«GEOMORPHIC FEATURES AND SOIL FORMATION OF ARID LANDS IN NORTHEASTERN JORDAN GEOMORPHOLOGISCHE MERKMALE UND BODENBILDUNG IN TROCKENGEBIETEN IM ...»
Archives of Agronomy and Soil Science,
December 2004, Vol. 50, pp. 607 – 615
GEOMORPHIC FEATURES AND SOIL FORMATION
OF ARID LANDS IN NORTHEASTERN JORDAN
GEOMORPHOLOGISCHE MERKMALE UND BODENBILDUNG
IN TROCKENGEBIETEN IM NORDOSTEN JORDANIENS
S. A. KHRESATa,*, Z. RAWAJFIHa, B. BUCKb and H. C. MONGERc
Jordan University of Science and Technology, Irbid-22110-Jordan; bUniversity of Nevada Las Vegas, Las Vegas Nevada, USA; cNew Mexico State University, Las Cruces, New Mexico, USA (Received 31 March 2004) Arid and semiarid lands occupy about one-third of the Earth’s land surface. Interpretation of soil formation and geomorphic features of arid lands is needed to assess their soil ecological potentials, limitations, problems and management needs. The objective of this paper was to study the geomorphic features and soil formation of the arid lands in northeastern Jordan, to provide information that could be used by land managers in the study area and other arid land areas. Five representative soil pedons were excavated and described in the ﬁeld.
Soil samples from each horizon per pedon were taken to the laboratory for chemical and physical analyses.
Geomorphic features of the area were also studied. Most of studied land surfaces are plains where eolian deﬂation has exposed loose gravels consisting predominantly of pebbles forming desert pavements. Desert pavements cover most of the land surface, excluding the mud playas, and are composed of basalt clasts.
The accumulation of calcium carbonate and gypsum within these soils create problems for their agricultural development. Accumulation of eolian ﬁne-grained silt has resulted in the formation of a vesicular horizon. The climatic variations during the late quaternary and the late Holocene periods contributed to the development of the desert pavement and the vesicular horizons. Clay illuviation and argillic horizon development within these soils is assumed to be a relict feature from more humid climates during the Quaternary. Sustainable agricultural development of such arid lands may not be easy. In general, these soils have high erodibility, high runoﬀ generation potential, high susceptibility to seal and crust formation, poor water-holding capacity, pedon hardening and structural instability.
Keywords: Desert pavement; Gypsum; Calcium carbonate; Playa; Land management
INTRODUCTIONArid and semiarid lands occupy about one-third of the Earth’s land surface. They are sources and sinks for atmospheric CO2, sources and sinks of global dust, and substrate that support a high biodiversity of plants and animals. The arid and semiarid lands of *Corresponding author: Department of Natural Resources and the Environment, Jordan University of Science and Technology, P.O.B. 3030, Irbid-22110-Jordan. E-mail: email@example.com ISSN 0365-0340 print; ISSN 1476-3567 online # 2004 Taylor & Francis Ltd DOI: 10.1080/
Jordan, locally known as the Badia, are one of the major dry areas in the world (Allison, 1997). The Badia encompasses a wide and signiﬁcant part of Jordan. It covers an area of approximately 72 600 km2, which constitutes 81% of the total area (89 400 km2) of Jordan (Figure 1).
The Jordanian Badia is a part of the ‘Mediterranean Sahara’ because it is less arid than Arabian or African Sahara and has a smaller diurnal temperature range with all the sparse precipitation concentrated in the winter months. It is classiﬁed as a semiarid to arid steppe environment and falls in the arid climate zone (Dutton et al., 1998).
The greatest part of eastern Jordan is desert, exhibiting the land forms and other features associated with aridic environments. Land use in the Badia region is mainly applied for agriculture (rainfed or irrigated), animal husbandry, and mining. These areas have also been important grazing lands for the local population over the years (Juneidi and Abu-Zanat, 1993).
Limited understanding of the processes involved in the formation of these arid lands makes it very diﬃcult to fully utilize their soils in a sustainable manner. This research is a part of continued soil and geomorphic research eﬀorts to investigate and evaluate the processes of arid lands formation. The objective of our research, therefore, is to study the geomorphic features and soil formation of these soils to provide information that can be used by land managers in this area and in other areas with similar conditions.
