaking occurs when the aggregate is not strong enough to withstand the stresses produced by differential swelling, the pressure of entrapped air, the rapid release of heat during wetting, and the mechanical action of moving water (Emerson 1977; Collis-George and Green 1979; Kay and Angers 1999). These stresses are termed slaking forces. The slaking process is controlled by the wetting rate of the soil: the faster the wetting, the stronger the slaking forces and the greater the proportion of aggregates that undergo slaking.

Most of the published studies on the effects of structural degradation on the [K.sub.s] of disturbed soil samples were conducted on samples with aggregate sizes mainly <2 mm (e.g. Frenkel et al. 1978; Pupisky and Shainberg 1979; Shainberg and Letey 1984; Suarez et al. 1984; Abu-Sharar et al. 1987; Mace and Amrhein 2001; Shainberg et al. 2001 ; Levy et al. 2005; Dikinya et al. 2007). However, the structure stability of the aggregates could be affected by their size (Tisdall et al. 1997; Six et al. 2004; Park and Smucker 2005a, 2005b). Aggregates in soil are primarily formed by physical, chemical, and biological processes, such as wetting and drying or freezing and thawing cycles, temperature changes, cultivation, soil biota biota /bi·o·ta/ (bi-o´tah) all the living organisms of a particular area; the combined flora and fauna of a region.

bi·o·ta
n.
The flora and fauna of a region.
 activities, and plant growth. The stabilisation of the aggregates, however, takes place mainly due to cementing action of inorganic agents such as silicate silicate, chemical compound containing silicon, oxygen, and one or more metals, e.g., aluminum, barium, beryllium, calcium, iron, magnesium, manganese, potassium, sodium, or zirconium. Silicates may be considered chemically as salts of the various silicic acids.  clays, calcium carbonate calcium carbonate, CaCO3, white chemical compound that is the most common nonsiliceous mineral. It occurs in two crystal forms: calcite, which is hexagonal, and aragonite, which is rhombohedral. , and sesquioxides, and of organic compounds (Six et al. 2004). Tisdall and Oades (1982) and Tisdall et al. (1997) proposed a multi-conceptual model that describes the hierarchical organisation of particles within soil aggregates. Oades and Waters (1991) indicated that the binding agents between particles in the aggregates vary according to according to
prep.
1. As stated or indicated by; on the authority of: according to historians.

2. In keeping with: according to instructions.

3.
 the aggregates' sizes, in which micro-aggregates are being held together strongly and can form macro-aggregates, while the macro-aggregates are being held together by weaker forces than the micro-aggregates. In this case, the aggregate sizes could influence the slaking, swelling, and dispersion processes, and consequently the way in which these processes affect the soil K.

Lado et al. (2004b) studied the effects of organic matter contents on [K.sub.s] in sandy loam soils with low exchangeable sodium percentage (ESP (1) (Enhanced Service Provider) An organization that adds value to basic telephone service by offering such features as call-forwarding, call-detailing and protocol conversion. ) (<2%) and with aggregate sizes of <2, 2-4, or 4-6 mm. They found that the effect of aggregate slaking on [K.sub.s] reduction was more pronounced in soils with aggregate sizes of 2-4mm than in those with aggregate sizes <2 mm, suggesting that there is an interaction between aggregate size and aggregate slaking in their combined effects on soil [K.sub.s]. Furthermore, Abu-Sharar et al. (1987) passed 3 dry, disturbed Californian soils containing 8.8, 14.2, and 19.5% clay through a sieve with a mesh size of <2 mm, and found that these soils had significantly different dry-aggregate size distributions. Abu-Sharar et al. (1987) hypothesised that this variation in the aggregate size distribution was the main reason for the differences in the structure and the [K.sub.s] of these soils during their leaching with successively diluted electrolyte solutions. Therefore, the objective of the present study was to investigate the effects of aggregate sizes and soil texture, and their interactions, on the structural degradation and [K.sub.s] of smectitic soils under different saline and sodic so·dic  
adj.
Relating to or containing sodium.



[sod(ium) + -ic.]


sodic  

Relating to or containing sodium.
 conditions. In addition, a set of aggregate stability tests was proposed in order to determine the sensitivity of soils, with different salinity and sodicity levels, to slaking, swelling, and dispersion processes.

