Proceedings of the International Conference Soil and Desertification – Integrated Research for the Sustainable Management of Soils in Drylands 5-6 May 2006, Hamburg, Germany www.desertnet.de/proceedings/start.htm
SALINE SOILS OF IRAN WITH EXAMPLES FROM THE ALLUVIAL PLAIN OF KORBAL, ZAGROS MOUNTAINS MARTIN KEHL Institute of Crop Production and Resource Conservation, Soil Science Division, University of Bonn, Nussallee 13, D-53115 Bonn Abstract The factors of salt enrichment, the distribution of salt-affected soils and the significance of secondary soil salinization in Iran are briefly discussed. In a second part, the properties and salinity status of three Calcic Solonchaks (S1, S3, S4) and one Calcic Gypsisol (S2) from the plain of Korbal are described. The heavy textured calcareous soils are moderately to extremely saline related to differences in natural drainage and salt contents of irrigation water (S1, S2) and groundwater (S3, S4). Salt contents in the calcium dominated and well drained S1 and S2 could comparatively easy be controlled if high quality irrigation water was available in sufficient amounts. The extremely saline S3 and S4 are sodium dominated and would require artificial drainage, extensive leaching and careful management in order to reduce salt contents to an acceptable level and to prevent structural deterioration. These soils should remain under pasture. Given the importance of irrigation farming for food production in Iran, secondary salinization should receive more attention in the country. More data on soil and groundwater salinity and on the depth of the groundwater table are needed to monitor salt balances of the soil as a prerequisite of sound irrigation practises. Key words: Saline soils, secondary salinization, Iran 1. Salt-affected soils and secondary salinization in Iran Salt accumulation in Iranian soils is mainly related to a dry climate, salt-rich parent materials of soil formation, insufficient drainage and saline groundwater or irrigation water. About 75 % of the total land area (1,648,000 km2) is semi-arid or arid. While the average annual rainfall is at 252 mm, 2/3 of the country receives less than 250 mm of precipitation (FAO, 2000). In contrast, potential evaporation is generally high ranging from 500 to 4000 mm/year. Besides the lack of precipitation and excessive evaporation, strong winds that blow for instance across the Central Plateau (Fig. 1), might redistribute salts from salt crusted kavir areas onto the surfaces of soils. The salts originate from evaporitic rocks covering large areas in southern and Central Iran. Among those rocks are the "Infracambrian" Hormuz salts of the southern Zagros and Persian Gulf region or the saliferous and gypsiferous marls of the Upper Red Formation (Dewan and Famouri, 1964; NIOC, 1975-1978). In addition, salt enriched Quaternary alluvial and lacustrine sediments occur in closed basins. Except for the Persian Gulf and Gulf of Oman watersheds, the hydrology of Iran is characterised by internal drainage. Mineralized runoff from channel, sheet and groundwater flow dissipates into saline marshes, salt flats and salt crusted playas, where salts accumulate by evaporation of stagnant surface water or groundwater occurring close to the land surface. These salt-rich areas cover vast depressions of the Central Plateau. Bordering these depressions large areas are covered by weakly to strongly saline soils (Fig. 1). Other major occurrences of saline soils are in the delta regions and coastal lowlands along the Persian Gulf and the Caspian Sea, where high groundwater tables and seawater intrusion promote the enrichment of salts. In interior basin, intrusion of saline groundwater might also be caused by excessive groundwater extraction for irrigation purposes. In an early country-wide assessment, Dewan and Famouri (1964) estimated the extent of saline soils in Iran at about 25 Mio ha including saline alluvial soils, Solonchak and Solonetz, salt marsh soils and saline desert soils. According to the recently published soil map at the scale of 1:1,000,000 (SWRI,
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Proceedings of the International Conference Soil and Desertification – Integrated Research for the Sustainable Management of Soils in Drylands 5-6 May 2006, Hamburg, Germany www.desertnet.de/proceedings/start.htm
