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Hydraulic conductivity of a swelling clay in relation to irrigation water quality
CATENA
Vol. 12, 121-127
Bmunschweig 1985
HYDRAULIC CONDUCTIVITY OF A SWELLING CLAY IN RELATION TO IRRIGATION WATER QUALITY R.K. Gupta and S.K. Verma, Indore
SUMMARY The effect of soil water quality on the unsaturated hydraulic conductivity of a clay soil in the 0.18-0.30 g/g moisture content range was evaluated. The ca~i0n concentration and SAR of the soil wlried from 0.5 me/l to 500 me/l and 5 to 35 (me/l) -l'~, respectively. The relative hydraulic conductivity changes ascribed to solution salinity level were independent of soil moisture content but highly dependent of SAR and cation concentration. The decrease in hydraulic conductivity associated with increase in SAR and decrease in salinity level of the soil solution were attributed mainly to Ex-Na+-induced swelling. Based on the threshold concentration curves, an empirical relationship relating reduction in hydraulic conductivity and soil solution properties has been established for Wertisols having smectite clay mineralogy. The pore structural stability model proposed by CASS & SUMNER (1982) for saturated systems was applied successfully to an unsaturated system.
1. INTRODUCTION The hydraulic conductivity of a soil fundamentaly depends on pore size distribution and the degree of saturation. In a swelling type clay soil the pore size distribution is considerably altered by changes in the degree of saturation (GUPTA& VERMA 1984) and by the susceptibility of the solid matrics to deform during wetting. The latter depends on the quality of irrigation water, content and type of clay mineral and soil solution parameters (QUIRK & SCHOFIELD 1955, McNEAL et al. 1966). Reduction in saturated hydraulic conductivity of sodic soils has been ascribed to initial swelling followed by particle dispersion and translocation at lower concentration (McNEAL et al. 1966). McNEAL et al. (1966) suggested that swelling was the dominant mechanism causing decreased flow in confined soils containing appreciable quantities of smectite. As observed by CASS & SUMNER (1982) the possibility of aggregate failure existed even in confined soil samples. Though voluminous data on hydraulic conductivity reduction in sodic soils in relation to composition of equilibrating solution are reported (McNEAL & COLEMAN 1966, McNEAL 1968, McNEAL etal. 1968, VAN DER PLUYM et al. 1973, FRANKEL et al. 1978) and the mechanism of such reductions postulated, no efforts have been directed to consider hydraulic conductivity reductions in the unsaturated moisture regime. The information on the latter is essential for proper irrigation management of Vetisole and associated soils. This manuscript reports the effect of soil moisture content and solution composition on hydraulic conductivity reductions. Further the mechanism of reduction in saturated hydraulic conductivity has been discussed and the sodium stability model proposed by CASS & SUMNER (1.982) extended to the unsaturated zone. ISSN 0341 - 8162 c~ Copgright lg85 bg CATENA VERLAG. D- 3302 Cremlingen- Desledt, W. 8ermang
122
GUPTA &. VERMA
loo
0--0.30
g/g
#/
80
,, SAR
"7.0
JAW
,, sAR l o o
/t//~
= s A R Zl.0 * SAR
O
////l
17-O
a SAR
so
/I/
, "-
Z5.0
//1#
7 /
//~)//
,I
~"
,L
I
,
""
l
L
100
;_~ eo so
g 40 u
~
zo
~" -r"
0
-
,oor ~6
8o
60 40 20 0
,. 1
10 Cotton FiB. I:
1OO
Concentration
1000 (me/l)
Relationship between relative hydraulic conductivity (Kr), SAR and total cation concentration
at different moisture contents for montmorillonitic clay soil.
HYDRAULICCONDUCTIVITY:WATERQUALITY 2.
123
MATERIAL AND METHODS
The soil subjected to irrigation with water of varying sodium adsorption ratio (SAR) for a few years was collected from field plots. The soil is a member of fine, montmorillonitic hyperthermic t~lmily of typic chromusterts. The soil (sand 8.2%, silt 37.3%, clay 54.5%, CEC 48 me/100 g soil, organic carbon 0.4% and CaCO3 3.250/0) was ground and passed through 2 mm sieve after oven drying. The SAR, ESP and associated properties of the soil are summarised in table 1. The clay fraction of soil has about 78% smectite, 9% kaolinite and 7% illite (CHATTERJEE & RATHORE 1976).