Materials and methods

Soil samples

Two different smectitic soils were collected from the 0.05-0.15 m layer in 2 different locations in Israel: a clay soil (Typic Haploxerert) from the Yezre'el Valley in the north and a loamy sand (Typic Rhodoxeralf) (Soil Survey Staff 1999) from the Coastal Plain in the centre of the country. The soil samples were collected from several plots that had been individually irrigated with water of various qualities, and that had, therefore, acquired differing ESPs. In each soil, 2 ESP values were studied: 4.3 and 10.6 in the clay soil, and 4.6 and 9.3 in the loamy sand. Some physical and chemical properties of the soils are presented in Table 1. The particle size Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials.  analysis was determined with a hydrometer hydrometer (hīdrŏm`ətər), device used to determine directly the specific gravity of a liquid. It usually consists of a thin glass tube closed at both ends, with one end enlarged into a bulb that contains fine lead shot or mercury to  (Day 1956); the organic matter content by the Walkley-Black method (Allison 1965), the CaCO3 content by a volumetric volumetric /vol·u·met·ric/ (vol?u-met´rik) pertaining to or accompanied by measurement in volumes.

vol·u·met·ric
adj.
Of or relating to measurement by volume.
 method (Allison and Moodie 1965), the cation exchange capacity In soil science, cation exchange capacity (CEC) is the capacity of a soil for ion exchange of positively charged ions between the soil and the soil solution. A positively-charged ion, which has fewer electrons than protons, is known as a cation due to its attraction to cathodes.  (CEC (Central Electronic Complex) The set of hardware that defines a mainframe, which includes the CPU(s), memory, channels, controllers and power supplies included in the box. Some CECs, such as IBM's Multiprise 2000 and 3000, include data storage devices as well. ) by extraction with ammonium acetate Ammonium acetate is a chemical compound with the formula NH4C2H3O2. It is a white solid, which can be derived from the reaction of ammonia and acetic acid. It is available commercially, and depending on grade, can be rather inexpensive.  at pH 7 (Chapman 1965), and ESP by standard methods (U.S. Salinity Laboratory Staff 1954).

The soil samples were air-dried, visible roots and organic residues were removed, and the dry samples were then sieved to obtain 2 groups of aggregates with size ranges of: <1 mm (small aggregates); and 2-4 mm (large aggregates). The mechanical compositions, organic matter and CaC[O.sub.3] contents, and ESPs of the 2 soil samples were also determined for each aggregate size range by the methods described above. No significant differences in these soil properties were found between the various aggregate sizes in either soil (results not presented). Samples of each soil at each ESP were subjected to the 4 tests described below.

Saturated hydraulic conductivity experiment

The [K.sub.s] measurements were carried out in 0.15-m-long, plastic, cylindrical columns with an internal diameter of 0.05 m. A 0.1-m layer of the soil sample was placed in the column, on top of a 0.02-m layer of acid-washed, coarse, quartz sand. The soil sample for each column was divided into 3 equal portions, each of which was packed gently into the column in order to achieve a nearly uniform bulk density. The bulk densities of the 2 soils, each with its 2 ESP values and 2 aggregate size ranges, when packed in the columns are presented in Table 2. A filter paper (Whatman 42) was placed on the top of the soil layer in each column, to prevent mixing and turbulence when water was added.

Each column was initially wetted from the bottom with saline water Saline water is a general term for water that contains a significant concentration of dissolved salts (NaCl). The concentration is usually expressed in parts per million (ppm) of salt.  with an electrical conductivity (EC) of 5.0 dS/m and a sodium adsorption adsorption, adhesion of the molecules of liquids, gases, and dissolved substances to the surfaces of solids, as opposed to absorption, in which the molecules actually enter the absorbing medium (see adhesion and cohesion).  ratio (SAR (Segmentation And Reassembly) The protocol that converts data to cells for transmission over an ATM network. It is the lower part of the ATM Adaption Layer (AAL), which is responsible for the entire operation. See AAL.

