SUMMARY AND CONCLUSION Laboratory studies were conducted on clay, clay loam and sandy loam soils to study the effect of irrigation water qualities on hydraulic, dispersion and swelling properties. Twenty-seven water quality combinations with three levels of SAR (10,20, and 30 mmol1/2 L'l/2) and three levels TEC (10, 20, and 30 me L'1) with three Ca:Mg ratios (1:2, 1.5:2 and 1:1) was synthesized using chloride, carbonate and sulphate salts of calcium, magnesium and sodium. Soil samples were equilibrated with different quality of synthetic waters and analyzed for hydraulic properties. Dispersion and swelling properties of the equilibrated soil were also recorded to assess their contribution in deterioration of hydraulic properties. The equilibrium solutions were analyzed for their ionic compositions, ph, ECe, ESP and Dispersion index. The reduction in saturated hydraulic conductivity at various SAR and TEC levels of irrigation water were determined. The significant correlations were observed by developing regression equation on different hydraulic properties with different water qualities and soil properties. 6.1 ph ph showed a regular increase with increasing SAR and TEC of the equilibrating solution. The range of ph was very high in carbonate series as compared to sulphate and chloride series in three textured soils (clay, clay loam, and sandy loam) with three series (chloride, carbonate, and sulphate) along with three Ca:Mg ratios of 1:2, 1.5:2, and 1:1 respectively. Variation in ph was observed in presence of alkaline salts of carbonate which showed the sharply increased in ph of equilibrium solution. Such behaviour was mainly due to different nature of soluble salts. An increase in ph under high SAR water is mainly due to sufficient increase in exchangeable sodium causing more hydrolysis effect. 6. 2 Electrical Conductivity of Extract (ECe) Soil salinity was compared in terms of soluble salt concentration and electrical conductivity. Total soluble salt concentration was approximately doubled when soil was equilibrated with low salt water (10 me L"1), thus increasing when soil was equilibrated with higher TEG levels such as in 20 and 30 me L'1. Electrical conductivity was also found 180
in similar fashion as in ph. In general magnitude of ECe followed the order: clay > clay loam > sandy loam. This was mainly because of hydrolysis of Ca and Mg in clay and clay loam soils in presence of low salt concentration. 6.3 Exchangeable Sodium Percentage (ESP) ESP of different soils increased with increasing SAR and TEC of equilibrating solution. Higher ESP build-up in clay over clay loam and sandy loam soils was attributed to higher SAR and TEC of these soils. The increased ESP was higher irrespective of TEC as compared to SAR indicating that ESP was more effective with increase in SAR than TEC of water quality. The variation of increased value of ESP (irrespective of SAR) was higher in all series of clay soil than clay loam and sandy loam soil. 6.4 Dispersion Index (DI) Dispersion index of all the soils increased with increasing SAR and decreasing TEC of irrigation water in three series along with three Ca:Mg ratios in all soils. Dispersion index was higher in clay soil than clay loam and sandy loam soil indicating more dispersivity of clay and clay loam soils than sandy loam. Highest dispersion was observed in combination of high SAR and low TEC (30 mmol1/2l'1/2 and 10 me L'1), while lowest dispersion was in low SAR and high TEC (10 mmo\xuum and 30 me L'1). On an average, the variation in percentage of increased DI was lower with increase in sodicity (irrespective of TEC) of irrigation water and percentage of variation in reduction of DI was higher with increase in salinity (irrespective of SAR). 6.5 Water Retention Water quality affected the soil water characteristic curve of the soils. With water salinity decrease and/or sodicity increase, water retention especially at high matric suctions increased. The effect of water sodicity on soil water characteristic curve was greater when water salinity was not high. Saturated water content and water retention at similar matric suctions were increased with increasing ECe and SAR. The effect of water quality was more evident for the clay soil than for the sandy clay loam especially at low matric suction. The water retention was the highest with water quality having low electrolyte concentration 181
