Vol. 35 – 2009 - Ecologia Mediterranea - Université d'Avignon et des ...

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MUSTAPHA DADDI BOUHOUN, LOUHICHI BRINIS, MOHAMED LAKHDAR SAKER, MARC COTE, JACQUES RABIER 46 Impact of shallow crusts-water tables The present study showed that 10.8% of the palm plantations, with a rooting above 120 cm depth, undergo two types of obstacles, namely the groundwater and the gypseous crust in Bamendil, Mekhadma and, gypseous calcareous crust in Ksar and Beni Thour, above Figure 4 – Relationship between date palm rooting and gypseous crusts in areas of deep water tables. Figure 5 – Relationship between date palm rooting and shallow water table in areas with crusts. Figure 6 – Relationship between date palm rooting and gypseous crusts in areas of shallow water tables. 120 cm depth. The crust and water table constituted a physical and water obstacle for rooting of the date palm. There were significant positive correlations (Figures 5 and 6) between root depth of the date palm (R.d.) and, depths of the water table (W.t.d.) and the gypseous crust (G.c.d.). We obtained the following relations: 1. R.d. = 0.9636 W.t.d. + 6.5374; R 2 = 0.9283. 2. R.d. = 1.0057 G.c.d. + 2.2422; R 2 = 0.6658. Therefore, we hypothesized that the shallow groundwater and the gypseous calcareous crusts in the Ouargla basin constituted physical and chemical obstacles, which limit the date palm root development in soil depths. Impact of shallow water tables in areas without crusts Our results showed that 43.1 % of the palm plantations, with a rooting above 120 cm depth, were undergoing a water obstacle of groundwater in the studied areas. There were positive correlations (Figure 7), highly significant between root depth of the date palm (R.d.) and water table depth (W.t.d.). We obtained the following relation, with R 2 = 0.6509, R.d. = 0.8069 W.t.d. + 21,275. The most significant regressions were observed in Bamendil, Mekhadma and in Beni Thour, with respectively the following relationships: 1. R.d. = 0.9243 W.t.d. + 4.4106; R 2 = 0.9609, 2. R.d. = 0.9013 W.t.d. + 8.4535; R 2 = 0.9492, 3. R.d. = 0.9725 W.t.d. + 12.193; R 2 = 0.855. However, in Ksar and Said Otba, R 2 was very weak. These areas are the oldest of the Ouargla basin. Rooting of the date palms there seemed to precede the groundwater rise. The study showed that the groundwater level limited the rooting by asphyxiation and salinization of date palm areas. Discussion The salinity and hydromorphy of groundwater at Ouargla constituted a physical and chemical obstacle for rooting and growth of Deglet Noor date palms. However, according to previous works, strong salinity causes the reduction of Fusarium population density (Amir et al. 1989), and salts induce a soil resistance to vascular fusariose (Amir & Riba 1990). The laboratory tests showed that 1% and 2% salt contents in the soil (E.C. e1:5 of 3.5 and 7 dS.m -1 ) strongly affects the pathogenic ecologia mediterranea – Vol. 35 – 2009
density which are respectively 0.5 and 0% of its initial level at the end of the experiment (Amir et al. 1996). Rooting of the date palms in the Ouargla basin is subjected to a salinity of phreatic origin, above 1.5 to 6 times the threshold of disappearance of the pathogen, which confers to this area a resistance to the bayoud disease. The plant root system cannot easily penetrate the hardened massive calcareous horizon. Moreover, water is mainly confined in the zone above the massive horizon, so that the geometrical model of root growth changes. Roots, seeking water, tend to grow laterally, favouring the development of the horizontal secondary roots (Esteban & Klappa 1983; Alonso-Zarza et al. 1989). Practically, in oasis, these gypseous formations of water table constitute a very important limiting factor, in particular for the shrubby cultures (date palms and fruit trees). Sometimes, they brutally block root depth increase which come to be crushed on a true sandy bank (Durand 1959; Pouget 1968). Gypseous calcareous crusts are compact and not very porous. Because of mechanical resistance posed by the crusts, roots of the palm tree have difficulty penetrating soil. Root depth decreases in soil and rooting takes a lateral orientation. The groundwater rise exasperates the problem, because the root zone is impoverished in oxygen, which produces asphyxiation and root rot. According to Callot et al. (1982), root development of plants is not only related to soil porosity, but also to moisture and soil resistance to penetration. Our study showed that date palm tolerated high moisture because, according to Went & Darley (1953), date palm does not have the capacity to form a fine root mass. In spite of its tolerance to moisture, description of roots of the palm trees in Arizona showed that root depth in contact with the groundwater does not exceed 2.2 m, and horizontal lengthening increases to 8.3 m. Vertical development of the roots is slowed down by the level of the water table (Simmons 1926). Bad drainage of salted water from the water table increases the concentration of salts in soil solution; soil becomes toxic and incompatible with normal date palm vegetation (Monciero 1954). Salinity of groundwater in the root zone exceeds the standard limit of the salinity threshold, estimated at 3 dS.m-1 by Ayers and Westcot (1976). Contact of the roots with very salted groundwater, causes fall of the outputs, then ecologia mediterranea – Vol. 35 – 2009 Effects of hydro-edaphic environment on the rooting of date palm (Phoenix dactylifera L.) Deglet Noor in the Ouargla Basin (south-east algeria) Figure 7 – Relationship between date palm rooting and shallow water table in areas without crusts. drainage becomes paramount under these conditions (Weeks 1957; Furr 1975). To counter all these difficulties, it is necessary to maintain the water level at approximately 1.3 m in sandy soils, possibly up to 1.5 m in silty soils (Dubost 1991). Conclusion In light of the results obtained from different soil classes, we can conclude that the shallow groundwater in the Ouargla basin constituted the main obstacle to the root development of Deglet Noor date palms in soil depth. Roots avoided both water saturated zone and mechanical resistance in crusts, causing them developping in a horizontal direction. Salinity of the groundwaters proved to be less important to rooting of the date palm compared to level of the groundwater. Long term, clogging of the soils by water can cause hypoxia problems, root rot, salinization, dropping crop yields, and, deterioration and degradation of palm plantations in certain extreme situations. Lastly, to avoid all these problems for date palm crops, it is necessary to maintain the level of the water table below root zone of date palms by using a good drainage network in the Ouargla basin. Taking into account the prevailing situation, it is advised that groundwater salinity levels be reduced by implementing suitable hydroagricultural improvements. However, it is necessary for groundwater salinity to remain above the threshold which allows the date palm to resist the bayoud disease. In crust zones, it is recommended to break soil before plantation of young date palms using subsoi- 47
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