SAR - segmentation and reassembly
) which corresponded to the ESP of the soil under study. This solution with relatively high electrolyte concentration (EC 5.0 dS/m) was used to determine the [K.sub.s] of the leached soils under no clay dispersion and minimum soil swelling (Shainberg and Letey 1984). In all cases except one, this initial wetting was conducted at 1 of 2 rates: slow, the entire soil column was wetted in 65 75 min; and fast, the entire soil column was wetted in 3540 min. The exception was for the fast wetting of the small aggregates of the clay soil with ESP 10.6, which was wetted in 55 min. After saturation was reached, the direction of the flow was reversed so that the water entered from the top of the column, and about 4 pore volumes of saline water, followed by deionised water, were percolated through each column under saturation conditions. The [K.sub.s] values of each soil, for each aggregate size range and for each ESP, were determined by means of a constant-head device (a Mariotte bottle), and were calculated using Darcy's Law Darcy's law  

A law in geology describing the rate at which a fluid flows through a permeable medium. Darcy's law states that this rate is directly proportional to the drop in vertical elevation between two places in the medium and indirectly proportional to
. The out-flowing solution (leachate) was collected continuously, in timed intervals, using a fraction collector, and its volume and EC were measured.

Slaking test

The aggregate stability of each of the 2 soils, for each ESP, was determined after wetting at each of 2 rates: fast wetting and slow wetting under vacuum.

(i) Fast wetting: 5 g of oven-dried (40[degrees]C) aggregates 2-4 mm were immersed in a beaker containing 50 mL of deionised water. After 10 min, during which the aggregates remained at rest, the water was carefully removed under suction using a pipette pipette /pi·pette/ (pi-pet´) [Fr.]
1. a glass or transparent plastic tube used in measuring or transferring small quantities of liquid or gas.

2. to dispense by means of a pipette.
. The soil fragments were transferred to a 50-[micro]m sieve which had previously been immersed in ethanol, and were then gently moved up and down 5 times in ethanol to separate fragments <50 [micro]m from larger ones. The >50 [micro]m fraction was oven-dried and then gently sieved by hand through a column of sieves of mesh sizes 2.0, 1.0, 0.5, 0.25, and 0.1 mm. The weight of each fraction was measured and that of the <50-[micro]m fraction was calculated as the difference between the initial weight and the sum of the weights of the other 6 fractions. The aggregate stability of each soil sample was expressed in terms of the mean weight diameter (MWD MWD Metropolitan Water District of Southern California
MWD Measurement While Drilling (oil drilling)
MWD Morgan Stanley Dean Witter (stock symbol)
MWD Molecular Weight Distribution
MWD Military Working Dog
), which was calculated by using Eqn l:

MWD = [7.summation over (i=1)] [[bar.x].sub.i][W.sub.i] (1)

in which [w.sub.i] is the weight fraction of aggregates in the size class i with a mean diameter [x.sub.i].

(ii) Slow wetting: 5g of oven-dried (40[degrees]C) aggregates 2-4 mm were placed on a cotton cloth, the edges of which were immersed in deionised water, inside a desiccator des·ic·cate  
v. des·ic·cat·ed, des·ic·cat·ing, des·ic·cates

v.tr.
1. To dry out thoroughly.

2. To preserve (foods) by removing the moisture. See Synonyms at dry.

3.
. The aggregates were wetted slowly under vacuum in the desiccator for 24 h. The wet aggregates were then transferred to a 50-[micro]m sieve, which had previously been immersed in ethanol, and were then sieved as described above for the fast wetting test. The MWD values of the aggregates after the slow wetting procedure were calculated with Eqn 1. The slaking value (SLV SLV
abbr.
standard launch vehicle
) of each soil sample with each ESP value was calculated by using Eqn 2:

SLV = [MWD.sub.s]/[MWD.sub.f] (2)

in which [MWD.sub.s] and [MWD.sub.f] are the mean weight diameters under slow and fast wetting conditions, respectively. This test was applied only to the aggregates 2-4 mm, because those <1 mm were too small to yield reliable results.

Swelling test

For each soil sample, 20 oven-dried aggregates of size 2-4 mm were placed in a Petri dish pe·tri dish
n.
A shallow circular dish with a loose-fitting cover, used to culture bacteria or other microorganisms.



