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DIVISION S-1-SOIL PHYSICS

Hydraulic Gradient and Wetting Rate Effects on the Hydraulic Conductivity of Two Calcium Vertisols

^a Université catholique de Louvain, Unité de Génie Rural, Place Croix du Sud 2, bte 2, B-1348 Louvain-la-Neuve, Belgium
^b Inst. of Soils, Water, and Environmental Sciences, Agricultural Research Organization (ARO), The Volcani Center, P.O. Box 6, Bet Dagan 50-250, Israel

marylene.moutier{at}skynet.be

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Swelling Physical Compression of the... Development of Cohesive Bonds Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Understanding the combined role of intrinsic (e.g., clay content) and extrinsic soil conditions in determining the saturated hydraulic conductivity (K) is a key factor in improving soil and irrigation management. Hydraulic gradient effects on K have been investigated on two Ca vertisols (Chromic Haploxerert) from Yizreel and Kedma, Israel. Samples were packed in columns and subjected to two prewetting rates (4.5 and 70 mm h^-1) and two hydraulic gradients (2.8 and 11.6 for Yizreel; 3.1 and 13.6 for Kedma). Saturated K was determined during leaching with CaCl₂ solutions having total electrolyte concentrations (TEC) of 0.5, 0.01 M Cl^-, and deionized water (DW). The average hydraulic conductivity at the end of the leaching with the 0.5 M solution of the two vertisols increased with a decrease in prewetting rate. The effect of prewetting rate was more pronounced in Yizreel, where the high clay content (70.3% clay) resulted in a more stable structure. Upon leaching with the 0.01 M solution, _r0.01 first decreased and then increased. This increase in _r0.01 was explained by cohesive bond formation, which increased with an increase in soil clay content and with increased proximity between the clay particles. High hydraulic gradient enhanced clay to clay contacts, and a steep increase in _r0.01. Following fast prewetting, the effect of the hydraulic gradient on cohesive bond formation was more pronounced in Yizreel than in Kedma (46.5% clay). Leaching with DW decreased the _rDW, regardless of the prewetting treatment or soil clay content, suggesting that swelling was the governing mechanism in the reduction of _rDW.

Abbreviations: DW, deionized water • EC, electrical conductivity • SAR, Na adsorption ratio • TEC, total electrolyte concentration

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Swelling Physical Compression of the... Development of Cohesive Bonds Materials and methods Results and discussion Summary and conclusions REFERENCES

 
THE SATURATED HYDRAULIC CONDUCTIVITY (K) of soils depends on soil permanent properties (e.g., soil texture, clay mineralogy), soil structure, presence of entrapped air (e.g., Christiansen, 1944; De Backer, 1967; Constantz et al., 1988; Faybishenko, 1995), type of exchangeable cations, and salt concentration of the leaching solution. Saturated hydraulic conductivity tends to decrease with increasing exchangeable Na percentage and decreasing TEC (Quirk and Schofield, 1955). Recently, aggregate stability, infiltration rate, and erosion studies demonstrated that soil structure and its hydraulic properties depended on, in addition to sodicity and water quality, the antecedent water content, aging, and prewetting rate (Kemper and Rosenau, 1984; Shainberg et al., 1996; Levy et al., 1997).

The prewetting rate influences aggregate stability (Le Bissonnais, 1988, 1989; Levy et al., 1997) and, hence, K. Whereas slow prewetting minimizes degradation of the original structure of the soil, fast prewetting disintegrates the aggregates and deteriorates the hydraulic properties of soils (Kemper and Koch, 1966). Aggregate breakdown (slaking) results from the development of internal pressures that cause them to explode (Kemper and Rosenau, 1984). These pressures can arise from a differential hydration and swelling of the clay fraction (Le Bissonnais, 1989) or the compression of occluded air in the capillary pores (Panabokke and Quirk, 1957; Boiffin, 1984). Murray and Quirk (1990) suggested that aggregate slaking was due to both the weakening of the cohesive interparticle binding forces by wetting and the appearance of repulsive interparticle forces. The presence of organic matter and inorganic cementing agents such as Fe oxides will contribute to the stability of aggregates. The extent of slaking and the relative importance of each process controlling it were found to depend on TEC, Na adsorption ratio (SAR) (Abu-Sharar et al., 1987), and clay content (Le Bissonnais, 1988).

