|
A. Tedeschi (corresponding author). Department of Agronomy Sciences and Plant Genetics, University of Naples, Italy, W. Hamminga. Department of Soil Physical Transport Phenomena, The Winand Staring Centre for Integrated Land, Soil and Water Research, Wageningen, The Netherlands, L. Postiglione, Department of Agronomy Sciences and Plant Genetics, University of Naples, Italy and M. Menenti, Department of Water Management in Arid Zones, The Winand Staring Centre for Integrated Land, Soil and Water Research, Wageningen, The Netherlands |
SUMMARYIn the Mediterranean environment characterized by hot, dry summers, since 1988 soil salinity research has been carried out on a clay loam soil classified as Haplustolls. Irrigation took place with commercial NaCl in solution at five levels: 0 gNaCl l-1; 1.25 gNaCl l-1; 2.5 gNaCl l-1; 5 gNaCl l-1; and 10 gNaCl l-1 on spring-summer vegetable crops. Three irrigation intervals of 2, 5 and 10 days were studied. On the extreme treatments of 0% and 1% of NaCl and with 2- and 10-day irrigation frequency, soil samples were taken in duplicate at depths of 0-10 and 20-30 cm. The water retention, h(q), and the unsaturated conductivity K(h) characteristics were measured for each sample and the parameters of the Van Genuchten functions determined. Significant differences on the h(q) and K(h) curves were observed. The h(q) curve of the 1% treatment in both irrigation frequencies had lower values of q than the 0% treatment at the same pressure head. The K(h) curve at the same pressure head considered for the h(q), also showed a lower value for the 1% treatment than on the 0% treatment. Significant differences were observed between the mean values of most of the Van Genuchten's parameters, particularly between the (0%; 2 days) and (1%; 2 days) treatments.
About 10% of all land is affected by salinity problems. They occur in every continent in different proportions; more frequently in arid and semi-arid areas than in humid and semi-humid areas (Szabolcs, 1994). The inorganic salts, mainly sodium salts, that cause soil salinity, are highly mobile. The salts influence the pedogenetic processes, physical, chemical and biological soil properties.
Soils with a high clay fraction are more readily affected. The clay fraction has the greatest specific surface area and is therefore most active in the physico-chemical processes such as swelling and dispersion (Shainberg and Levy, 1992). The difference between swelling and dispersion processes are important. Swelling of reference clays (Shainberg et al., 1971) in soil (Emerson and Bakker, 1973) is not greatly affected by low exchangeable sodium percentage (ESP) values but increases markedly as the ESP increases above 15. Instead, the dispersion of clay is very sensitive to low levels of sodicity and increases markedly at the low ESP range (Shainberg and Levy, 1992).
Rowell et al. (1969) found that at the same electrolyte concentration and with an increase of ESP the permeability decreases as a result of the increased clay swelling. Frenkel et al. (1978) have shown that clay dispersion and clogging of pores within a soil column reduce hydraulic conductivity. For them, the dispersion and swelling of clays within the soil matrix are interrelated phenomena, and either can reduce soil hydraulic conductivity. The exact levels of exchangeable sodium and electrolyte concentration at which hydraulic conductivity is appreciably reduced vary with mineralogy, clay content and soil bulk density (Frenkel et al., 1978). Shainberg et al. (1981) concluded that when high-quality water is used, an ESP of five can be detrimental to the physical properties of the soil. However, when waters of higher salinity are used, an ESP of 15 is required for damage to the physical properties of soils.
On the other hand, yield of agricultural crops and also horticultural crops is severely affected even at relatively low solute concentrations. In arid and semi-arid zones, water is insufficient and of poor quality. The correct management of this water is important, especially in areas where saline water is a long-term limiting factor to crop production. The saline water has to be applied by the best irrigation strategy to minimize or avoid reductions in crop yield. This requires precise knowledge of soil water and solute flow and, therefore, of soil physical properties, h(q) and K(h). Since these are both modified by solute concentration, development of improved irrigation strategies requires a quantitative evaluation of these modifications.
The objective of this paper is to assess solute-induced modifications of h(q) and K(h) by determining and comparing the values of the Van Genuchten parameters obtained for a range of irrigation strategies.