2000) slightly and moderately saline soils occupy approximately 25.5 Mio ha and strongly saline soils cover about 8.5 Mio ha (FAO, 2000). Large areas covered by saline soils are not used for agricultural purposes because of water shortage particularly in the Central Plateau. Arable land covers 10 % of the total land area of Iran (1,648,195 km2), while 27 % and 11 % are covered by pasture and forest, respectively. The extent of arable land is limited by the availability of water (e.g., Ghassemi et al., 1995), and could be considerably increased, if water storage and distribution would be improved. Today, about 46 % of arable land (=7.3 Mio ha) is irrigated (FAO, 2005) producing about 90 % of agricultural crops in Iran (Siadat, 1999). Agricultural purposes consumed 91.6 % of the annual water demand (70 km3 in 1993, FAO 2000). The country thus heavily depends on effective and sustainable irrigation practices. In many areas primarily salt-affected soils are irrigated. In addition, irrigation often causes secondary soil salinization depending on a variety of factors including the salt content and composition of irrigation waters, distance to ground water table raised by excessive irrigation and conveyance losses and insufficient drainage or water scarcity hampering effective leaching. According to estimates for 1997, secondary salinization affected 28 % or 2.7 Mio ha of irrigated lands and also occurred on 0.6 Mio ha in rain-fed areas (FAO, 2000). For the year 1974, 38 % of irrigated lands were likely affected by secondary salinization and waterlogging (Ghassemi et al., 1995). A systematic study on the extent of secondary salinization in Iran is still missing (Ghassemi et al., 1995), and data on salinization-related yield decreases or on the abandonment of agricultural fields and desertification is scarce. Nevertheless, the problem of secondary salinization is being faced. Prevention strategies focus on increasing the water use efficiency in irrigated agriculture, which is probably as low as 30 %, as a consequence of 60 % conveyance efficiency and 50 % application efficiency (Siadat, 1999).
Central Plateau
Figure 1: Distribution of salt-affected soils in Iran (from SWRI 2000, changed)
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Proceedings of the International Conference Soil and Desertification – Integrated Research for the Sustainable Management of Soils in Drylands 5-6 May 2006, Hamburg, Germany www.desertnet.de/proceedings/start.htm
2. Saline soils in Korbal plain 2.1 Introduction The about 60 km long and 30 km wide plain of Korbal (Fig. 2) is the lower part of the large alluvial plain of Marvdasht, which is located about 60 km north-east of the city of Shiraz (Fig. 1). The plain of Korbal is mainly used for cropping of wheat, rice, maize and sugar beets. Climate is semiarid with mean annual temperatures of about 16-17 °C and mean annual rainfall ranges between 320 mm in the west and about 200 mm at Lakes Baghtegan and Tashk. In the same direction, potential evaporation increases to more than 2.200 mm a-1. The lakes have a maximum water depth of 1.5 m and may fall dry after dry winter seasons (Nadji, 1997). Due to tectonic uplift, the plain of Korbal has a mean gradient of about 0.6 ‰. It is drained by the Kor River and by artificial drains running parallel to the river along the marginal depressions of the plain (Fig. 2). The river and drains discharge to the lakes. Irrigation water is diverted from Kor River via six diversion structures (e.g., Band-e Amir, Band-e Tilakhan) and distributed to the fields by lined and unlined irrigation canals. In addition, groundwater extracted by shallow and deep wells and qanats as well as the discharge of springs and drains are used for irrigation. Recent statistics on the amount and quality of water extracted in Korbal plain are sparse. Data cited in literature generally cover the whole plain of Marvdasht, where EC values in µS cm-1 range from 500 to 8,140 for wells, 380 to 12,000 for qanats and 460 to 1470 (Ghassemi et al., 1995, 360). Salinization of the water resources increases downstream towards the salt lakes, whereas water table depth decreases to less than 2 m in the same direction. The shallow water table results from poor natural drainage of heavy textured alluvial deposits, excessive irrigation and seepage from irrigation canals. During the past, groundwater tables have risen as documented for the years 1971 to 1985 (Nadji 1997). In the last two decades, they possibly decreased because of the construction of drains and lined canals in combination with excessive extraction of groundwater by wells (Fatemi, personal comm., Javan, personal comm.). Further details on the irrigation system and water quality of Marvdasht plain are given by Ghassemi et al. (1995).
Spring
S3, S4
Drains Lake Tashk Salt plug
Drain
Kor River
S2 S1 0
5
10 km
Lake Baghtegan
Kherameh
Figure 2: The plain of Korbal (Landsat ETM, 7-3-1, 20.05.2000) with locations of soil pits Many soils of Korbal plain are heavily affected by salinization as indicated by efflorescence of salts on soil surfaces, halophytic vegetation and poor growth of agricultural crops. However, little information is available about the salinity status of cultivated and rangeland soils in Korbal plain. The objective of this study was to describe salt-affected soils in the area and to evaluate their salinity status.