Hydraulic properties are also affected by the presence of entrapped air, the amount of which depends on the method used for the initial saturation (Faybishenko, 1995). Following an upward initial saturation, which occurs in soils during the rise of the groundwater table, the volume of entrapped air is <5% and mobile air is almost absent. Upon wetting a dry soil from the top, the volume of entrapped air is large and it blocks a significant volume of the water-conducting pores. As entrapped air dissolves, K increases. Increases of K by a factor of 5 to 10 were reported by Constantz et al. (1988), whereas Christiansen (1944) determined that the relative permeability increased by 2 to 40 times. It should be noted that when soil columns are wetted from below, the volume of entrapped air is small and most of it is immobile; changes in K upon air dissolution are therefore insignificant (Faybishenko, 1995).

An increase of aggregate mechanical strength with time has been reported by several authors (e.g., Blake and Gilman, 1970; Singer et al., 1992; Shainberg et al., 1996). Blake and Gilman (1970) observed significant thixotropic changes in artificial aggregates (Webster clay loam [fine-loamy, mixed, superactive, mesic Typic Endoaquoll] with 34.9% clay) within 20 to 30 h. Significant development of cohesive forces in aggregates of a vertisol (46.5% clay) were reported during 24 h of aging (Shainberg et al., 1996). The development of cohesive bonds has been recently inferred from K measurements in a smectitic vertisol (Moutier et al., 1998).

The hydraulic conductivity of a disturbed vertisol (A horizon, Chromic Haploxerert) wetted by capillary rise was shown to be affected by aging and by the hydraulic gradient applied to the soil column (Moutier et al., 1998). The relative hydraulic conductivity (K_r) initially decreased before increasing as a function of leaching volume or time. A combination of three mechanisms was suggested to explain the observed changes in saturated K_r (Moutier et al., 1998):

	Swelling

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Swelling of the quasi-crystals (reduction in the intraaggregate porosity) was offered to explain the decrease of K (Blackmore and Miller, 1961) when a more dilute solution replaced a more concentrated one.

	Physical Compression of the Clay Fabric

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Kemper et al. (1972) observed the physical compression of the clay fabric at the outflow side of a clay column. This compression of clay resulted in lower K values in the outflow side of the column, and thus, lower average K values for the whole column. A similar observation was made in our laboratory in a leaching experiment on Ca-saturated Yizreel soil samples packed in columns equipped with piezometers (I. Shainberg, 1997, unpublished data). It was noted that with leaching, the hydraulic gradient in the lower part of the column increased (i.e., a decrease of K). Because water flow in such a column is controlled by the layer with the lowest K (i.e., a bottleneck for the water flow), the average K decreased. It should be noted that the decrease in K was not accompanied by a measurable change in the soil column length. The degree of compression increased with increasing hydraulic gradient and decreasing SARs (Moutier et al., 1998), as greater repulsion forces develop with higher SAR values (Kemper et al., 1972).

	Development of Cohesive Bonds

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The development of cohesive bonds between clay fabric structural units and microaggregates in close contact (Kemper and Rosenau, 1984; Bresson and Boiffin, 1990; Shainberg et al., 1996; Levy et al., 1997), unlike the former two mechanisms, leads to an increase in the average size of transmission pores, and hence an increase in K_r (Moutier et al., 1998). This concept of "reaggregation" is similar to the "coalescence" idea presented by Bresson and Boiffin (1990). Coalescence is regarded as the welding of initially loose aggregates into larger, stronger structural units by plastic deformation that is induced by the prewetting procedure. Such structural changes in seedbeds of a red-brown earth wetted by capillary rise were also observed by Bresson and Moran (1995).

The rate of cohesive bonds development, as inferred from the increase of K_r with time, was also shown to depend on the hydraulic gradient applied and water quality (Moutier et al., 1998). The higher the hydraulic gradient and the lower the SAR of the leaching solution, the steeper the increase of K_r with time of aging. This suggested that the proximity of the clay particles was essential for the formation of the cohesive forces at the aggregate level (Zubkova, 1998). The "proximity" consideration explains some conflicting results related to cohesive bond development under saturated conditions where water layers surrounding the clay particles may prevent close contacts (Blake and Gilman, 1970; Kemper and Rosenau, 1984; Shainberg et al., 1996).