MATERIALS AND METHODSThe environmental conditions associated with the Badia arid environment, including low or sporadic moisture availability and high temperatures, are not ideal for plant growth. The vegetation of the Badia is sparse but enormously diverse. Despite that over 300 plant species have been identiﬁed, most of the Badia area is bare and lacking of vegetation cover (Cope and El-Eisawi, 1998; Dutton and Shahbaz, 1999). The grassland steppe; Artemesia herba-alba, Poa sinaica and Carex pachystylis form a distinctive, shallow rooting turf, with a root mat zone that provides protection against water and wind erosion. In overgrazed areas, these protective mats disappeared leaving soil more susceptible for erosion.
The Badia region falls in an arid climatic zone. Diurnal temperature ranges from mean minimum of 108C to mean maximum of 24.58C with mean daily temperature of
17.58C. Precipitation is variable both spatially and temporally. Occasional heavy showers cause surface run-oﬀ and soil erosions that decrease the amount of water stored in the soil. A high evaporation demand caused by strong wind gusts and high temperatures exceed the amount of precipitation and therefore, decreases the water available for plant growth. Five soil pedons of diﬀerent land use and diﬀerent parent material types were chosen for the study. Those were excavated and sampled following Schoeneberger et al. (1998). Soil colour was deﬁned using a Munsell Soil Colour Chart.
Undisturbed soil samples were taken for particle size analysis.
The pedons were described according to Guthrie and Witty (1982). The bulk soil samples were air dried, crushed with a mortar and pestle, and sieved to remove coarse (4 2 mm) fragments. Particle size distribution was determined by the hydrometer method (Gee and Bauder, 1986). Organic matter was determined using the WalkleyBlack method (Nelson and Sommers, 1982). Soil pH was measured on 1 : 1 soil : water suspensions (McLean, 1982). Calcium carbonate (CaCO3) equivalent values were obtained using the acid neutralization method (Richards, 1954). Gypsum content values were obtained by precipitation with acetone method (Richards, 1954). Chemistry data can be found in Rawajﬁh et al. (2002). Descriptions of the ﬁve pedons and their classiﬁcation according to the Soil Survey Staﬀ (1998) and FAO (1998) systems are given in Table I.
RESULTS AND DISCUSSIONGeology and geomorphology Most of study area land surfaces are plains where eolian deﬂation has exposed loose gravels consisting predominantly of pebbles but with occasional cobbles forming desert pavement, a sheetlike surface of rock fragments that remains after wind and water have removed the ﬁne particles. Desert pavements cover most of the land surface excluding the mud playas (locally known as Marabs) and are composed of basalt clasts that range in size from cobble to granule near pedons 1 and 2; and chert clasts that range in size from cobble to granule in pedon 5. The predominate geomorphic features found near pedons 1, 2, 3, and 4 are desert pavements composed of basalt clasts, and outcrops of other more recent sediments with varying degrees of development. These desert pavements have not been well studied, however the data in this study suggest that these 610 S.A. KHRESAT et al.
TABLE I Classiﬁcation (USDA; FAO) and Morphological characteristics of the studied sites*
* The abbreviations are according to the Soil Survey Staﬀ (1951).
desert pavements have formed through accretion of ﬁne-grained eolian sediments similar to the ﬁndings of McFadden et al. (1987) and Anderson et al. (2002).
The soil parent material in the study area is diverse. Pedons 1, 2, 3 and 4 are located in areas of geologically recent (5 13 million years) basalt ﬂows and the parent materials for these soils include alluvium and basalt fragments. Parent material for pedon 5 is limestone, marl, and chert alluvium.
Many of the surﬁcial basalt clasts near pedons 1 and 2 show extensive eolian abrasion and are classic examples of ventifacts. In contrast, the chert clasts near pedon 5 do not show these features. The desert pavement plays an important role in the geomorphic, hydrologic, pedologic and ecosystem processes. Manganese, iron oxides, and hydroxides deposited on pebbles and cobbles resulted in dark black mineral staining (Desert varnish) on surfaces of the study area rocks.
