الفهرس 
ACKNOWLEDGEMENTS
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Abstract
The area under study is located at the north-western part of El-Dakhla oasis in the western desert of Egypt and is called Gharb El-Mawhoub . It lies between latitudes 25◦ 44′ 00″ and 25◦56′ 00″ N and longitudes 28◦ 26′ 00″ and 28◦ 40′ 00″ E. The total area covers about 235.57 km2 representing about 23557 hectares. It is a part of the geological formation and includes two main formations, Dawi formation and El-Dakhla formation. The current study aims to: determine and map the major soil chemical and physical properties of the study area using geostatistical analysis, identify soil mapping units of this area, carry out the land capability and suitability evaluation for the study area, and classify the soils of this area according to Soil Taxonomy (Soil Survey Staff, 2014). According to the Digital Elevation Model (DEM) of Gharb El-Mawhoub area at the depression showed that the elevations ranged from about 87 to 179 m above sea level (asl).
Thirty-four soil profiles were selected according to the morphological observation of the area. The points of the soil profiles of the investigated area were located using Global Position System (GPS). The soil profiles representing the study area were morphologically described and sampled according to FAO (2006) and Schoenberger et al. (2012). One hundred-thirty one soil samples were collected from the profile layers for laboratory analysis. Physical and chemical analysis were performed using the standard methods. Some physical and chemical soil characteristics are produced by using geostatistical analysis tools extension available in ArcGIS 10.2.2. Soil Taxonomy was used to classify the soils of the study area into taxonomic classes starting from soil order level down to the soil family level according to Soil Survey Staff, (2014).
Land capability of the study area was done using different programs (ASLE, CERVATANA and Modified Storie Index, 2008). Land suitability of the study area was preformed using ASLE and ALMAGRA programs.
Based on the specific soil of attributes soil texture, depth and salinity, the soils of the studied area were classified to eleven mapping units as follow:
1- Coarse-textured, moderately saline, deep soils which this soil mapping unit occupied an area of about 12% of the total study area.
2- Coarse-textured, strongly saline, deep soils. That covered an area of 5% of the total study area.
3- Coarse-textured, very strongly saline, deep soils. Which covered an area of 11% of the total study area.
4- Coarse-textured, moderately saline, moderately deep soils. That occupied an area of 2% of the total study area.
5- Coarse-textured, strongly saline, moderately deep soils. The area of this unit was 6% of the total study area.
6- Coarse-textured, very strongly saline, moderately deep soils. The soils of this unit occupied 5% of the total study area.
7- Fine-textured, moderately saline, deep soils. These soils covered an area of 12% of the total study area.
8- Fine-textured, strongly saline, deep soils. That covered an area of 17% of the total study area.
9- Fine-textured, very strongly saline, deep soils. The soils of this unit represented 19% of the total area.
10- Fine-textured, moderately saline, moderately deep soils which occupied an area of about 2% of the total area.
11- Fine-textured, very strongly saline, moderately deep soils. The area of this unit covered 9% of the total study area.
The obtained results indicated that the soil coarse-textured area represented 41% while the soil fine-textured one illustrated 59% of the total study area. Most of these soils had a sandy loam, loamy sand, sandy clay loam and clay textures. The common soil structure of these soils was single grains in the surface layers of most soil profile and massive to sub angular blocky in the subsurface layers. The calcium carbonate content varied from 2.7 to 83.5%.
The gypsum content of these soils varied from 0.0 to 10%. The saturation percentage (SP) ranged from 21 to 125%. The soil reaction was (neutral to moderately alkaline) as indicated by pH values of 7.18 to 8.36. Soil salinity values ranged widely between 1.4 to 178.5 dS/m. In general, sodium was the dominant soluble cation in most soil profiles, followed by calcium, magnesium and then, potassium. However, soluble anions were dominated by chlorides in most cases, followed by sulphates and then, bicarbonates. Exchangeable cations were dominated by Ca+2 followed Mg+2, Na+ and K+. The cation exchangeable capacity (CEC) of these soils varied from 5.17 to 63.39 cmol(+) / kg-1. The exchangeable sodium percentage (ESP) of these soils ranged from 0.89 to 59.17%. The sodium adsorption ratio (SAR) values differed from 2.4 to 228.1. The organic matter content was low and their values ranged between 0.02 to 2.44%.