Tab. 1: PHYSICO-CHEMICAL PROPERTIES OF EXPERIMENTAL SOIL SAR (Soil Extract) Ime/l )- ~-' 7.0 10.0 17.0 21.0 26.0 35.0
SOIL ESP
pH of extract
EC (mS, cm-I) at 2 5 ° C
6.0 10.0 15.0 22.0 37.0 58.0
7.9 8.1 8.1 8.2 8.3 9.3
0.76 1.37 1.80 3.09 6.36 11.61
Saturated hydraulic conductivity (cm/hr) 1.10 0.79 0.39 0.17 Negligible Negligible
MWD Dispersion ratio (mm) (Water dispersible clay/ Total clay) 1.52 1.39 1.08 0.97 0.88 0.23
0.63 0.76 0.83 0.83 0.85 0.85
The unsaturated hydraulic conductivity in the 0.18 to 0.30 g/g moisture content range was computed from soil water diffusivity determined by using BRUCE & KLUTE (1956) method and the differential water capacity function estimated from moisture retention curve. The soil of each SAR was packed uniformly at a bulk density of 1.25 ( + 0.05) gcm -3 in 38 m m diameter cylinders, t~abricated by holding together 1 cm thick acrylic segments. Each solution was allowed to infiltrate the horizontally positioned soil solumn at a 2 cm inlet suction for about 6 to 10 hours depending on soil and electrolyte concentration of permeating solution. Details ofthe procedure have been reported elsewhere (GUPTA& VERMA 1984). The mixed ion solutions of CaCl2 and NaC1 with ionic strength of 5, 50, 100, 200, 300 and 500 me/1 and SAR7, 10, 17, 21, 25, and 35 were used as permeating solutions. The solution SAR were choosen to represent SAR of the saturation extract of the soil used, so as to maintain chemical equilibrium between soil and the solution.
3.
RESULTS AND DISCUSSION
Fig. 1 depicts the changes in the relative unsaturated hydraulic conductivity (Kr) ascribed to sodium adsorption ratio (SAR) and salinity level of soil water. While increase in SAR tends to reduce Kr, the effect is counteracted by the presence of solute in the infiltrating solution. Thus Kr was reduced at higher electrolyte levels in the soil of higher ESP (corresponding to higher solution SAR) as also observed by FRANKEL et al. (1978) in case of saturated hydraulic conductivity. The adverse effect of SAR under unsaturated conditions resulted from alteration in pore size distribution brought about by structural deterioration.
124
GUPTA & VERMA
Asharp decrease in Kr with increase in SAR at lower electrolyte level was because of higher swelling of clay platelets at the expense of macro pores which is a characteristics ofsmectitic clay soils. The pattern of relative hydraulic conductivity changes ascribed to the solution composition was similar throughout the moisture content range studied, although the absolute hydraulic conductivity (K) decreased as much as 10 fold with decrease in soil moisture content. At soil SAP, greater than 17, the relative hydraulic conductivity increased almost in proportion with salinity levels of the soil solution. Three to fourfold increase in K resulting from a raise in salinity level of the solution from 5 to about 450 me/l even at low SAR suggests that the main mechanism involved in K reduction was microscopic swelling enhanced by adsorbed Na ÷ (GUPTA & VERMA 1984, MUSTAFA & HAMID 1977, McNEAL 1968). In a partially confined clay soil, the macroscopic swelling would tend to alter the pore size distribution in favour of micro pores which in turn reduced K; whereas the chances of dispersion of even deflocculated clay particle would be meagre. Regardless of soil moisture content, the effect of SAR and salinity of the solution on unsaturated hydraulic conductivity was similar as also was the percentage reduction in K. The data revealed that the effect of soil moisture content on swelling in 0.18-0.30 g/g range was considerably masked by the SAR effect. The relative unsaturated hydraulic conductivity data (Kl00 being for the most concentrated solution) at varying solution concentration (C) and SAR observed in the present case (Fig. 2) conform to: Logio [(Kloo - Kr)/Kr] = a - bC ...