Petri dish

a shallow, circular, glass or disposable plastic dish used to grow bacteria on solid media such as agar.
 and scanned using a flat-bed scanner. The image area of each aggregate was then measured with the UTHSCSA UTHSCSA University of Texas Health Science Center at San Antonio  ImageTool Software (University of Texas Health Science Center, San Antonio, Texas “San Antonio” redirects here. For other uses, see San Antonio (disambiguation).
San Antonio is the second most populous city in Texas, the third most populous metropolitan area in Texas, and is the seventh most populous city in the United States. As of the 2006 U.S.
). The aggregates were considered to be spherical, and their volumes were calculated from the radius of a circle with an area equal to the area of the scanned aggregates. After scanning, the aggregates were wetted slowly with either deionised water or with saline water with an EC of 5.0 dS/m and an SAR which corresponded to the ESP of the soil under study, as described above, under vacuum in a desiccator, and were then scanned again. The swelling value (SWV SWV Sisters With Voices (Singing Group)
SWV Sisters With Voices (R&B Group)
SWV Something Weird Video (DVD supplier)
SWV Square Wave Voltammetry
) for each soil sample was determined by using Eqn 3:

SWV = [n.summation over (i=1)] ([I.sub.wi] - [I.sub.di])/[I.sub.di]/n (3)

in which n is the number of aggregates, [I.sub.wi] is the calculated volume of the aggregate i after wetting, and [I.sub.di] is the calculated volume of the dry aggregate i. This test was applied only to the aggregates 2-4 mm, because those <1 mm were too small to yield reliable results.

Dispersion test

The soil dispersivity was determined according to Gupta et al. (1984). A 2-g sample of each soil for each ESP was suspended in 0.07 L of deionised water in a 0.1-L centrifuge centrifuge (sĕn`trəfyj), device using centrifugal force to separate two or more substances of different density, e.g., two liquids or a liquid and a solid.  tube. The tubes containing the suspension were shaken on a reciprocal shaker for 30 rain at 20 rpm, and were then immediately centrifuged at a relative centrifugal force of 960G for 5 min. The concentration of dispersed clay in the turbid tur·bid
adj.
Having sediment or foreign particles stirred up or suspended; muddy; cloudy.



tur·bidi·ty n.
 supernatant supernatant /su·per·na·tant/ (-na´tant) the liquid lying above a layer of precipitated insoluble material.

supernatant

the liquid lying above a layer of precipitated insoluble material.
 was determined by measuring the absorbance absorbance /ab·sor·bance/ (-sor´bans)
1. in analytical chemistry, a measure of the light that a solution does not transmit compared to a pure solution. Symbol .

2.
 at 420 nm with a spectrophotometer spectrophotometer, instrument for measuring and comparing the intensities of common spectral lines in the spectra of two different sources of light. See photometry; spectroscope; spectrum. , and then comparing the result with a calibration curve of absorbance v. suspended clay concentration that had previously been prepared for each soil type. The dispersion value (DV) for each soil sample was determined by using Eqn 4:

[FIGURE 1 OMITTED]

DV = [M.sub.d]/[M.sub.t] x 100 (4)

in which [M.sub.d] is the mass of the dispersed clay in the turbid supernatant per 1 g of tested soil sample, and [M.sub.t] is the total clay mass in 1 g of tested soil sample.

Statistical analysis

All the measurements were conducted with 3 replicates, and the differences among the means and the interactions between the parameters were subjected to analysis of variance (ANOVA anova

see analysis of variance.

ANOVA Analysis of variance, see there
) as a complete randomised Adj. 1. randomised - set up or distributed in a deliberately random way
randomized

irregular - contrary to rule or accepted order or general practice; "irregular hiring practices"
 design. The differences among the means were subjected to Tukey's Honestly Significant Difference test (Steel and Torrie 1981). All tests were performed at P-0.05.

Results and discussion

The saturated hydraulic conductivity of each aggregate size, for each of the 2 ESP values which were leached with water of 2 qualities after pre-wetting at 1 of 2 rates, are presented as functions of accumulated leachate volume in Fig. 1 for the clay soil, and in Fig. 2 for the loamy sand soil. The initial [K.sub.s] values (i.e. the initial [K.sub.s] values that were measured after the flow direction was reversed so that the percolating solution entered from the top of the column) were greater in the large (2-4 mm) than the small (<1 mm) aggregate samples for all treatments (Figs 1 and 2). This was, most likely, because of the differences in the initial bulk densities of the soils as packed in the columns, which changed the total porosity of the soils: the total porosity thus ranged from 47.2 to 53.6% for the small aggregates and from 56.6 to 60.4% for the large aggregates (Table 2). In addition, the initial [K.sub.s]. values of the loamy sand soil (Fig. 2) were much greater than those of the clay soil (Fig. 1) for both ESP values and for the 2 wetting rates for each aggregate size. This occurred even though the total porosity values of the 2 soils were similar for each aggregate size (Table 2). These results suggested that the loamy sand contained more pores of larger size than the clay soil. The coarse texture (sand content ~86%) of the loamy sand (Table 1) was most likely the main reason for the relatively large number of pores of larger size in this soil. The effects of the initial aggregate sizes, when packed in the columns, on the [K.sub.s] values during the leaching of the 2 soils with the different solutions and wetting rates are discussed below.