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Proceedings of the International Conference Soil and Desertification – Integrated Research for the Sustainable Management of Soils in Drylands 5-6 May 2006, Hamburg, Germany www.desertnet.de/proceedings/start.htm
2.2. Materials and methods Four soils located on different topographic positions of the plain were described according to standard field methods (AG Boden, 1994) and classified following . The soils S1 and S2 were sampled on two agricultural fields located on the distal part of alluvial fans near the town of Kherameh (Fig. 2). At time of sampling in April (S1) and September (S2) 2001, the groundwater tables were at > 3.5 m and 8 m below surface, respectively. The soils S3 and S4 are located on a pediment and a clay pan, respectively, within the northern depression of Korbal plain about 150 m away from each other. Both soils are poorly vegetated and are under grazing with sheep. Sampling was done in September 2001 (S3) and September 2002 (S4), when groundwater was at 1.1 m depth and > 1.9 m depth, respectively. The laboratory analyses were conducted on the fine earth fraction (< 2.0 mm in diameter). Grain size distribution, pH(H2O), calcium carbonate equivalent (CCE) and gypsum content were determined according to standard laboratory procedures (cf. Kehl et al., 2005). The total amount of readily soluble salts (S), the electrical conductivity of a 1:5 soil-water-extract (EC1:5) and of a soil saturated paste extract (ECe) were determined according to Schlichting et al. (1995). In order to characterize the ionic composition of the saturation extract, Na+, K+, Ca2+ and Mg2+ cations were measured using atomic absorption spectrometry, and Cl-, NO3-, SO42-, PO43- were analysed by ion chromatography. HCO3was indirectly deferred from dissolved inorganic carbon. The exchangeable sodium percentage (ESP*) was estimated from the sodium absorption ratio (SAR) of the saturation extract (Van Reeuwijk, 1995) according to the following equation: ESP* (%)
100 x (-0.0126 + 0.01475 x SAR) = —————————————— 1 + (-0.0126 + 0.01475 x SAR)
2.3 Results and discussion Morphologic descriptions of the soil profiles are given by Börgens (2005). Here, we focus on the analytical data (Table 1) and give an evaluation of the salinity status (Table 2). According to WRB (1999), the soils classify as Calcic Solonchaks (S1, S3, S4) or Calcic Gypsisol (S2). All soils are slightly alkaline as indicated by pH(H2O) values between 7.1 and 8.0. They are calcareous throughout with CCE ranging from 17.2 to 42.3 %. Small amounts of gypsum are detected in all horizons, whereas samples of S2 and S3 show elevated gypsum contents with maximum values of 22 % in S2. The granulometric composition of soils S1 and S2 is dominated by the silt fraction (50-63 %). S3 and S4 are very rich in clay with maximum clay contents of 82 %. In soil S4, significant variations in clay content with soil depth and high Corg values in buried topsoil horizons result from layering of sediments. Slightly soluble salts range from 0.3 to 2.6 % and from 2.6 to 27.0 % in the cultivated and the rangeland soils, respectively. ECe values are moderately high to extremely high reaching maximum values of 28.4, 12.2, 211 and 167 dS m-1 in soils S1, S2, S3 and S4. Overall, EC1:5 gives a similar picture and shows a good correlation with ECe (ECe = -0.035 EC1:52 + 5.297 EC1:5 – 6.678, r2 = 0.851, n = 41). If gypsum bearing samples are excluded, the data scatter increases and the correlation coefficient decreases likely due to the small number of observations. In accordance with ECe the concentrations of kations and anions in the saturation extract of the different horizons and soils vary to a large extent. Calcium and magnesium are the main cations in S1 and S2, whereas sodium has the major share in S3 and S4. Potassium generally takes low concentrations. The anionic composition is dominated by chloride in S1, S3 and S4. Higher sulphate than chloride contents occur in S2 due to the presence of higher amounts of gypsum. Compared with these major anions, nitrate and bicarbonate are found in negligible concentrations, only. The measured SAR and calculated ESP* values fit into the picture shown by S, ECe and EC1:5. (cf. Table 1 for further details).
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Proceedings of the International Conference Soil and Desertification – Integrated Research for the Sustainable Management of Soils in Drylands 5-6 May 2006, Hamburg, Germany www.desertnet.de/proceedings/start.htm
Table 1: Physical and chemical properties of the soils S1 – S4.