Soil classification was carried out according to Soil Taxonomy (Soil Survey Staff, 2014). Accordingly, the studied soils were classified into two orders, Entisols and Aridisols. Two suborders were recognized under the order Entisols namely Psamments and Orthents. Two suborders could be distinguished under the order Aridsols namely Calcids and Salids. The soils under consideration were classified according to soil taxonomy down to the family level as:
a) Sandy, mixed, hyperthermic, Typic Torripsamments; Sandy, mixed, hyperthermic, Sodic Torripsamments. b) Fine-loamy, mixed, hyperthermic, Typic Torriorthents; Loamy, mixed, hyperthermic, Typic Torriorthents; Fine-clayey, mixed, hyperthermic, Typic Torriorthents; Fine-loamy over clayey, mixed, hyperthermic, Typic Torriorthents; Coarse-loamy, mixed, hyperthermic, Sodic Torriorthents; Loamy, mixed, hyperthermic, Sodic Torriorthents; Fine-silty over clayey, mixed, hyperthermic, Sodic Torriorthents; Fine-loamy, mixed, hyperthermic, Sodic Torriorthents; Fine-clayey, mixed, hyperthermic, Sodic Torriorthents; Coarse-loamy, mixed, hyperthermic, Salic Torriorthents; Loamy, mixed, hyperthermic, Salic Torriorthents; Fine-loamy, mixed, hyperthermic, Salic Torriorthents; Fine-loamy, mixed, hyperthermic, Vertic Torriorthents; Fine-loamy over clayey, mixed, hyperthermic, Vertic Torriorthents; Fine-clayey, mixed, hyperthermic, Vertic Torriorthents. c) Sandy, mixed, hyperthermic, Sodic Haplocalcids; Loamy, mixed, hyperthermic, Sodic Haplocalcids. d) Loamy, mixed, hyperthermic, Sodic Haplosalids ; Fine-loamy, mixed, hyperthermic, Sodic Haplosalids.
Land capability and land suitability for agricultural use were evaluated by using three programs (ASLE, MicroLEIS:CERVATANA model and Modified Storie Index 2008). The results revealed that the soils of the study area according to ASLE program varied from “Excellent capability” to “non-agricultural” due to different limiting factors.
The results showed that about 12% of the evaluated soils had an excellent capability, about 21% showed a good capability, 17% were of a fair capability, 6% had a poor capability and about 44% were not suitable for the agricultural use. According to CERVATANA model, the soils of the studiy area realized two classes, moderate (S3) and marginal (N) capabilities. The results showed that about 50% of the evaluated soils had a moderate capability (S3) and about 50% showed a marginal capability (N). Considering, the parametric approach (modified Storie index 2008), the soils of the study area were of a poor capability (grade 4) representing about 50% of the total evaluated soils and non-agricultural (grade 5) that occupied an area 50% of the total evaluated soils. These findings were fully compatible with those obtained by using MicroLEIS- CERVATANA model.
According to the ASLE program, most of the studied area was considered suitable for the current land use type. It is noticed that the highly suitable (S1) area represented 14, 12, 31, 14 and 12% of the total area for wheat, cotton, sunflower, sugar beet and alfalfa crops, respectively. The suitable (S2) area was 42, 16, 28, 14, 12, 38, 25, 42, 33, 16 and 50% of the total study area for wheat, maize, watermelon, potato, soybean, cotton, sunflower, sugar beet, alfalfa, citrus and olive crops, respectively. The moderately suitable (S3) area was 12, 5, 14, 14, 6, 11 and 12% of the total study area for maize, watermelon, potato, soybean, cotton, alfalfa and citrus crops, respectively.
The results also revealed that most of the study area were considered not suitable for the current land use according to ALMAGRA model. It was noticed that only 12% of the total study area were suitable (S2) for wheat, potato, soybean, sunflower and alfalfa crop production and 14% of the total study area were suitable for cotton and sugar beet. Also, the results showed that nearly 4% of the total study area were moderately suitable (S3) for wheat, potato, soybean, sunflower and alfalfa, 16% were moderately suitable for maize and watermelon, and 2% were moderately suitable for cotton, sugar beet, citrus and olive crops production.
Comparing the results of the ASLE program and MicroLEIs-Almagra model with the current agricultural situation in the study area showed that the ASLE was more competent in evaluating the soil suitability than Almagra model. So, it could be recommended not to use Almagra model in land suitability assessment under Egyptian conditions. The results were indicated that the most suitable crops are wheat, alfalfa, cotton, sunflower, olive, sugar beet and watermelon.
Based on the results of this study, it could be recommended that:-
1- This study suggests that the ASLE program is suitable for the evaluation of land suitability for agricultural proposes due to it is compatible with many Egyptian conditions.
2- Salinity limitation can be eliminated by reclaiming these soils through leaching, especially if good quality irrigation water is available, application of gypsum and other management practices besides the proper cropping pattern that could be followed in this area.
3- Most of the farmers are capable to improve their fields through an application of organic matter and using proper fertilization program.
4- This study also showed that geostatistical analyses proved powerful analytical tools to map the horizontal variability of soil properties.