[FIGURE 2 OMITTED]

In the absence of raindrop impact and external compacting pressures, there are 3 main mechanisms that could degrade the soil structure during wetting and leaching: aggregate slaking, swelling, and clay dispersion. In the clay soil with the large aggregates and for both ESP values, the [K.sub.s] values after fast pre-wetting were significantly smaller than those after slow pre-wetting, and these differences were apparent from the beginning of the leaching run (Fig. 1). In contrast, differing pre-wetting rates had no significant effect on the [K.sub.s] values of the clay soil with small aggregates for both ESP values (Fig. 1), or of the loamy sand with either aggregate size for both ESP values (Fig. 2).

Slaking is strongly affected by the wetting rate of the soil: the faster wetting occurs, the greater the slaking forces (Rengasamy and Olsson 1991). The MWD of 2-4 mm aggregates from the clay soil and the loamy sand, for both ESP values and after either slow or fast wetting, and their SLVs as determined by the slaking test (Eqns 1 and 2), are presented in (Fig. 3). The MWD values after fast wetting were small, and did not differ significantly between the soils (Fig. 3). In contrast, the MWDs after slow wetting were significantly larger than those after fast wetting, and were significantly greater for the clay soil than for the loamy sand (Fig. 3). No significant effects of the ESP on MWD values were found for either soil undergoing either wetting rate (Fig. 3). When the aggregates were wetted slowly, the slaking forces were weak, which in turn allowed for the preservation of large MWD values (Fig. 3). In contrast, when the dry aggregates underwent fast wetting, the strong slaking forces arising from differential swelling and explosion caused by rapid compression of entrapped air caused extensive aggregate breakdown, which led to smaller MWD values. The SLVs of the clay soil were greater than those of the loamy sand (Fig. 3), indicating that the former soil was more sensitive to slaking than the latter. Similar results were obtained previously by Lado et al. (2004a), who found that the SLV of soil increased linearly with increasing clay content.

The large SLVs of the clay soil (Fig. 3) suggest that the reason why [K.sub.s] values of this soil, with the large aggregates, were smaller after fast pre-wetting than after slow pre-wetting (Fig. 1) was the extensive aggregate slaking that took place when this soil underwent fast pre-wetting. However, in the case of the small aggregates for this soil, the differential swelling and entrapment entrapment, in law, the instigation of a crime in the attempt to obtain cause for a criminal prosecution. Situations in which a government operative merely provides the occasion for the commission of a criminal act (e.g.  of air during fast wetting were probably minor factors, because of the very small size of the aggregates and, therefore, no significant differences were found between the [K.sub.s] values of the fast and the slow pre-wetted samples of this soil (Fig. 1). In the loamy sand, the relatively large number of pores of larger size, resulting from the high sand content in this soil, probably negated the effects of the pre-wetting rates on the [K.sub.s] (Fig. 2).

[FIGURE 3 OMITTED]