MATERIALS AND METHODS
Experiment design and soil samples
At the University of Naples in a Mediterranean environment characterized by hot, dry summers, since 1988 a research has been carried out at the experimental field of Torre Lama (province of Salerno) on a clay loam soil (around 30% of clay) classified as Haplustolls in an alluvial environment with a very well developed pedogenic with an epipedon mollic over A horizon and clayey B horizon. In such a clay environment the sand fraction is dominated by volcanic minerals (Soil Survey Staff, 1992; Keys to Soil Taxonomy, 1994). Irrigation took place with NaCl solute concentration at five levels (gNaCl l-1): 0 gNaCl l-1 1.25 gNaCl l-1; 2.5 gNaCl l-1; 5 gNaCl l-1; and 10 gNaCl l-1 on spring-summer vegetable crops and three irrigation intervals of 2, 5 and 10 days; the layout of the experiment is shown in Table 1. The objective of the experiment was to study the effects of the saline water on some characteristics of the soil-water-plant system.
Soil samples were taken to measure the physical characteristics. The sampling was done on fields with 0% and 1% saline irrigation water in combination with 2- and 10-day irrigation intervals. To measure simultaneously the h(q) and K(h) characteristics the Wind evaporation method (Wind, 1968; Boels et al., 1978) was used to measure the soil physical characteristics.
Experimental procedures
Undisturbed samples to measure h(q) and K(h) were taken with the sampling system provided by Agro Research Instruments. The instrument is composed of three parts: a steel tube with a helical thread to anchor the installation in the soil; a lever with a press down construction; and an adapter on which a PVC cylinder is assembled and pressed into the soil. After pressing the PVC cylinder completely into the soil, the cylinder filled with soil is excavated carefully. Duplo samples were taken in PVC cylinders (inner diameter 110 mm, height 80 mm), from the layers 0-10 and 20-30 cm.
The samples were saturated by leaving them standing in water for at least four weeks. Four tensiometers (outer diameter 1.8 mm, length 18 mm) were horizontally placed into pre-bored holes at heights of 10, 30, 50 and 70 mm from the bottom of the soil sample. The bottom was closed with a lid to prevent evaporation. The tensiometers were connected to stopcocks and a pressure transducer. The soil sample was placed on a balance. The top of the sample was covered and the saturated sample and tensiometers equilibrated for at least 12 h. The top lid was removed at the beginning of the experiment. The soil sample could evaporate only from the top in a controlled environment (T = 20 ± 20°C, relative humidity = 50 ± 5%). At regular intervals the hydraulic heads, hh, and the total weight of the soil sample were measured. The experiment ended when air entered the top tensiometer. The soil was removed from the cylinder, weighed and dried at 105 °C to determine the average water content at the end of the experiment. Assuming that evaporation was equal to the liquid flux density through the samples, K(h) values were obtained using the Darcy equation with each pair of measurements: evaporation and hydraulic head gradient.
TABLE 1 - Layout of the research carried out at the experimental field of Torre Lama; each cell in the table represents the plot on which the treatment indicated in the cell was applied
Salinity = gr. l-1 of NaCl in addition of normal water, Irr. freq. = irrigation frequency. The bold treatments are the ones from which samples were taken.
|
salinity 0% |
salinity 0.125% |
salinity 0.25% |
salinity 0.5% |
salinity 1% |
|
salinity 0% |
salinity 0.125% |
salinity 0.25% |
salinity 0.5% |
salinity 1 % |
|
salinity 0% |
salinity 0.125% |
salinity 0.25% |
salinity 0.5% |
salinity 1 % |
To obtain the bulk density the samples have to be taken with a minimum of disturbance. This was possible by using the improved soil core sampler to prevent compaction of the soil (Hendrickx et al., 1991). To determine the bulk density, soil core samples were taken on the same treatments and irrigation intervals mentioned above.
Data analysis
The measured K(h) values were subsequently used to estimated a parametric model of the K(h) characteristics.
RETC is a computer program to describe the unsaturated soil hydraulic properties for monotonic drying or wetting in homogeneous soils. Soil water retention data are described with the equation of Brooks and Corey and Van Genuchten, whereas the pore-size distribution models of Burdine and Mualem are used to parametrize h(q) and K(h) characteristics (Van Genuchten et al., 1992). This model was used to analyse the data from the experimental field of Torre Lama.