Mud flats (Marabs) These are sediment deposits which are parallel to the line of wadis and are known locally as Marab (also known as playa). Marabs (pedons 1 and 2) form where wadis exhibit relatively large discharges and are able to spread out across a wide area. There may be incision along the Marab, particularly towards the down-stream end where water becomes channel bounded.
Pedons 3 and 4 are located in a large low lying Marab (playa), forming an alluvial depression with highly gypsiferous soils. This area receives primarily sediments
SOIL FORMATION AND GEOMORPHIC FEATURES OF ARID LANDS 611composed of limestone and chert from many of the major wadi systems draining from various directions. It forms a major part of the Azraq basin (Northeastern Badia). The drainage network is coarse and incised. In many areas, soil relief provides no eventual outlet to the sea, so that sedimentary deposits accumulate in basins (e.g., Azraq basin).
In these basins, moisture evaporates leaving an accumulation of deposits such as anhydrite, gypsum, halite, calcite and other types of salts.
Surﬁcial deposits in the study area cover most of the land surface. Coarse-grained granules, pebbles, cobbles and boulders of diﬀerent lithologies are present above the modern day drainage levels in the area. The gravels are poorly sorted, angular to subangular, and sub-rounded. The geology varies greatly from basalt in pedons 1 and 2 to chert in pedons 3, 4 and 5.
Soil genesis The formation, types, and properties of the studied soils are closely controlled by parent material, topography and climate. The major groups broadly follow geological variations, in particular the spatial distribution of desert pavements. While there are some characteristics that may be attributed to the present-day precipitation gradient, most of the diﬀerences seen in these pedons are the result of diﬀerences in their parent material and/or the diﬀerent geomorphic processes experienced during the Quaternary.
Diﬀerent investigations suggested that climate in this zone had changed several times during the Quaternary.
Mineralogical studies (Irani, 1992) showed that the clay in this region has been subjected to extreme weathering conditions in previous wetter climates. The last of these changes was the present aridic climate (Rognon and Williams, 1977). The last episode of climatic changes is responsible for the development of unfavourable soil properties that accelerated the degradation of many plant species. Coupled with the eﬀect of continuing drought incident, removal of plant cover was greatly enhanced (Taimeh, 1991). The climatic variations were as follows: pluvial period (40 000 – 20 000 years B.P.), dry interval (20 000 – 13 000 years B.P.), minor pluvial period (13 000 – 7000 years B.P.) and dry interval (7000 years B.P. – present).
Accumulation of eolian ﬁne-grained silt has resulted in the formation of a vesicular horizon in pedons 2 and 3, and possibly in pedon 1. In addition, much of the silt, clay, carbonates, and soluble salts that have accumulated in the studied soils could be attributed to incorporation of eolian materials rather than to chemical weathering of soil parent materials.
Silt content increases towards the surface indicating eolian activity. However, clay content increased with depth indicating that enough illuviation of clay occurred so that argillic horizons were formed in some pedons (Table II). The presence of palygorskite and plagioclase at the surface indicates the weakness of chemical weathering and is attributed to eolian activities. High silt content leads to unfavourable soil properties (structure and crusting) and eventually unfavourable plant growth conditions.
The geomorphic position of the landscape greatly aﬀects the soil characteristics. The main soil divisions are related to topographic and weathering diﬀerences. Pedons 1 and 2 have been strongly inﬂuenced by argillipedoturbation in the upper horizons. The presence of maﬁc rocks (basaltic) and the deposition of alluvium in Marabs favour the formation of smectites (Irani, 1992). The deep argillic horizons are considered to be inherited from past more humid climates. These soils exhibit cracks, but because of the 612 S.A. KHRESAT et al.
TABLE II Particle-size distribution (carbonate free) of the studied soils
low shrink-swell potential and the size of cracks (less than 3 cm), these soils did not meet the requirements to be classiﬁed as Vertisols. The soil moisture regime for these two pedons is more properly xeric-aridic transitional rather than truly aridic since occasional ﬂoodwaters are carried into these basins. This moisture regime results in the accumulation of calcium carbonate and gypsum in these soils. The mean organic matter in the studied soils is less than 1.0% (Table III). The low soil organic matter content resulted in the presence of ochric epipedons in all of the studied soils. Therefore, a wide range of soils (Gypsiols, Calcisols) occur on older surfaces.