(1)
regardless ofsoil moisture content. Where 'a' and 'b' are constants whose values depend on SAR. The data obtained are substantiated by previous reports on relatively coarse structured soils (SHA1NBERG et al. 1981, MUSTAFA & HAMID 1977). The data were used to relate SAR and salinity level ofthe solution to associated reduction in unsaturated hydraulic conductivity. Regardless of the variations in soil moisture content employed in the study, a linear relationship (Fig. 3) was observed between critical SAR and critical salinity level, C, required for a specified reduction in K. That is: SAR c = a c + b c C
(2)
Where the parameters b c (slope of threshold concentration curve) depends on the acceptable reduction in K, "ac' was estimated to be about 5 for the soil used. A plot ofbc against A K has been shown in Fig. 4. The linearity of the relationship between b c and A K for the present case is expressed as below: bc =
- 0 . 0 2 + 8.7 x 1 0 - 4 A K
(3)
A combination ofequation (2) and (3) leads to a relationship among a K, SARand Cas below: SAR = 5 + (-0.02 + 8.7 x 1 0 - 4 A K ) C
(4)
The parameter b s characterises the soil for its structural stability. Its value in the present case was observed to be 8.7 x 10-4 (me~l) -1/2. The relationship r expressed in eq. (4) furnishes a guidline for quantitative assessment of
H YDRAU LIC CON DUCTIVITY: WATER QUALITY
i----n
125
i[
i
o
171
O ..J
.1 -2
0
100
ZOO, 300 400 Concentrations (me / i )
500
Fig. 2: Transformation of the curves of Fig. 1 at 0.30 g/g moisture content to linear form, with Log ((100 - Kr)/Krl plotted as a function of(C).
40
A. K (%) -8O
30 ~N ¢=1
E
-65 ZO -50
nc
<~ o~
10
0
-30 -15
100
300
500
Fig. 3: Threshold concentration curves determined from the curves of Fig. I for a montmorillonitic clay soil.
the critical composition of irrigation water for permissible reduction in hydraulic conductivity of Vertisols and associated soils which are inherently susceptible to water-logging and subsequent development of salinity and alkalinity.
126
GUPTA & VERMA
0-1 > t..
u
I
I
~u ~.O
n
•,
(3. _o tit
•
o
zb
~0
Reduce|on in Hydraulic
6b
80
100
C o n d u c t i v i t y AK(Ol.)
Fig. 4: The relationship between the slope ofthreshold concentration curves ofFig. 3 and the corresponding hydraulic conductivity reduction.
4.
CONCLUSIONS
The empirical relationship expressed in eq. (4) enables one to predict reduction in K o f a montmorillonitic clay soil even under unsaturated conditions, if SAR and salinity levels of soil solution are specified. Further, the relationship between slope of threshold concentration and percentage reduction in hydraulic conductivity shown in Fig. 4 characterizes the soil for its susceptibility (mostly swelling) to irigation water leading to a decline in K. Since the relationship is linear the slope, b s constitutes a parameter to serve as an index of structural susceptibility of soils as demonstrated by CASS & SUMNER (1982), for a variety of soils. The quantitative knowledge of the effect of irrigation water on soil condition under a given set of environmental and drainage conditions could be used to assess irrigation water quality in relation to structural stability.
BIBLIOGRAPHY CASS, A. & SUMNER, M.E. (1982): Soil pore structural stability and irrigation water quality. I. Sodium stability model. Soil Sci. Soc. Amer. J. 46, 503-506. CASS, A. & SUMNER, M.E. (1982): Soil pore structural stability and irrigation water quality. !I. Sodium stability data. Soil Sci. Soc. Amer. J. 46, 507-512. CHATTERJEE, R.K. & RATHORE, G.S. (1976): Clay mineral composition, genesis and classification of some soils from basalts in Madhya Pradesh. J. Indian Soc. Soil Sci. 24, 144-157. FRANKEL, H., GOERTZEN, J.O. & RHOADES, J.D. (1978): Effects of clay type and content, exchangeable sodium percentage and electrolyte concentration on clay dispersion and soil hydraulic conductivity. Soil Sci. Soc. Amer. J. 42, 32-39. GUPTA, RAM K. & VERMA, S.K. (1984): Influence of soil ESP and electrolyte concentration of permeating water on hydraulic properties ofa vertisol. Z. Pflanzenern/ihrung Bodenk. 147, 77-84. McNEAL, B.L. (1968): Prediction of the effect of mixed salts solutions on soil hydraulic conductivity. Soil Sci. Soc. Amer. Proc. 32, 190-193. McNEAL, B.L. & COLEMAN, N.T. ( 1966): Effect of solution composition and soil hydraulic conductivity. Soil Sci. Soc. Amer. Proc. 30, 308-312. McNEAL, B.L., LAYFIELD, D.A., NORVELL, W.A. & RHOADES, J.D. (1968): Factors influencing hydraulic conductivity in the presence of mixed salt solutions. Soil Sci. Soc. Amer. Proc. 32, 187-190.
