Karak Resources Project

The Karak Plateau is located about 75 km south of Amman, the capital of Jordan, and encompasses approximately 500 square miles with a wide variety of soils and landscapes. This wide variety of soils and landscapes has resulted from the complex geologic history, climatic variation, and other environmental factors. The northern and southern boundaries of the Karak Plateau are the Wadi al-Mujib and Wadi al-Hasa, respectively. The western boundary is composed of the steep slopes adjacent to the Dead Sea, and the eastern boundary is the desert highway.

The satellite image of the study area shows the Dead Sea on the western side of the image and the pronounced dissection of the landscape on the north and south are Wadi al-Mujib and Wadi al-Hasa, respectively.

The objectives of the environmental study on the Karak Plateau are: (1) acquaint students to the geography of a portion of Jordan, (2) get an appreciation for soils and landscapes of semi-arid and arid regions and factors influencing soil variability, and (3) understand the challenges of this environmentally sensitive region.

Regional Soil Characteristics

Soil and landscape variability is generally ascribed to Jenny's (1941) factors of soil formation; these factors include climate, vegetation, topography, geology, and length of time that these factors have been operating on a given parent material. The effects of man's activities have sometimes been added as another factor influencing soils. Erosion resulting from tillage is a primary impact on man's effect on soils and landscapes. The following brief discussion outlines the major characteristics of the factors of soil formation for the Karak Plateau.

Climate

The Karak Plateau has a Mediterranean climate with mainly winter precipitation and very dry summers; the precipitation from May through September may be negligible at many stations on the plateau. The present-day precipitation ranges from 325-350 mm in the more humid highlands on the western side of the plateau to <100 mm in the eastern part of the plateau. The average annual temperature ranges from 15-20 degrees Celsius. The precipitation factor is important in soil development as a result of water's impact on mineral weathering and leaching action through geologic materials. This is one reason for the generally shallow nature of soils in desert environments. Older soils (>20,000 years) in arid regions, however, may reflect past climatic patterns and contrast greatly with soils developed under present climatic conditions; this was observed on some soils in the Karak Plateau.

Vegetation

The native vegetation of the region probably included both forests and grasslands. Although there is considerable debate on the native vegetation of the Karak Plateau, many investigators believe forests did occur in the more humid areas while brush and grass occurred in the drier areas. Some reforestation is taking place on the plateau with satisfactory results. The native vegetation, whether forests or grasslands, can have a profound influence on the soils; soils formed on grassland or prairie have a deeper topsoil and higher organic matter than forest soils. The roots of prairie type vegetation contributes a great deal of organic materials to surface horizons. In soils of desert regions, the vegetation is sparse and thus the soils will have low organic matter.

Geology

The dominant rock types on the Karak Plateau are sedimentary, with limestone being the most common. Although the limestone was formed during the Cretaceous period (80 million years ago), soils are generally very shallow on the limestone as a result of erosion of landscapes during uplift and crustal disturbance. The shallow soil profile in [Figure 3 ] is formed in limestone colluvium. Other rock types include sandstone and shale (both sedimentary rocks), and basalt; volcanic activity during the Pleistocene and probably earlier resulted in basalt flows on some landscapes. The most important soil-forming geologic material, however, is loess; this is wind-deposited silty sediment that generally results in a very productive soil given an environment to develop deep soils. Soils in our productive U.S. "cornbelt" are commonly formed in loess. A major limitation of loessial soils, however, is the potential erosion; these soils are very erodible where slopes are greater than 3 to 5%. Even with the limited rainfall on the Karak Plateau, erosion was still deemed to be a major problem on the more sloping landscapes.

Topography

The shape and magnitude of slopes have a major impact on soil formation. On steep slopes (>20%), many soils are shallow because natural erosion removes soil more rapidly than weathering can break down the geologic materials. The steep slopes of Wadi al-Mujib, for example, have resulted in many shallow soils in the region. On the more level landscapes of the loess plains (e.g. Units 2, 3, 4, and 8), the soils have a thickness of 1 to 2 meters; this is related to the topography and the original thickness of the loess deposits. The loess thickness decreases from west to east; thus, the loess is quite thin and discontinuous in the desert region (Unit 12).

Time

The length of time that a parent material has been weathering is a major factor in the morphology of soils. Alluvial areas, for example, may contain sediments deposited in the last 200-500 years and these soils would show minimal weathering effects. On the other hand, soils formed in loess that was deposited >40,000 years ago and on nearly level stable surfaces would have developed a profile exceeding 1-2 meters. Figure 4 [Photo ] illustrates a soil profile developed under the conditions above. In addition, the climatic factor in the past 40,000 years has fluctuated widely with rainfall greatly exceeding the limited amount normally expected in today's conditions. As weathering time increases, the amount of clay in the profile will increase, there will be greater iron translocation, and increased leaching of soluble constituents in soils (e.g. Na, Ca).