Horizon
Depth
color, moist
cm
pH (H2O)
-
CCE
Gypsum
Particle-size distribution S
Corg
Sand
Silt Clay
————————— % —————————
Soluble cations ECe
EC1:5
— dS m-1 —
Ca
Mg
Na
K
Cl NO3
SO4 HCO3 SAR ESP*
——————————— me l-1 ———————————
Apz Bkz1 Bkz2 Bk
-25 -90 -120 -140
7.5YR 4/6 7.5YR 5/6 7.5YR 5/4 7.5YR 4/4
7.1 7.4 7.4 7.7
40.6 41.3 30.2 36.4
0.01 0.01 3.32 0.04
0.4 1.8 2.6 0.3
0.42 0.16 0.16 0.10
S1, alluvial fan, Chloridi-Calcic Solonchak 4 55 42 12.2 3.8 36 40 6 60 34 28.4 7.5 92 112 3 57 40 28.3 12.3 80 108 1 63 36 3.7 1.9 27 14
Apz1 Apz2 Bkz Bkyz1 Bkyz2
-15 -35 -65 -95 -150
7.5YR 4/4 5YR 4/4 5YR 4/4 5YR 4/4 7.5YR 4/6
7.2 7.2 7.2 7.7 7.4
32.0 35.0 42.3 19.8 19.9
3.80 4.50 0.15 22.1 17.2
1.0 1.1 0.8 0.7 0.9
1.05 0.34 0.27 0.25 0.20
S2, alluvial fan, Sali-Calcic Gypsisol 11 53 36 8.8 4.4 32 8 55 37 12.2 6.3 46 11 50 40 5.5 4.8 20 n.d. n.d. n.d. 4.5 3.3 30 5 63 32 7.9 4.4 30
Byz1 Byz2 Byz3 Bwz Bzg1 Bzg2
-40 -65 -90 -115 -125 -145
7.5YR 5/4 10YR 4/3 7.5YR 5/4 7.5YR 5/4 10YR 5/4 10YR 5/4
8.0 7.6 7.9 7.7 7.9 7.9
21.7 18.8 21.7 26.0 29.6 29.3
16.5 10.6 11.5 3.50 0.09 0.06
2.3 27.0 3.6 6.7 5.7 6.9
0.22 0.56 0.17 0.25 0.28 0.28
Ahz1 Ahz2 Bzg Azb Czg Azgb
-8 -22 -40 -51 -62 -70
10 YR 6/2 2.5 Y 6/2 10 YR 6/3 2.5 Y 4/2 10 YR 5/3 10 YR 4/2
7.4 7.4 7.3 7.4 7.3 7.2
35.5 36.1 31.7 41.8 28.1 17.2
0.40 0.11 0.01 0.02 0.02 0.02
6.9 3.8 8.0 9.5 10.9 9.8
0.84 0.77 0.70 1.48 0.93 2.34
%
62 122 146 2
1 2 3 0
91 276 261 9
4 6 6 1
30 29 51 34
2 2 1 1
10.0 12.1 15.0 0.4
11.9 14.2 17.3 0.0
22 24 10 10 21
55 74 23 12 48
0 0 0 0 0
37 77 34 8 25
2 3 1 1 2
61 46 18 47 64
2 1 1 1 1
10.6 12.6 6.0 2.7 9.6
12.6 14.8 7.0 2.7 11.4
42 240 80 206 165 140
390 2,874 692 1,033 1,265 1,429
4 34 11 18 14 10
445 3,294 840 1,433 1,521 1,606
7 54 15 27 26 26
72 119 76 78 77 69
1 0 1 0 1 0
52.2 180 70.4 68.7 98.5 124
43.1 72.6 50.6 50.0 59.0 64.5
S4, alluvial plain, Gleyi-Calcic Solonchak 1 39 61 130 22.0 154 154 1 39 60 76.6 11.8 74 81 1 18 81 100 20.2 90 120 6 45 50 131 22.1 136 162 2 43 56 167 30.0 195 197 0 25 74 138 29.1 141 165
1,434 726 937 1,384 1,940 1,542
12 6 7 8 11 7
1,580 821 1,165 1,599 2,337 1,740
27 18 13 28 28 29
91 55 34 60 66 62
1 1 1 1 0 0
115 82.4 91.4 113 138 124
62.8 54.6 57.2 62.4 67.0 64.6
S3, pediment, Gypsi-Calcic Solonchak 3 39 57 47.1 16.5 70 3 39 59 211 75.6 269 3 37 60 76.7 22.9 114 3 37 60 118 36.5 245 3 50 47 124 32.5 165 3 40 57 130 36.1 125
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Proceedings of the International Conference Soil and Desertification – Integrated Research for the Sustainable Management of Soils in Drylands 5-6 May 2006, Hamburg, Germany www.desertnet.de/proceedings/start.htm
In order to evaluate the salinity status of the soils, weighed means of salinity parameters for the profile depth 0-1 m were calculated (Table 2). Following the evaluation schemes of Abrol et al (1988) and Driessen et al. (2001), soils S1 and S2 classify as very strongly saline or moderately saline, respectively, whereas both S3 and S4 are extremely saline. The ECe values are slightly lower than those reported for cultivated Aquollic or Typic Salorthids, which were investigated in the flood plain of Kor River about 50 km northeast of the city of Kherameh (Abtahi, 1977; Givi & Abtahi, 1985). The ratio of (Ca2++Mg2+)/(Na++K+) is larger than 1 for soils S1 and S2 and smaller than 1 for soils S3 and S4. Since the ratio Ca2+/Mg2+ is close to or above 1 and the Na+/K+ ratio is larger or very much larger than 1, the soils S1 and S2 can be classified as calcium dominated, whereas S3 and S4 are sodium dominated (Driessen et al., 2001).