HYDRAULIC CONDUCTIVITY: WATERQUALITY
127
McNEAL, B.L., NARVELL, W.A. & COLEMAN, N.T. (1966): Effect of solution composition on the swelling ofextracted soil clays. Soil Sci. Soc. Amer. Proc. 30, 313-317. MUSTAFA, M.A. & HAMID, K.S. (1977): Comparision of two models for predicting of the relative hydraulic conductivity of salt affected swelling soils. Soil Sci. 123, 150-154. QUIRK, J.P. & SCHOFIELD, R.K. (1955): The effect of electrolyte concentration on soil permeability. J. Soil Sci. 6, 163-178. SHAINBERG, I., RHOADES, J.D. & PRATHER, ILJ. (1981): Effect of low electrolyteconcentration on clay dispersion and hydraulic conductivity ofa sodic soil. Soil Sci. Soc. Amer..J. 45, 273-277. VAN DER PLUYM, H.S.A., TOOGOOD, J.A. & MILNE, 1LA. (1973): Reclamation of a saline sodic soil by the high salt water dilution method. Can. J. Soil Sci. 54, 473-480.
Address of authors: Ram K. Gupta and S.K. Verma, Co-ordinated Research Project on Management of Salt Affected Soils, J.N. Agricultural University, Campus - lndore - 452001 (M.P.), India
Vol. 12, 121-127
Bmunschweig 1985
HYDRAULIC CONDUCTIVITY OF A SWELLING CLAY IN RELATION TO IRRIGATION WATER QUALITY R.K. Gupta and S.K. Verma, Indore
SUMMARY The effect of soil water quality on the unsaturated hydraulic conductivity of a clay soil in the 0.18-0.30 g/g moisture content range was evaluated. The ca~i0n concentration and SAR of the soil wlried from 0.5 me/l to 500 me/l and 5 to 35 (me/l) -l'~, respectively. The relative hydraulic conductivity changes ascribed to solution salinity level were independent of soil moisture content but highly dependent of SAR and cation concentration. The decrease in hydraulic conductivity associated with increase in SAR and decrease in salinity level of the soil solution were attributed mainly to Ex-Na+-induced swelling. Based on the threshold concentration curves, an empirical relationship relating reduction in hydraulic conductivity and soil solution properties has been established for Wertisols having smectite clay mineralogy. The pore structural stability model proposed by CASS & SUMNER (1982) for saturated systems was applied successfully to an unsaturated system.