The combined effect of intrinsic soil properties and extrinsic conditions in the measurement of saturated K should be studied in greater detail. We hypothesize that the hydraulic gradient effect on controlling the saturated hydraulic conductivity might be affected by the initial structural state of the soil sample. Thus, this study aims to investigate the effect of a hydrostatically induced pressure on saturated K, as measured with the constant head method, for two Ca-saturated vertisols of differing clay contents. The contrasting initial structure was achieved by varying the prewetting treatment prior to the K measurements.

	Materials and methods

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Disturbed samples of two smectitic soils (Chromic Haploxerert) were taken from the A horizon (0–250 mm) of two cultivated fields from the Yizreel Valley and the Pleshet Plains (Kedma), Israel. Soil samples were physically and chemically characterized by standard methods (Klute, 1986; Page et al., 1986) (Table 1) . The clay fraction of these soils contained 64% smectite, 13% kaolinite, 4% illite, 12% calcite, and 7% sesquioxides (Banin and Amiel, 1970).

Following leaching with the 0.5 M Cl^- solution, the soil columns were leached with 80 pore volumes (5 L) of dilute CaCl₂ solution (0.01 M Cl^-) or DW. The 0.01 M Cl^- solution was chosen in order to induce some swelling (but no clay dispersion), whereas DW was used to simulate rainwater infiltration. As development of cohesive forces were reported to be time dependent (e.g., Blake and Gilman, 1970; Shainberg et al., 1996), leaching duration was limited to 20 h. However, the treatment was much shorter for soil columns exposed to a high hydraulic gradient and slow prewetting rate as it only took 200 min to infiltrate 80 pore volumes through these columns. In the few cases where steady-state hydraulic conductivities were not maintained, the experimental results were extrapolated to steady-state values, which are presented in Table 2 .

View this table: [in this window] [in a new window]   Table 2 Mean absolute (0.5) and relative hydraulic conductivities (r 0.01, rDW) with their standard deviations () calculated at the end of leaching with CaCl2 solutions (0.5, 0.01 M Cl-) and deionized water. For a given treatment, mean absolute and relative hydraulic conductivities of replicates were calculated at the same pore volume

The column leachates were collected in tubes with a fraction collector and electrical conductivity (EC), pH, and volume of leachate were measured. Results for both vertisols were compared in terms of relative hydraulic conductivity, K_r, defined as the ratio of the saturated hydraulic conductivity calculated for a given solution (K_0.01 or K_DW) to the reference hydraulic conductivity calculated at the end of the conditioning stage (K_0.5). Up to five replicates were carried out and the results were analyzed by one-way analysis of variance using paired Student t test (comparison of two means with unknown but equal variance). A significance level of 0.05 was chosen for the tests.

	Results and discussion

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Effects of Prewetting Rate
Mean reference hydraulic conductivities , mean relative hydraulic conductivities measured at the end of the corresponding leaching period, and their standard deviations are summarized in Table 2. Hydraulic conductivities for the fast prewetting treatment of the Yizreel vertisol were not affected significantly by the hydraulic gradient. As expected, the slow prewetting treatment minimized structure degradation and led to significantly higher values when compared with the fast prewetting treatment. Relative hydraulic conductivities for the slow prewetting treatment for the Yizreel soil samples exposed to a high hydraulic gradient were significantly higher _0.5, than those exposed to a low hydraulic gradient for both leaching solutions.