During the leaching of the soils with saline water, the high EC (>4.7 dS/m) of the leachate (Fig. 4), which is greater than the flocculation flocculation /floc·cu·la·tion/ (flok?u-la´shun) a colloid phenomenon in which the disperse phase separates in discrete, usually visible, particles rather than congealing into a continuous mass, as in coagulation.  value of the clay component of the studied soils (Van Olphen 1977; Oster et al. 1980), prevented clay dispersion. Therefore, during this leaching stage, the differences between Ks values associated with ESP changes (Figs 1 and 2) could have been caused by differences in the degree of clay swelling. The average SWVs, as determined by the swelling tests (Eqn 3) on the clay and loamy sand soils, each with 2 ESP values after wetting with water of 2 different qualities, are presented in Fig. 5 for the 2-4 mm aggregates. The SWVs of the loamy sand were small, i.e. <20% (Fig. 5), for both ESP values and for both water qualities, most likely because of the low clay content (12.5%) of this soil (Table 1). In this case, the increase in the soil ESP had no significant effects on the SWVs when the soil was wetted with deionised water, but had significant effects when it was wetted with saline water (Fig. 5). The SWVs of the clay soil with either ESP value for both water qualities were large, i.e. >30% (Fig. 5), because of the high clay content (>52%) of this soil (Table 1). The increase in the ESP of this soil increased the SWVs significantly during wetting with either deionised or saline water (Fig. 5). It can be concluded from these SWVs (Fig. 5) that, in the clay soil during leaching with saline water, the smaller [K.sub.s] values of the higher ESP samples (Fig. 1) resulted mainly from increased clay swelling caused by the greater soil sodicity (Fig. 5). This effect on the [K.sub.s] was more pronounced in the soil with small aggregates than in that with the large ones (Fig. 1); the [K.sub.s] values decreased on average by a factor of ~1.5 in the large aggregates and by an order of magnitude A change in quantity or volume as measured by the decimal point. For example, from tens to hundreds is one order of magnitude. Tens to thousands is two orders of magnitude; tens to millions is three orders of magnitude, etc.  in the small ones. The movement of water in soil is mainly through the inter-aggregate pores, and the soil swelling mainly decreases the volume of these pores. Therefore, in the case of the clay soil with small aggregates, in which the inter-aggregate pores were already small, the decrease in the volume of these pores, which resulted from soil swelling, decreased markedly the [K.sub.s] values. In contrast, in the clay soil with the large aggregates, although the clay swelling decreased the volume of the inter-aggregate pores, they still remained large, so that the decrease in [K.sub.s] was less pronounced. In the loamy sand, however, the small degree of clay swelling (<20%) (Fig. 5) and the relatively greater number of pores of large size in this soil probably prevented the occurrence of any significant and consistent effects of the increased ESP on the [K.sub.s] during the leaching with saline water (Fig. 2).

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

For both soils, replacing the percolating saline water with deionised water decreased the electrolyte concentrations in the leachates to low values (Fig. 4), possibly to less than the flocculation value of the clay component of the soil, and in such cases, clay dispersion could occur that may then decrease the [K.sub.s] of the soil. The average dispersion values as determined by the dispersion test (Eqn 4) and the ECs of the deionised water suspensions of the clay soil and the loamy sand, each with 2 ESP values, are presented in Table 3. Both soils, for either ESP, were very dispersive dispersive /dis·per·sive/ (-per´siv)
1. tending to become dispersed.

2. promoting dispersion.
: the minimum DV was 46.2% (Table 3). The reason why the DVs were smaller for the higher than for the lower ESP, in both soils (Table 3), was probably attributable to the higher electrolyte concentrations in the high-ESP suspensions than in those of the low-ESP (Table 3), which decreased clay dispersion in the case of the former.

When the clay soil with aggregates of either size range and with either ESP value was leached with deionised water, the [K.sub.s] values decreased significantly (Fig. 1). The results in Table 3 suggest that, in the clay soil, the decrease in the [K.sub.s] values during leaching with deionised water was partly because of clay dispersion. The dispersed clay particles that moved with the percolating water would have subsequently plugged some of the pores in the bulk soil (Frenkel et al. 1978, 1992; Keren and Singer 1988; Keren and Ben-Hur 2003), which would, in turn, decrease the [K.sub.s]. This decrease in the [K.sub.s] during leaching with deionised water could also be caused by an increase in the soil swelling as the electrolyte concentration in the soil solution decreased (Shainberg and Letey 1984).

In spite of the relatively high dispersivity of the loamy sand (Table 3), replacing the percolating saline water with deionised water did not significantly decrease the [K.sub.s] values in this soil for either ESP value for either aggregate size (Fig. 2). The absence of [K.sub.s] reduction was probably because the pores in the loamy soil were big enough to allow the dispersed particles to move downwards with the percolating water without plugging the pores (Keren and Singer 1988; Keren and Ben-Hur 2003).