(10 me L'1) and high SAR (30 mmol1/2 L"1/2) and least having high electrolyte concentration (30 me L"1) and low SAR (10 mmol1/2 L'1/2). The decrease of drainage water percentage was highest in clay soil and least in sandy loam soil, whereas increase in available water content was highest in clay and clay loam and decrease in available water content in sandy loam soil was observed as compare to normal soil in three series with respective Ca:Mg ratios. The result revealed that considering water quality and soil properties, the influence of SAR and DI on available water content was highly significant in chloride and sulphate series and ESP was significant in all series, whereas TEC was in chloride series in clay soil. In clay loam soil, AW was significantly affected by SAR and TEC in carbonate and sulphate series, and ESP in carbonate series only. In sandy loam soil there was no significant effect of water quality and soil parameters together. 6. 6 Saturated Hydraulic Conductivity (Ks) Saturated hydraulic conductivity in all the soil decreased with increasing SAR and increased with increasing TEC. Highest value of Els was observed when equilibrated with water having SAR 10 mmol1/2 L'm and TEC 30 me L'1, whereas lowest value was with water at SAR 30 mmol1/2 If1/2 and TEC 10 me L"1. Ks as a function of SAR (irrespective of TEC) showed that the percentage of reduction (from 10 to 30 mmol1/2 L"1/2) was lower in clay and clay loam soil, whereas was higher in sandy loam soil. In case of Ks as a function of TEC (irrespective of SAR) the percentage of increase in TEC (from 10 to 30 me L'1) was observed with the respective increase in Ks in all the series of clay and clay loam soil and decrease in sandy loam soil as compared to normal soil. The study indicated that the combined effect of water quality and soil properties on Saturated hydraulic conductivity (Ks) was significantly influenced by SAR and TEC in carbonate and sulphate series, whereas ESP and DI was in chloride and sulphate series in clay soil. In clay loam soil Ks was highly significantly dependent on DI in chloride series only. In sandy loam soil, influence of SAR was highly significant in sulphate series and significant in carbonate series, whereas TEC and ESP had significant effect in sulphate series only. 182
6.7 Unsaturated Hydraulic Conductivity (ko) Unsaturated hydraulic conductivity (Kg) decreased according to the decrease of water content; such a decline was higher in clay soil followed by clay loam and silt loam soil. At equilibrium SAR, Kg increased with increasing TEC at any 0V in all three soils (clay, clay loam and sandy loam) with three anionic series (chloride, carbonate, and sulphate) and Ca:Mg ratios (1:2, 1.5:2, and 1:1). Ke was maximum in the samples equilibrated with water having SAR 10 mmol1/2 L'1'2 with TEC 30 me L'1 and minimum in with water having SAR 30 mmoli/2l"1/2 and TEC 10 me L'1. The results of the study suggest that the water quality parameter and ESP, and DI of clay and clay loam soils influenced unsaturated flow significantly in chloride and sulphate series, whereas carbonate series did not show any significant effect on clay soil. However, in sandy loam soil the flow was more dependent on water quality parameters as compared with ESP and DI. Because of the predominance of expanding type of clay minerals in clay loam soil, Kg-SAR relationship was highly significant as compared to Kg-ESP. The clay soil was observed to be the most sensitive soil to the unsaturated flow at SAR 30 mmol1/2 L'1/2. 6.8 Soil-Water Diffusivity (Dg) The effect of water qualities on D0 was maximum at SAR 10 mmol1/2 IT1/2 with TEC 30 me L'1 and minimum SAR 30 mmol1/2 L'1/2 with 10 me L'1. As compare to 0V, D0 was higher in sandy loam, followed by clay loam and least in clay soil at all combination of SAR and TEC in all series (chloride, carbonate and sulphate) with respect to three Ca:Mg (1:2,1.5:2, and 1:1) ratios respectively. Considering the effect of water quality and soil parameters both, De was significantly dependent on SAR of water and DI in chloride series of clay. TEC was highly significant in sulphate series of clay loam soil,'whereas SAR, TEC, ESP, and DI in sulphate series of sandy loam soils. Unsaturated flow through clay, clay loam and sandy loam soils has shown different patterns of its dependence on water quality parameters and ESP at high (33 kpa) and low (in available water range) water contents. It is inferred from the study that the transmission of water during unsaturated conditions is adversely affected if the irrigation is practiced with poor quality waters. In arid and semi-arid parts of India, where 183