A comparison of _r0.01 as a function of pore volume (Fig. 1a) and cumulative time (Fig. 1b) is given for Yizreel soils samples exposed to fast and slow prewetting and two different hydraulic gradients. When the 0.5 M Cl^- solution was replaced by the 0.01 M Cl^- solution _r0.01 decreased, and attained a minimum before increasing as a function of pore volume or time. However, the amplitude and the rate of increase of _r0.01 differed among the treatments. Results for Yizreel soil samples exposed to fast prewetting were discussed elsewhere (Moutier et al., 1998) and only the main conclusions are presented. The decrease in _r0.01 was attributed to the combination of two mechanisms, namely swelling upon dilution of the leaching solution and physical compression of the clay fabric at the outflow end of the soil column (Kemper et al., 1972; Moutier et al., 1998). Physical compression increases with an increase in the hydrostatically induced pressure, and hence, with an increase in hydraulic gradient. The initial decrease was followed by an increase in _r0.01. Increases of the hydraulic conductivities were reported by several authors (Christiansen, 1944; Constantz et al., 1988; Faybishenko, 1995). Faybishenko (1995) attributed his observations to air entrapment and its subsequent dissolution. Our soil samples were prewetted by capillary rise, and only immobile entrapped air remained in the column, representing probably <5% of the total porosity (Faybishenko, 1995). This was also confirmed by flushing soil columns with CO₂ prior to the prewetting treatment (data not presented). The small amount of entrapped CO₂ dissolved readily during the soil conditioning phase when the soil was leached with the 0.5 M solution. The trends exhibited by _r0.01 (Fig. 1) were similar for both air and CO₂ flushing prior to soil saturation, with no significant difference between the two treatments. The experiment with and without CO₂ flushing confirmed (i) that the effect of entrapped air following the fast prewetting procedure applied to the soil columns was minimal and (ii) that dissolution of entrapped air was not responsible for the increase in _r0.01. Thus, the development of cohesive bonds between clay fabric structural units and microaggregates in close contact has been proposed to explain the observed increase in _r0.01 (Kemper and Rosenau, 1984; Shainberg et al., 1996; Moutier et al., 1998).

View larger version (25K): [in this window] [in a new window]   Fig. 1 Relative hydraulic conductivities as a function of (a) pore volume and (b) accumulated time for Yizreel leached with 0.01 M Cl- solutions (data for the fast prewetting treatment were taken from Moutier et al., 1998)

The rate of development of interparticle bonding forces, as expressed by the rate of increase in _r0.01, was evaluated by plotting _r0.01 as a function of cumulative time (Fig. 1b). The assumption that prolonged leaching was needed for cohesive forces to develop is not necessarily true (Moutier et al., 1998). A sharp increase in _r0.01 was observed for soil columns exposed to slow prewetting and high hydraulic gradient treatment in spite of the fact that leaching with the 0.01 M Cl^- solution lasted only 200 min (Fig. 1b). It is suggested therefore that the rate of development of bonds between clay particles in close proximity (e.g., when high hydraulic gradient is used) is very fast. Conversely, the formation of bonds in soils exposed to low hydraulic gradient is slow because of the relatively large distance between the clay particles.

The observed interaction between the rate of bond formation, as expressed by the increase in _r0.01, and the proximity of clay particles, as determined by the hydrostatically induced pressure, was verified by studying the effect of aging at zero hydraulic gradient. The experiment at zero gradient mimics experiments at zero compaction such as those recently published by Ben-Hur et al. (1998). Following leaching with a 0.01 M Cl^- solution (five pore volumes), the slow prewetting treatment for Yizreel soil samples were exposed to a zero gradient and allowed to age for 16 h (960 min). The flow was then reestablished and a high hydraulic gradient of 11.8 was applied (50 min). The rate of increase in _r0.01 in Yizreel soil samples exposed to a zero hydraulic gradient for 16 h was much lower than the subsequent rate of increase when high hydraulic gradient was applied (Fig. 2) . _r0.01 increased from 1.14 to 1.6 within 16 h (equivalent to a rate of 0.03 h^-1) and from 1.6 to 2.2 within the subsequent 50 min (equivalent to a rate of 0.72 h^-1) when the high hydraulic gradient was applied. The latter was close to the rate of increase of _r0.01 observed in the slow prewetting, high hydraulic gradient treatment (0.54 h^-1) (Fig. 1b). This experiment supported the hypothesis that the rate of hydraulic conductivity increase and the formation of cohesive bonds strongly depended on the mutual proximity of the clay particles. This is also clearly demonstrated in Fig. 1b. For a given prewetting treatment, the rate of hydraulic conductivity increase was higher in columns exposed to high hydraulic gradient than those exposed to low hydraulic gradient. This is in agreement with previously published data (Moutier et al., 1998) and confirms the fact that the rate of particle bond formation depends on the hydraulic gradient; the higher the hydraulic gradient, the closer the clay particles and the higher the rate of cohesive forces development. This confirms recently published findings that the "closeness" of clay particles is a prerequisite for the manifestation of these cohesive forces at the aggregate level (Zubkova, 1998). It also explains some conflicting evidence that cohesive interparticle bonds might develop under saturated conditions where water layers surrounding the clay particles impede close contacts (Blake and Gilman, 1970; Kemper and Rosenau, 1984; Shainberg et al., 1996).