Although significant structural degradation (high SLV, SWV, and DV, Figs 3 and 5, and Table 2) occurred during the wetting of the clay and loamy sand soils, the [K.sub.s] values of these soils throughout the entire leaching runs were smaller in the soils with small aggregates than in those with large aggregates, for each of the 2 ESPs, water qualities, and pre-wetting rates (Figs 1 and 2). It can be concluded from these results that the initial aggregate size is an important parameter, and that it affects the [K.sub.s] even in soils that are relatively prone to structural degradation.

Soils in the field comprise various aggregate sizes, and therefore, measuring the effects of structural degradation on [K.sub.s] by using disturbed soil samples with an initial aggregate size of <2 mm alone could lead to erroneous conclusions, since the effects of slaking, swelling, and dispersion processes on [K.sub.s] reduction during soil wetting and leaching could be affected by the initial aggregate size distribution in the soil. For example, large aggregates (>2 mm), which are prone to disintegration by the slaking process in particular, are not taken into account in such measurements of [K.sub.s] reduction in soil columns with an initial aggregate size of <2 mm. Moreover, these effects of initial aggregate size distribution on [K.sub.s] reduction could be altered in soils with differing textures. Therefore, in order to avoid erroneous interpretations of the results of the effects of soil wetting and leaching on [K.sub.s] reduction under various saline and sodic conditions, which are obtained from columns of disturbed soil, larger ranges of the initial aggregate sizes of the soil packed in the columns need to be studied, and more details of this initial aggregate size distribution should be reported.

Summary and conclusions

In the clay soil with large aggregates (2-4 mm), aggregate slaking led to a significant decrease of its [K.sub.s] values when the soil underwent fast wetting. In contrast, differing wetting rates had no significant effect on the [K.sub.s] values of the clay soil with small aggregates (<1 mm). In this case, the differential swelling and entrapment of air during fast wetting were probably minor factors, because of the very small size of the aggregates.

The [K.sub.s] values of the clay soil with high ESP (10.6) were significant lower than with low ESP (4.3) during soil leaching with saline water (EC >4.7 dS/m). The smaller [K.sub.s] values of the higher ESP samples resulted mainly from an increase in clay swelling caused by the greater soil sodicity. This effect of ESP on clay swelling and reduction of the [K.sub.s] was more pronounced in the soil with small aggregates than in that with large ones. Soil swelling decreases mainly the volume of the inter-aggregate pores. Therefore, in the case of the soil with small aggregates, in which the inter-aggregate pores were already small, the decrease in the volume of these pores, which resulted from soil swelling, decreased markedly the [K.sub.s] values. In contrast, in the soil with the large aggregates, although clay swelling decreased the volume of the inter-aggregate pores, they still remained large, so that the decrease in [K.sub.s] was less pronounced.

When clay soil with aggregates of either size range and with either ESP value was leached with deionised water, the [K.sub.s] values decreased significantly. This decrease in the [K.sub.s] was due to clay dispersion and soil swelling as the electrolyte concentration in the soil solution decreased.

In the loamy sand soil, the relatively small degree of aggregate slaking and clay swelling, and the greater number of pores of large size which were big enough to allow the dispersed particles to move downwards with the percolating water without plugging the pores, prevented the occurrence of any significant and consistent effects of the wetting rates, increased ESP, and decreased the electrolyte in the soil solution on the [K.sub.s] during the soil leaching.

The initial aggregate size in soil is an important parameter, and that it affects the [K.sub.s] even in soils that are relatively prone to structural degradation. Therefore, measuring the effects of structural degradation on [K.sub.s] by using disturbed soil samples with an initial aggregate size of <2 mm alone could lead to erroneous conclusions, since the effects of slaking, swelling, and dispersion processes on [K.sub.s] reduction could be affected by the initial aggregate size distribution in the soil. Moreover, these effects of initial aggregate size distribution on [K.sub.s] reduction could be altered in soils with differing textures.

The effects of aggregate size distribution and wetting rate on soil structure degradation and [K.sub.s] reduction could also have practical agronomic impacts. The size of the aggregates in the field could be controlled by cultivation, and the wetting rate of the soil in irrigated fields could be changed by altering irrigation irrigation, in agriculture, artificial watering of the land. Although used chiefly in regions with annual rainfall of less than 20 in. (51 cm), it is also used in wetter areas to grow certain crops, e.g., rice.  management and methods. Preparation of a proper aggregate distribution according to the soil texture in the field by means of cultivation and the use of drip or mini-sprinkler irrigation systems, which maintain slow wetting rates, could minimise soil structural degradation and the reduction of the [K.sub.s] of sensitive soils.