, (1)
where: a , n and m are empirical constants affecting the shape of the retention curve.
From different studies it was concluded (Van Genuchten et al., 1992) that the equation gives an excellent fit to the observed data for most soils.
The model by Mualem (1976a) for predicting the relative hydraulic conductivity, K, uses the Se (relative saturation) term:
, (2)
where: the q s and q r refer to the soil water content at saturated and residual value.
The K(h) is expressed as a function of pressure head when (m = 1 - 1/n):
, (3)
Inspection of (1) and (2) shows that the soil water retention curve, q (h), contains five parameters: the residual water content, q r, the saturated water content, q s, and where a, n and m are parameters which determine the shape of the curve. The residual water content, q r, refers to the water content where the gradient dq /dh becomes zero. In practice, q r, is the water content at some large negative value of the water pressure head. The dimensionless parameter n determines the rate at which the S-shape retention curve turns toward the ordinate for large negative values of h, thus reflecting the steepness of the curve, while a (cm-1) equals approximately the inverse of the pressure head at the inflection point where dq /dh has its maximum value (Wösten and Van Genuchten, 1988). The predictive equation for K introduces two additional unknowns: the pore connectivity parameter, l, and Ks, the saturated hydraulic conductivity.
The RETC code may be used to fit any one, several, or all of these parameters simultaneously to observed data. RETC uses a non-linear least-squares optimization approach to estimate the unknown model parameters from observed retention and/or conductivity or diffusivity data. The approach is based on the partitioning of the total sum of squares of the observed values into a part described by the fitted equation and a residual part of observed values around those predicted with the model. The aim of the curve fitting process is to find an equation that maximizes the sum of squares associated with the model, while minimizing the residual sum of squares, SSQ. The residual sum of squares reflects the degree of bias and the contribution of random errors. This is what will be estimated later as s q r; s a , etc.; and using the four samples.
RESULTS
Postiglione et al. (1994), found, after six years, clear effects on the soil physical properties by irrigation with saline waters. As shown in Table 2, the ECe, pH and SAR increase over time, with higher values on the 1% treatment than the 0% treatment.
The index of stability of the structure showed that the deterioration was higher in the 1 % treatment, specially in the first 30 cm of the profile and in summertime when a crust developed, than in the 0% treatment. A further confirmation of the damage caused by NaCl was the infiltration rate.
On the 0% treatment in winter and summer the values of infiltration rate are higher than on the 1% treatment. On the same experiment, Ruggiero et al. (1994) found the crop growth rate was severely reduced due to the toxic effects connected to the higher NaCl concentration in the soil.
Table 3 shows the Van Genuchten parameters estimated by the RETC model for the layer 0-30 cm and for each treatment. This layer is constructed by a combination of four samples with the same soil physical characteristics, from the layer 0-10 cm (in duplo) and 20-30 cm (duplo). For each parameter the average and the standard deviation out of four samples are presented in Table 3. The q r value is much higher in the 0% treatment than in the 1% treatment. The a for the 1% treatment is much higher than at the other treatments. Instead the n and m values are much lower for the 1% than the 0%. The Ksat was not measured but was obtained by model optimization.
The plots of the water retention (WR) curve h(q) and the hydraulic conductivity (HC) curve K(h) were obtained. The 0% and 1% treatments of 2-day irrigation frequencies (Figures 1 and 3), and the 0% and 1% treatments of 10-day irrigation frequencies (Figures 2 and 4), show the different forms of the WR and the HC. The WR curve of the 1% treatment in both the irrigation frequencies has lower values of q than the 0% treatment at the same pressure head. The HC curve at the same pressure head considered for the WR also show lower values for the 1% treatment than on the 0% treatment. The 1 % treatment with 2 days of irrigation frequencies has a higher Ksat value that is in contrast with the infiltration rate shown in Table 2. One reason could be that the Ksat was not measured but obtained by optimization.