View larger version (15K): [in this window] [in a new window]   Fig. 2 Relative hydraulic conductivities as a function of accumulated time for Yizreel exposed to slow prewetting and high hydraulic gradient treatment (50 min) following a zero hydraulic gradient treatment (16 h)

View larger version (28K): [in this window] [in a new window]   Fig. 3 Relative hydraulic conductivities as a function of (a) pore volume and (b) accumulated time for Yizreel leached with deionized water (data for the fast prewetting treatment were taken from Moutier et al., 1998)

Effects of Clay Content
Mean reference hydraulic conductivities , mean relative hydraulic conductivities measured at the end of leaching and their standard deviations for the vertisol with a lower clay content (Kedma soil with 46.5% clay) are also summarized in Table 2. The reference values of the Kedma exposed to fast prewetting were 1.07 and 2.11 cm h^-1 under high and low hydraulic gradient treatments, respectively, and were similar to those for Yizreel despite the difference between their clay contents. As expected, slow prewetting resulted in higher _0.5. However, this increase in _0.5 was lower than the corresponding increase for Yizreel (Table 2). The more pronounced effect of prewetting rate on _0.5 of Yizreel was attributed to the more stable structure that its higher clay content imparted to this soil (Kemper and Koch, 1966); the higher the clay content, the more stable the aggregates. Slow prewetting did not cause aggregate breakdown in the Yizreel soil; therefore _0.5 was high. Due to its lower clay content, Kedma soil aggregates were less stable and some slaking probably occurred during the slow prewetting treatment, resulting in the lower _0.5 values. When fast prewetting was applied, aggregate disintegration occurred in both soils, resulting in comparable _0.5 values.

As cohesive forces are more prone to develop in soils with higher clay contents (Singer et al., 1992), the impact of hydraulic gradient on hydraulic conductivity was expected to be less pronounced in Kedma soil samples. All treatments (Fig. 4a) , with the exception of the slow prewetting, low hydraulic gradient treatment, resulted in trends similar to those of Yizreel (Fig. 1a) but were less pronounced. For the Kedma vertisol, both prewetting treatments resulted in final _r0.01 values that were not significantly affected by the hydraulic gradient.

View larger version (25K): [in this window] [in a new window]   Fig. 4 Relative hydraulic conductivities as a function of (a) pore volume and (b) accumulated time for Kedma leached with 0.01 M Cl- solutions

Data in Fig. 4a support our hypothesis that the initial structural state of Kedma affects the impact of hydraulic gradient on relative hydraulic conductivity. The increase in _r0.01 for Kedma soil columns exposed to a low hydraulic gradient was more pronounced following a slow prewetting than following a fast prewetting treatment (Fig. 4a). Due to the lower clay content in Kedma soil samples, fast prewetting resulted in a more extensive breakdown of soil aggregates, and the effects of aging and the increase in _r0.01 were less pronounced. Conversely, under slow prewetting, the increase in _r0.01 was more pronounced because aging was more extensive in partially broken aggregates. These results (Fig. 4a) supported a previously published assumption (Levy et al., 1997) that aging is more effective in partially disintegrated aggregates than in aggregates where extensive slaking has taken place. Thus, the increase of _r0.01 was greatest when Kedma columns were exposed to slow prewetting and a low hydraulic gradient. Unlike the other three treatments, this treatment seemed to be more effective in improving _r0.01 in the Kedma than in the Yizreel soil.

The change in _r0.01 with time for Kedma soil is shown in Fig. 4b. The results confirmed the strong dependence between the rate of development of interparticle binding forces, as inferred from the rate of increase in _r0.01, and the mutual proximity of clay particles within the clay fabric, as determined by the hydraulic gradient applied to the soil column for a given treatment. The rate of bond formation increased with increasing hydraulic gradient. Because of the more stable structure resulting from slow prewetting, a similar hydraulic gradient treatment resulted in a higher rate of cohesive bond development than that which followed fast prewetting.