DOI (Digital Object Identifier) A method of applying a persistent name to documents, publications and other resources on the Internet rather than using a URL, which can change over time. : 10.1071/SR09009

Manuscript received 9 January 2009, accepted 8 July 2009

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M. Ben-Hur (A,E), G. Yolcu (B), H. Uysal (C), M. Lado (D), and A. Paz (D)

(A) Institute of Soil, Water and Environmental Sciences, the Volcani Center, Agricultural Research Organization, PO Box 6, Bet Dagan 50250, Israel.

(B) Menemen Research Institute of Rural Services, Menemen, Izmir, Turkey.

(C) Faculty of Agriculture, University of EGE EGE European Group on Ethics (in Science and New Technologies)
EGE Enhanced Greenhouse Effect
EGE Vail/Eagle, CO, USA - Eagle County Regional (Airport Code)
EGE Economic Globalization and the Environment
, Izmir, Turkey.

(D) Faculty of Sciences, University of A Coruna, A Zapateira s/n, 15071 A Coruna, Spain.

(E) Corresponding author. Email: meni@volcani.agri.gov.il

Contribution from the Soil Science Area of the University of La Coruna, Spain.
Table 1. Exchangeable sodium percentage (ESP), mechanical composition,
organic matter (OM, g/kg) and CaC[O.sub.3] contents (g/kg), cation
exchange capacity (CEC, cmol/kg), pH, electrical conductivity (EC,
dS/m), and sodium adsorption ratio (SAR) of the saturated soil paste
extracts of the studied soils


Soil         ESP    Mechanical composition   OM   CaC[O.sub.3]
                    (g/kg)

                    Clay   Silt   Sand

Clay soil     4.3    524    209    267       12       166
             10.6    566    200    234       11        86
Loamy sand    4.6    125     17    858        2         4
              9.3    125     17    858        2         4

Soil         CEC    Saturated soil paste
                    extract

                     pH    EC   SAR

Clay soil    44.9   7.7   1.2   3.7
             44.8   7.8   4.0   9.8
Loamy sand    8.8   7.7   1.1   5.5
              8.7   7.9   1.1   8.3

Table 2. Bulk density and total porosity of the 2 studied soils with
low (4.3 or 4.6) and high (10.6 or 9.3) exchangeable sodium
percentages (ESP) and aggregate sizes of <1 or 2-4 mm, as packed in
the leaching columns

Values are means-k standard deviation

Soils        Low ESP
             <1 mm                2-4 mm

                  Bulk density (Mg/[m.sup.3])

Clay soil    1.23 [+ or -] 0.02   1.13 [+ or -] 0.01
Loamy sand   1.4 [+ or -] 0.02    1.05 [+ or -] 0.02

                       Total porosity (%)

Clay soil    53.6 [+ or -] 0.87   57.4 [+ or -] 0.51
Loamy sand   47.2 [+ or -] 0.67   60.4 [+ or -] 1.15

Soils        High ESP
             <1 mm                2-4 mm

                  Bulk density (Mg/[m.sup.3])

Clay soil    1.32 [+ or -] 0.01   1.15 [+ or -] 0.02
Loamy sand   1.38 [+ or -] 0.01   1.08 [+ or -] 0.01

                       Total porosity (%)

Clay soil    50.2 [+ or -] 0.38   56.6 [+ or -] 0.98
Loamy sand   47.9 [+ or -] 0.35   59.3 [+ or -] 0.55

Table 3. Clay dispersion values (%) and electrical conductivities (EC,
dS/m) in the deionised water suspensions of the 2 studied soils with
low (4.3 or 4.6) and high (10.6 or 9.3) exchangeable sodium
percentages (ESPs)

For each parameter, values followed by different letters arc
significantly different at P = 0.05

Soils             Low ESP               High ESP

             Dispersion     EC     Dispersion     EC
               value                 value

Clay soil      97.0a      0.055b     81.8b      0.072a
Loamy sand     70.9b      0.011d     46.2c      0.030c
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Author:Ben-Hur, M.; Yolcu, G.; Uysal, H.; Lado, M.; Paz, A.
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:7ISRA
Date:Nov 1, 2009
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