1. INTRODUCTION The hydraulic conductivity of a soil fundamentaly depends on pore size distribution and the degree of saturation. In a swelling type clay soil the pore size distribution is considerably altered by changes in the degree of saturation (GUPTA& VERMA 1984) and by the susceptibility of the solid matrics to deform during wetting. The latter depends on the quality of irrigation water, content and type of clay mineral and soil solution parameters (QUIRK & SCHOFIELD 1955, McNEAL et al. 1966). Reduction in saturated hydraulic conductivity of sodic soils has been ascribed to initial swelling followed by particle dispersion and translocation at lower concentration (McNEAL et al. 1966). McNEAL et al. (1966) suggested that swelling was the dominant mechanism causing decreased flow in confined soils containing appreciable quantities of smectite. As observed by CASS & SUMNER (1982) the possibility of aggregate failure existed even in confined soil samples. Though voluminous data on hydraulic conductivity reduction in sodic soils in relation to composition of equilibrating solution are reported (McNEAL & COLEMAN 1966, McNEAL 1968, McNEAL etal. 1968, VAN DER PLUYM et al. 1973, FRANKEL et al. 1978) and the mechanism of such reductions postulated, no efforts have been directed to consider hydraulic conductivity reductions in the unsaturated moisture regime. The information on the latter is essential for proper irrigation management of Vetisole and associated soils. This manuscript reports the effect of soil moisture content and solution composition on hydraulic conductivity reductions. Further the mechanism of reduction in saturated hydraulic conductivity has been discussed and the sodium stability model proposed by CASS & SUMNER (1.982) extended to the unsaturated zone. ISSN 0341 - 8162 c~ Copgright lg85 bg CATENA VERLAG. D- 3302 Cremlingen- Desledt, W. 8ermang
122
GUPTA &. VERMA
loo
0--0.30
g/g
#/
80
,, SAR
"7.0
JAW
,, sAR l o o
/t//~
= s A R Zl.0 * SAR
O
////l
17-O
a SAR
so
/I/
, "-
Z5.0
//1#
7 /
//~)//
,I
~"
,L
I
,
""
l
L
100
;_~ eo so
g 40 u
~
zo
~" -r"
0
-
,oor ~6
8o
60 40 20 0
,. 1
10 Cotton FiB. I:
1OO
Concentration
1000 (me/l)
Relationship between relative hydraulic conductivity (Kr), SAR and total cation concentration
at different moisture contents for montmorillonitic clay soil.
HYDRAULICCONDUCTIVITY:WATERQUALITY 2.
123
MATERIAL AND METHODS
The soil subjected to irrigation with water of varying sodium adsorption ratio (SAR) for a few years was collected from field plots. The soil is a member of fine, montmorillonitic hyperthermic t~lmily of typic chromusterts. The soil (sand 8.2%, silt 37.3%, clay 54.5%, CEC 48 me/100 g soil, organic carbon 0.4% and CaCO3 3.250/0) was ground and passed through 2 mm sieve after oven drying. The SAR, ESP and associated properties of the soil are summarised in table 1. The clay fraction of soil has about 78% smectite, 9% kaolinite and 7% illite (CHATTERJEE & RATHORE 1976).
Tab. 1: PHYSICO-CHEMICAL PROPERTIES OF EXPERIMENTAL SOIL SAR (Soil Extract) Ime/l )- ~-' 7.0 10.0 17.0 21.0 26.0 35.0
SOIL ESP
pH of extract
EC (mS, cm-I) at 2 5 ° C
6.0 10.0 15.0 22.0 37.0 58.0
7.9 8.1 8.1 8.2 8.3 9.3
0.76 1.37 1.80 3.09 6.36 11.61
Saturated hydraulic conductivity (cm/hr) 1.10 0.79 0.39 0.17 Negligible Negligible
MWD Dispersion ratio (mm) (Water dispersible clay/ Total clay) 1.52 1.39 1.08 0.97 0.88 0.23
0.63 0.76 0.83 0.83 0.85 0.85
The unsaturated hydraulic conductivity in the 0.18 to 0.30 g/g moisture content range was computed from soil water diffusivity determined by using BRUCE & KLUTE (1956) method and the differential water capacity function estimated from moisture retention curve. The soil of each SAR was packed uniformly at a bulk density of 1.25 ( + 0.05) gcm -3 in 38 m m diameter cylinders, t~abricated by holding together 1 cm thick acrylic segments. Each solution was allowed to infiltrate the horizontally positioned soil solumn at a 2 cm inlet suction for about 6 to 10 hours depending on soil and electrolyte concentration of permeating solution. Details ofthe procedure have been reported elsewhere (GUPTA& VERMA 1984). The mixed ion solutions of CaCl2 and NaC1 with ionic strength of 5, 50, 100, 200, 300 and 500 me/1 and SAR7, 10, 17, 21, 25, and 35 were used as permeating solutions. The solution SAR were choosen to represent SAR of the saturation extract of the soil used, so as to maintain chemical equilibrium between soil and the solution.
3.