The effect of clay content on the rate of increase in _r0.01 is demonstrated by comparing the results shown in Fig. 1b and 4b. Following slow prewetting, the rate of development of interparticle binding forces increased in the following order: low hydraulic gradient (Yizreel) < low hydraulic gradient (Kedma) < high hydraulic gradient (Kedma) < high hydraulic gradient (Yizreel). The lowest and the highest rates were both ascribed to the high clay content of Yizreel. High clay content contributed to two opposing tendencies: (i) a limited effect of aging because of the more stable clay fabric and (ii) an enhanced number of potential clay to clay contacts, which increases with increasing hydraulic gradient. Because of its lower clay content, Kedma exhibited intermediate rates of bond formation for similar hydraulic gradient treatments. Following fast prewetting, the effect of hydraulic gradient on the rate of cohesive bond formation was more pronounced in Yizreel (Fig. 1b) than in Kedma because fast prewetting resulted in extensive slaking, which affected the Kedma to a greater extent, and the rate of cohesive bond formation was lower in the latter (Fig. 4b).

When the 0.01 M Cl^- solution was replaced with DW, _rDW of the Kedma decreased irrespective of the prewetting rate or the hydraulic gradient applied to the soil columns (Fig. 5) . The quasi-steady state ECs of the effluent are presented in Fig. 5b. As in the case of the Yizreel soil, the high EC in the Kedma was due to CaCO₃ dissolution and depended on flow rates. Despite similarities in the ECs of the effluents when the two soils were leached with deionized water, the following should be noted:

1. The fast prewetting and high hydraulic gradient treatment resulted in comparable relative hydraulic conductivity values in the two soils (Table 2). Evidently, swelling affected both soils in a similar way. Because conditions for cohesive forces development were not favorable when Kedma was subjected to the fast prewetting, low hydraulic gradient treatment, _rDW decreased to 0.14.
   
2. In the slow prewetting, high hydraulic gradient treatment, the hydraulic conductivity of the Yizreel soil at the end of the leaching with DW was similar to that at the end of the leaching with the 0.01 M Cl^- solution, and more than twice the initial value (Table 2). In the Kedma, _rDW decreased to 0.85. Following the slow prewetting treatment, aggregate breakdown in Yizreel was much less pronounced than in Kedma. During leaching with DW the adverse effect of swelling in this soil was probably offset by the development of cohesive forces, and the net effect was that the size of the water-conducting pores was not affected. Conversely, in the Kedma, in which some aggregate slaking took place during the slow prewetting, swelling of the microaggregates probably reduced the size of the conducting pores, and _rDW decreased.
   
3. In the slow prewetting, low hydraulic gradient treatment, the _rDW of the Yizreel soil decreased to 0.61 (Table 2). Under a low hydraulic gradient, formation of cohesive bonds between clay particles is not as effective; therefore, because of swelling, _rDW decreased. Similar conditions prevailed in the Kedma, and the final _rDW in the two soils reached similar values.
   

View larger version (30K): [in this window] [in a new window]   Fig. 5 Relative hydraulic conductivities as a function of (a) pore volume and (b) accumulated time for Kedma leached with deionized water

	Summary and conclusions

TOP NOTES ABSTRACT INTRODUCTION Swelling Physical Compression of the... Development of Cohesive Bonds Materials and methods Results and discussion Summary and conclusions REFERENCES

	ACKNOWLEDGMENTS

 
The first author is grateful to the Agricultural Research Organization, Bet Dagan, Israel; the Catholic University of Louvain, Louvain-la-Neuve, Belgium; and the Rotary Club of Wavre-Europe, Belgium, in providing her with grants enabling the realization of this work. This study was supported by grant no. TA-MOU-96-CA-16-016 U.S.-Israel Cooperative Development Research Program, Office of the Science Advisor, U.S. Agency for International Development (AID).

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Swelling Physical Compression of the... Development of Cohesive Bonds Materials and methods Results and discussion Summary and conclusions REFERENCES

 
Contribution from the Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50-250, Israel. no 604/99 series.

¹ The total electrolyte concentration (TEC) can be inferred from the EC using the following approximation: TEC (mmol_c L^-1) = 10 EC (dS m^-1).

Received for publication December 8, 1998.

	REFERENCES

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