Soil unit pH(H2O) Readily soluble salts (%) a) ECe (dS m-1) a) ESP* (%) a) Salt content (Ca2++Mg2+)/(Na++K+) a) Ca2+/Mg2+ a) Na+/Mg2+ a) Dominant cation Anions Drainage Suitability for crop cultivation a)
S1 Chloridi-Calcic Solonchak
S2 Sali-Calcic Gypsisol
S3 Gypsi-Calcic Solonchak
S4 Gleyi-Calcic Solonchak
7.1-7.4 1.5 24.3 14.0 very strongly saline
7.2-7.7 0.9 7.2 8.3 moderately saline
1.5 0.8 1.2 Ca Cl > SO4 > HCO3 = NO3 good salt tolerant crops only
1.7 2.2 1.7 Ca Cl = SO4 >> HCO3 = NO3 moderate yield reduction of salt-sensitive crops
7.6-8.0 9.3 102.5 53.0 extremely saline 0.3 1.4 8.9 Na Cl >> SO4 > NO3 > HCO3 poor pasture
7.2-7.4 10.3 116.5 60.6 extremely saline 0.2 0.9 6.9 Na Cl >> SO4 > NO3 = HCO3 very poor pasture
Weighed mean for samples from 0-1 m depth.
Table 2: Summary description of soil profiles and evaluation of soil salinity status. The salts likely accumulated from different sources. Increasing salt concentrations with depth (Table 1) and a large distance to the groundwater table in soils S1 and S2 indicate that salts derived from saline irrigation water. In soils S3 and S4 salts most probably originated from the shallow groundwater table, which was likely even closer to the land surface before artificial drainage. In addition, evaporation of ephemeral surface runoff might supply salts to S4, whereas aeolian inputs could be a source of salt for both S3 and S4 (cf. Kehl et al, 2005). If S3 and S4 shall be used for crop production, a more effective artificial drainage and extensive leaching of readily soluble salts would be necessary to lower groundwater tables and the degree of salinity significantly. During leaching of S3, calcium ions from gypsum dissolution could partly replace sodium preventing dispersion of clays, which causes deterioration of soil structure in calcareous heavy textured soils (Abtahi, 1985; Morshedi and Sameni 2000). However, given the large ESP values of S3 and S4, sodicity hazard is high. These soils are thus better kept under pasture. 3. Final remarks More data on the salinity status of soils in Korbal plain and on groundwater tables and salinization are needed in order to further defer the causes and spatial extent of the salinity problem. This would help to differentiate between primary and secondary salinization and to adequately address the problem of soil salinization in that area. Concerning the lack of data, Korbal plain appears to be a representative
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Proceedings of the International Conference Soil and Desertification – Integrated Research for the Sustainable Management of Soils in Drylands 5-6 May 2006, Hamburg, Germany www.desertnet.de/proceedings/start.htm
example of salinity affected alluvial plains in the country. Given the importance of irrigated agriculture on the one hand and the shortage of water supply on the other hand, more ground-based soil and groundwater salinity monitoring schemes are needed as a prerequisite of sound irrigation practises in the bread baskets of Iran.
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