RESULTS AND DISCUSSION
Fig. 1 depicts the changes in the relative unsaturated hydraulic conductivity (Kr) ascribed to sodium adsorption ratio (SAR) and salinity level of soil water. While increase in SAR tends to reduce Kr, the effect is counteracted by the presence of solute in the infiltrating solution. Thus Kr was reduced at higher electrolyte levels in the soil of higher ESP (corresponding to higher solution SAR) as also observed by FRANKEL et al. (1978) in case of saturated hydraulic conductivity. The adverse effect of SAR under unsaturated conditions resulted from alteration in pore size distribution brought about by structural deterioration.
124
GUPTA & VERMA
Asharp decrease in Kr with increase in SAR at lower electrolyte level was because of higher swelling of clay platelets at the expense of macro pores which is a characteristics ofsmectitic clay soils. The pattern of relative hydraulic conductivity changes ascribed to the solution composition was similar throughout the moisture content range studied, although the absolute hydraulic conductivity (K) decreased as much as 10 fold with decrease in soil moisture content. At soil SAP, greater than 17, the relative hydraulic conductivity increased almost in proportion with salinity levels of the soil solution. Three to fourfold increase in K resulting from a raise in salinity level of the solution from 5 to about 450 me/l even at low SAR suggests that the main mechanism involved in K reduction was microscopic swelling enhanced by adsorbed Na ÷ (GUPTA & VERMA 1984, MUSTAFA & HAMID 1977, McNEAL 1968). In a partially confined clay soil, the macroscopic swelling would tend to alter the pore size distribution in favour of micro pores which in turn reduced K; whereas the chances of dispersion of even deflocculated clay particle would be meagre. Regardless of soil moisture content, the effect of SAR and salinity of the solution on unsaturated hydraulic conductivity was similar as also was the percentage reduction in K. The data revealed that the effect of soil moisture content on swelling in 0.18-0.30 g/g range was considerably masked by the SAR effect. The relative unsaturated hydraulic conductivity data (Kl00 being for the most concentrated solution) at varying solution concentration (C) and SAR observed in the present case (Fig. 2) conform to: Logio [(Kloo - Kr)/Kr] = a - bC ...
(1)
regardless ofsoil moisture content. Where 'a' and 'b' are constants whose values depend on SAR. The data obtained are substantiated by previous reports on relatively coarse structured soils (SHA1NBERG et al. 1981, MUSTAFA & HAMID 1977). The data were used to relate SAR and salinity level ofthe solution to associated reduction in unsaturated hydraulic conductivity. Regardless of the variations in soil moisture content employed in the study, a linear relationship (Fig. 3) was observed between critical SAR and critical salinity level, C, required for a specified reduction in K. That is: SAR c = a c + b c C
(2)
Where the parameters b c (slope of threshold concentration curve) depends on the acceptable reduction in K, "ac' was estimated to be about 5 for the soil used. A plot ofbc against A K has been shown in Fig. 4. The linearity of the relationship between b c and A K for the present case is expressed as below: bc =
- 0 . 0 2 + 8.7 x 1 0 - 4 A K
(3)
A combination ofequation (2) and (3) leads to a relationship among a K, SARand Cas below: SAR = 5 + (-0.02 + 8.7 x 1 0 - 4 A K ) C
(4)
The parameter b s characterises the soil for its structural stability. Its value in the present case was observed to be 8.7 x 10-4 (me~l) -1/2. The relationship r expressed in eq. (4) furnishes a guidline for quantitative assessment of
H YDRAU LIC CON DUCTIVITY: WATER QUALITY
i----n
125
i[
i
o
171
O ..J
.1 -2
0
100
ZOO, 300 400 Concentrations (me / i )
500
Fig. 2: Transformation of the curves of Fig. 1 at 0.30 g/g moisture content to linear form, with Log ((100 - Kr)/Krl plotted as a function of(C).
40
A. K (%) -8O
30 ~N ¢=1
E
-65 ZO -50
nc
<~ o~
10
0
-30 -15
100
300
500
Fig. 3: Threshold concentration curves determined from the curves of Fig. I for a montmorillonitic clay soil.
the critical composition of irrigation water for permissible reduction in hydraulic conductivity of Vertisols and associated soils which are inherently susceptible to water-logging and subsequent development of salinity and alkalinity.