TABLE 2 - Summary data after six years of the experimental field of Torre Lama
|
Data |
Treatments |
|||
|
0% T2 |
0% T10 |
1% T2 |
1% T10 |
|
|
ESP |
2.2 |
- |
64 |
- |
|
ECe (w) (dS m-1) |
1.9 |
2.5 |
3.5 |
5.0 |
|
ECe (s) (dS m-1) |
2.5 |
2.0 |
20 |
19 |
|
pH (w) |
7.1 |
7.2 |
8.4 |
8.0 |
|
pH (s) |
7.7 |
7.3 |
8.2 |
8.0 |
|
ISS (w) (mm h-1) |
45 |
- |
12 |
- |
|
IR (s) (mm h-1) |
19 |
- |
0.0 |
- |
|
|
12 |
- |
0.6 |
- |
% = percentage of NaCl in addition in the normal water. T2,10 = Irrigation frequency 2 and 10 days. ISS = Index stability structure in %. IR = Rate infiltration in (w) = winter, and in (s) = summer. ECe = electric conductivity. ESP = Exchangeable Sodium Percentage.
TABLE 3 - Average values of the Mualem-Van Genuchten parameters of the soil samples
|
Description |
q 3 |
q 3 (std) |
q r |
q r(std) |
a |
a (std) |
n |
n(std) |
m |
m(std) |
Ksat |
Ksat(std) |
|
0 % T2 |
40.6 |
5.02 |
24.3 |
4.41 |
0.052 |
0.039 |
1.98 |
0.11 |
0.495 |
0.027 |
1.21 |
1.37 |
|
0 % T10 |
34.0 |
7.83 |
21.0 |
9.05 |
0.022 |
0.004 |
1.64 |
0.09 |
0.388 |
0.035 |
0.91 |
0.43 |
|
1 % T2 |
35.4 |
6.23 |
6.9 |
7.97 |
0.053 |
0.053 |
1.23 |
0.13 |
0.178 |
0.084 |
3.03 |
4.23 |
|
1 % T10 |
35.4 |
6.13 |
0.0 |
0.00 |
0.204 |
0.190 |
1.11 |
0.01 |
0.103 |
0.010 |
432 |
476 |
TABLE 4 - Available water and hydraulic conductivity under saline conditions
|
Treatment |
Parameters | ||||
|
|
q pF2.0 |
q pF4.0 |
Av. W. |
KpF2.0 |
KpF4.0 |
|
0% T2 |
0.28 |
0.24 |
0.04 |
0.90 |
3.6 · 10-5 |
|
0% T10 |
0.29 |
0.21 |
0.08 |
0.91 |
|
|
1% T2 |
0.26 |
0.16 |
0.10 |
0.98 |
9.1 · 10-5 |
|
1% T10 |
0.26 |
0.13 |
0.13 |
0.08 |
|
Table 4 shows the q values at the field capacity (log h=2.0) and at pressure head of 10.000 cm (log h=4.0). The wilting point (log h=4.2) was rapidly reached with the method used. The difference between the field capacity and the wilting point is the available water. The available water is an important parameter for the crops. The q value at the field capacity and of the wilting point for all the treatments decreases. The available water increases with salinity. The less water available on the 0% treatment can be explained by the higher percentage of macro-pores. From the graphs it can be concluded that the available water amount between pF 2.0 and 4.0 of samples with a 0% treatments will be lost more easily at low pressure heads, while on the 1% treatments the available water is constantly extracted at low and high pressure heads. A reason for this behaviour on the 1 % treatments could be explained by what Quirk and Schofield (1955) suggested: that swelling of clay particles, which increase in clay sodicity, could result in blocking or partial blocking of the conducting pores. Quirk (1986) found that the sodium causes the deterioration and flocculation of the clay colloids with a reduction in porosity and extremely unfavourable consequences on permeability. The fine pores contribute to having more water that is un-easily conducted and readily available in the soil. As shown in Table 4, K value decreases with salinity and is very low at a pressure head of 10 000 cm.