irrigation waters often contain salts in the range between 10-30 me L"1, field irrigation should be practiced with low SAR waters or attempts may be made for lowering down the SAR on medium to fine-textured soils. 6.9 Response Measurement of Crop During the seedling stage, gas exchange attributes were significantly reduced by salt ^stress, with a greater reduction with time of exposure. A substantial reduction in photosynthesis, stomatal conductance and transpiration was detected within only 10 days of exposure to salt stress. Salinity is known to inhibit photosynthesis in a number of plant species (Longstreth and Nobel, 1979; Flowers and Yeo, 1981; Yeo et al., 1985; Dionsio- Sese and Tobita, 2000). Possible reasons for this include stomatal closure, feedback inhibition due to reduced sink activity, decreased efficiency of Rubisco, displacement of essential cations from the endo-membrane.structure (leading to changes in permeability), and swelling and disorganization of the grana-(flowers and Yeo, 1981), or due to the direct effects of salt on stomatal conductance via a reduction in guard cell turgor and intercellular CO2 partial pressure (Dionisio-Sese and Tobita, 2000). In general, our results point to the idea that salinity effects on plant growth are mediated very largely by changes in the plant s water status, particularly that of the leaf. For example, it is our interpretation that leaf extension slowed when the lights came on in the growth chamber because leaf water status was diminished; as the radiant heat load increased, the evaporative demand would have been greater so that water losses would outstrip supply and reduce leaf water potential. In this pragmatic investigation, salinity decreased growth and photosynthetic pigments at 10 and 20 days due to decrease in osmotic potential level in root and shoot leading to decreasing photosynthesis in Vigna species. An increase in root zone salinity would exacerbate this effect by lowering the overall plant water status. When salinity was imposed for 20 days, leaf growth and leaf water potential measured hygrometrically declined. Salinity significantly impaired stomatal conductance, causing a reduction in transpiration rate and an elevation of leaf temperature. Though photosynthetic rates expressed per unit of chlorophyll decreased, photosynthesis per unit of leaf area was also diminished. Probably the increased amount of photosynthetic apparatus per unit area of the 184
thicker leaves compensated for the lowered internal carbon dioxide concentration. The reduction in leaf area, yield and yield components under saline conditions were also due to reduced growth as a result of decreased water uptake, toxicity of sodium and chloride in the shoot cell as well as reduced photosynthesis. Reduction in chlorophyll concentrations is probably due to the inhibitory effect of the accumulated ions of various salts on the biosynthesis of the different chlorophyll fractions. Salinity affects the strength of the forces bringing the complex pigment protein liquid, in the chloroplast structure. From the above discussion, we concluded that, the growth processes in the Vigna radiata were suppressed which was a result of the disturbed osmotic processes and toxic of Cl', Ca2+, Mg2+, and Na+. The concentration of Na with Ca:Mg was treated with chloride was probably a result, on the one hand, of the weaker inhibition of the water potential and the stomata closure, and on the other hand, of the higher concentration of sodium adsorption ratio SAR and total electrolyte concentration TEC in the tissue of the plant was highly effected. It appears that salinity affected normal physiological functions of mung plants highly in clay than clay loam and sandy loam soils. This was expressed by the imbalance in water relation and mineral ions accumulation in this variety. The higher reduction of WRC indicated a greater damage in cell structure due to salinity (Sangakkara et al. 1996). It is well-known that under saline conditions plant suffers from osmotic shock due to lower osmotic potential in the soil solution (Oreutt and Nilsen, 2000). 185