126
GUPTA & VERMA
0-1 > t..
u
I
I
~u ~.O
n
•,
(3. _o tit
•
o
zb
~0
Reduce|on in Hydraulic
6b
80
100
C o n d u c t i v i t y AK(Ol.)
Fig. 4: The relationship between the slope ofthreshold concentration curves ofFig. 3 and the corresponding hydraulic conductivity reduction.
4.
CONCLUSIONS
The empirical relationship expressed in eq. (4) enables one to predict reduction in K o f a montmorillonitic clay soil even under unsaturated conditions, if SAR and salinity levels of soil solution are specified. Further, the relationship between slope of threshold concentration and percentage reduction in hydraulic conductivity shown in Fig. 4 characterizes the soil for its susceptibility (mostly swelling) to irigation water leading to a decline in K. Since the relationship is linear the slope, b s constitutes a parameter to serve as an index of structural susceptibility of soils as demonstrated by CASS & SUMNER (1982), for a variety of soils. The quantitative knowledge of the effect of irrigation water on soil condition under a given set of environmental and drainage conditions could be used to assess irrigation water quality in relation to structural stability.
BIBLIOGRAPHY CASS, A. & SUMNER, M.E. (1982): Soil pore structural stability and irrigation water quality. I. Sodium stability model. Soil Sci. Soc. Amer. J. 46, 503-506. CASS, A. & SUMNER, M.E. (1982): Soil pore structural stability and irrigation water quality. !I. Sodium stability data. Soil Sci. Soc. Amer. J. 46, 507-512. CHATTERJEE, R.K. & RATHORE, G.S. (1976): Clay mineral composition, genesis and classification of some soils from basalts in Madhya Pradesh. J. Indian Soc. Soil Sci. 24, 144-157. FRANKEL, H., GOERTZEN, J.O. & RHOADES, J.D. (1978): Effects of clay type and content, exchangeable sodium percentage and electrolyte concentration on clay dispersion and soil hydraulic conductivity. Soil Sci. Soc. Amer. J. 42, 32-39. GUPTA, RAM K. & VERMA, S.K. (1984): Influence of soil ESP and electrolyte concentration of permeating water on hydraulic properties ofa vertisol. Z. Pflanzenern/ihrung Bodenk. 147, 77-84. McNEAL, B.L. (1968): Prediction of the effect of mixed salts solutions on soil hydraulic conductivity. Soil Sci. Soc. Amer. Proc. 32, 190-193. McNEAL, B.L. & COLEMAN, N.T. ( 1966): Effect of solution composition and soil hydraulic conductivity. Soil Sci. Soc. Amer. Proc. 30, 308-312. McNEAL, B.L., LAYFIELD, D.A., NORVELL, W.A. & RHOADES, J.D. (1968): Factors influencing hydraulic conductivity in the presence of mixed salt solutions. Soil Sci. Soc. Amer. Proc. 32, 187-190.
HYDRAULIC CONDUCTIVITY: WATERQUALITY
127
McNEAL, B.L., NARVELL, W.A. & COLEMAN, N.T. (1966): Effect of solution composition on the swelling ofextracted soil clays. Soil Sci. Soc. Amer. Proc. 30, 313-317. MUSTAFA, M.A. & HAMID, K.S. (1977): Comparision of two models for predicting of the relative hydraulic conductivity of salt affected swelling soils. Soil Sci. 123, 150-154. QUIRK, J.P. & SCHOFIELD, R.K. (1955): The effect of electrolyte concentration on soil permeability. J. Soil Sci. 6, 163-178. SHAINBERG, I., RHOADES, J.D. & PRATHER, ILJ. (1981): Effect of low electrolyteconcentration on clay dispersion and hydraulic conductivity ofa sodic soil. Soil Sci. Soc. Amer..J. 45, 273-277. VAN DER PLUYM, H.S.A., TOOGOOD, J.A. & MILNE, 1LA. (1973): Reclamation of a saline sodic soil by the high salt water dilution method. Can. J. Soil Sci. 54, 473-480.
Address of authors: Ram K. Gupta and S.K. Verma, Co-ordinated Research Project on Management of Salt Affected Soils, J.N. Agricultural University, Campus - lndore - 452001 (M.P.), India