TABLE 5 - Significant difference among the treatments of the q r, Ksat n and m parameters by the Van Genuchten equation (n.s. = non significant; h.s. = high significance)
|
Treatments/ |
0% T2 |
0% T10 |
1% T2 |
1% T10 |
|
0% T2 |
- |
ns |
hs |
hs |
|
0% T10 |
|
- |
ns |
hs |
|
1% T2 |
|
|
- |
ns |
|
1% T10 |
|
|
|
- |
|
Ksat |
||||
|
0% T2 |
- |
ns |
hs |
ns |
|
0% T10 |
|
- |
hs |
ns |
|
1% T2 |
|
|
- |
ns |
|
1% T10 |
|
|
|
- |
|
n |
||||
|
0% T2 |
- |
hs |
hs |
hs |
|
0% T10 |
|
- |
hs |
hs |
|
1% T2 |
|
|
- |
ns |
|
1% T10 |
|
|
|
- |
|
m |
||||
|
0% T2 |
- |
hs |
hs |
hs |
|
0% T10 |
|
- |
hs |
hs |
|
1% T2 |
|
|
- |
ns |
|
1% T10 |
|
|
|
- |
To assess whether the differences between the Van Genuchten parameters are significant, a Student's T-test was applied. This test does not indicate the cause of the difference (i.e., use of saline water); it simply indicates whether the observed differences are significantly larger than the random experimental errors. To estimate the random errors, the four samples were grouped and the standard deviation and the mean value calculated.
The Student's T-test was calculated on the 95 % confidence interval with three degrees of freedom (four samples in each group only) obtaining a critical value for t of 2.35. The results of the elaboration, only for the significant parameters, are presented Table 5.
There is high significance of the difference in the Van Genuchten parameters especially for the n and m parameters among the extreme treatments of 0% T2, 1% T2, and 1% T10, 0% T10. Differences in both q r and the other Van Genuchten parameters on the 1 % treatment but under different irrigation frequencies (2 and 10 days) are not significant.
TABLE 6 - Bulk density of the layer 0-30 cm (kg dm-3)
|
Treatments | ||||
|
Depth (cm) |
0% T2 |
0% T10 |
1% T2 |
1% T10 |
|
0-10 |
1.09 |
1.04 |
1.22 |
1.20 |
|
10-20 |
1.54 |
1.49 |
1.61 |
1.57 |
|
20-30 |
1.52 |
1.54 |
1.62 |
1.48 |
Another parameter measured was bulk density which changes through the profile. As Table 6 shows, the bulk density lower on the top layer for both extreme treatments and increases with depth. The bulk density value on the 1% treatments is higher than on the 0% treatments. In fact, the high ESP value and the formation of a surface crust due to a high sodium content cause dispersion of the clay with the consequent formation of fine pores and high bulk density. The sealing efficiency of the crust is achieved by suction forces that arrange the clay particles into a continuous dense skin, as well as the decrease in the HC. The suction forces at the soil-crust interface as a result of large differences in HC between the crust and the underlying soil (Frenkel et al., 1978; Kazmann et al., 1983; Shainberg and Letey, 1984).
CONCLUSION
The soil on which the research was carried out has a clay content of around 30% that is dispersed by salt water with the consequent formation of a crust that strongly influences the transporting of water in the deeper layers, especially under intense rainfall. In fact, the infiltration of the water is strongly reduced in the 1 % treatment (as shown in Table 2) where a crust seals the soil and leads to a large difference in hydraulic conductivity between the crust and the underlying soil (Kazmann et al., 1983).
On the other hand, the increase in bulk density from the 0% to 1 % confirms that the salt influences the dispersion of clay with a reduction of porosity especially for the top-layer (0-10 cm) but the effect decreases with depth.
The effects of saline water on the soil physical properties after six years are rather clear. These effects are most significant on the top layer, in the range 0-30 cm.
Even though visual inspection of the hydraulic characteristics did not reveal large qualitative differences, the Van Genuchten parameters show a significant differences between the treatments. This is confirmed by the highly significant differences observed for all parameters (except a) when comparing the 1% T2 with both 0% treatments.
Knowledge of the effects of the saline water on the soil physical properties is important for correct irrigation management with these waters.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support given by the European Environmental Research Organisation (EERO).
REFERENCES
Boels, D., Van Gils, J.B.H.M., Veerman, G.J. and Wit, K.E. 1978. Theory and system of automatic determination of soil moisture characteristics and unsaturated hydraulic conductivities. Soil Sci. 126: 191-199.
Emerson, W.W. and Bakker, A.C. 1973. The comparative effects of exchangeable calcium, magnesium, and sodium on some physical properties of red-brown earthsubsoils. Aust. J. Soil Res. 11: 151-157.
Frenkel, H., Goertzen, J.O. and 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. Am. J. 42: 32-39.
Hendrickx, J.M.H., Ritsema, C.J., Boersma, O.H., Dekker, L.W., Hamminga, W. and Van der Kolk, J.W.H. 1991. Motor-driven portable soil core sampler for volumetric sampling. Soil Sci. Soc. Am. J. 55: 1792-1795.
Kazman, Z., Shainberg, I. and Gal, M. 1983. Effect of low levels of exchangeable sodium and applied phospho-gypsum on the infiltration rate of various soil. Soil Sci. 135: 184-192.
Mualem, Y. 1976a. Anew model for predicting the hydraulic conductivity of unsaturated porous media. Water Resour. Res. 12: 513-522.
Postiglione, L., Barbieri, G. and Tedeschi, A. 1994. Long-term effects of irrigation with saline water on some characteristics of a clay loam soil. ESSC Conference on Problems and Management of Soil Salinization-alkalinization in Europe. Budapest, 26-30 April.
Quirk, J.P. 1986. Soil permeability in relation to sodicity and salinity. Phil. Trans. R. Soc. Lond. 316: 297-317.
Quirk, J.P. and Schofield, R.K. 1955. The effect of electrolyte concentration on soil permeability. J. Soil Sci. 6: 163-178.
Rowell, D.L., Payne, D. and Ahmad, N. 1969. The effect of the concentration and movement of solutions on the swelling, dispersion, and movement of clay in saline and alkali soils. J. Soil Sci. 20 (1): 176-188.
Ruggiero, C., De Pascale, S. and Barbieri, G. 1994. Effetto dell'irrigazione con acque a diverso contenuto salino sullo stato idrico, sull'accrescimento e sulla produzione della melanzana (Solanum melongena L.). Riv. d'Agron. 3: 222-234.
Shainberg, I., Bresler, E. and Klausner, Y. 1971. Studies on Na/Ca montmorillonite systems. I. The swelling pressure. Soil Sci. 111: 214-219.
Shainberg, I., Rhoades, J.D. and Prather, R.J. 1981. Effect of low electrolyte concentration on clay dispersion and hydraulic conductivity of a sodic soil. Soil Sci. Soc. Am. J. 45: 273-277.
Shainberg, I. and Letey, J. 1984. Response of soil to sodic and saline condition. Hilgardia 52: 1-57.
Shainberg, I. and Levy, G.J. 1992. Physico-chemical effects of salts upon infiltration and water movement in soils. R.J. Wagenet, P. Baveye and B.A. Stewart (eds.). Interacting Processes in Soil Science 38.
Soil Survey Staff. 1975. Soil Taxonomy, a basic system for soil classification for making and interpreting soil surveys. Agric. Handb. US Dep. Agric. Sci. Edn. Adm. 436. 754 p.
Szabolcs, I. 1994. Prospects of soil salinity for the 21st century. 15th World Congress of Soil Science. Acapulco, Mexico, July 10-16, 1994 Vol. I. pp. 123-141.
USDA. 1994. Keys To Soil Taxonomy. SMSS Technical Monograph No. 19, Pocahontas Press, Blacksburg, Virginia.
Van Genuchten, M.Th., Leij, F.J. and Yates, S.R. 1992. The RETC code for quantifying the hydraulic functions of unsaturated soils.
Wind, G.P. 1968. Capillary conductivity data estimated by a simple method. In: Water in the Unsaturated Zone. Proceedings of the Wageningen symposium, June 1966, Vol. 1. P.E. Rijtema and H. Wassink (eds.). IASH Gentbrugge/Unesco Paris. pp. 181-191.
Wösten, J.H.M. and Van Genuchten, M.Th. 1988. Using texture and other soil properties to predict the unsaturated soil hydraulic function. Soil Sci. Soc. Am. J. 52: 1762-1770.
