الفهرس 
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Abstract
Using aerial photo-technique and satellite images for soil mapping of Siwa oasis which is situated between longitudes 25° 16- - 26° T E and latitudes 29° T - 29° 21 N. Photo-analysis was first undertaken for the preparation of the photo-interpretation map using the stereoscope for analysis. The map was then checked in the field by the testing augers for boundary accuracy between the different mapping units. Four sample areas were selected representing the different mapping units. The soil profiles were carefully described in situ; their morphological features were recorded and samples were collected for laboratory analyses. Image processing techniques through using Landsat enhanced thematic mapper ”ETM+” were grouped into three general subjects: 1)- image enhancement. 2)- geometric correction and data fusion. And 3)- information extraction and image classification. Unsupervised classification, spectral profiles, data merge, and change detection maps were obtained.
physiographic units have been identified i.e. 1)- Sand sheets, 2)- Hummocks, 3)- Alkali flats, 4)- Overflow basins, 5)-Decantation basins, 6)- Mountain footslopes, and 7)- Hill footslopes, 8)- Marmarica plateau, 9)- Longitudinal sand dunes, 10)- Terrace sided hills, 11)- Corrosion forms, 12)- Mesas and butes, and 13)- Lakes, water bodies.
The main results that represent the characteristics of the different mapping units could be presented as follows:
1)-Sand sheets (SS), which contains many sub-units represented by high sand sheets, moderately high sand sheets, and low sand sheets. The particle size distribution results showed that, texture class is sand. Calcium carbonate content ranges between 6.2 and 14.7 %. Such values may be due to the transported residuals of limestone by wind action. Gypsum content is very low in these soils ranging from 0.001 to 1.6 %. Organic matter content is low as it ranges between 0.1 and 1.2 % reflecting Entisols characteristics. EC values are low, except in the moderately high sand sheets and increase dramatically with depth from 8 to 29 dS/m. This may be due to seepage from the adjacent lakes. pH values range between 7.8 and 8.5. CEC values range between 1.3 and 6.6 cmol/kg soil; these low values reflect the low contents of fine fractions and organic materials. Results of fertility status reveal that available nitrogen content ranges between 2.8 and 24.7 mg/kg. Phosphorus content ranges between 5.9 and 63.2 mg/kg. Potassium content ranges between 250 and 592 mg/kg.
2)-Hummocks (HA), which contains many sub-units that represented by high hummocky areas, moderately high hummocky areas, and low hummocky areas. The particle size distribution results showed that, texture classes are sandy loam, loamy sand, and sand. Calcium carbonate content ranges between 6.2 and 10.9 %. These values reflect the transported residuals of limestone by wind action. Gypsum content was less than 0.1 % in these soils. Organic matter content is low, except in moderately high hummocky areas, where it reaches 3.5 %, and such a high value may be due to the dense decayed roots. EC values range from 1.5 to 1.7 dS/m in the relatively high areas and
in the moderately high areas ranging between 25.4 and 100.7 dS/m; high salinity with other parameters meet the requirements of salic horizon. pH values range between 8.2 and 8.5. CEC values range between 3.8 and 30.3 cmol,./kg soil; these values reflect the organic materials and fine fractions content. Results of fertility status revealed that available nitrogen content ranges between 8.4 and 40.7 mg/kg. Phosphorus content is high as it ranges between 9.0 and 35.5 mg/kg. Potassium content is high as it ranges between 230 and 516 mg/kg.
3)- Alkali flats (AF), which contains two sub-units of wet sabkhas and dry sabkhas. The particle size distribution results showed that, texture classes are sand, loam, and silty loam. Calcium carbonate content ranges between 9.3 and as high as 55.3 %. These high values are reflection of dominant nodules and soft segregation of calcium carbonate found in the soil. Gypsum content ranges between 1.2 and 3.9 %. Organic matter content is high as it ranges between 1.8 and 7.4 %; these high values may be due to alluvial accumulation of fresh and humified fish and organic residuals, these residuals are coated and preserved by salts against decomposition. EC values are high as they range between 11.7 and 57.6 dS/m. pH values range between 7.8 and 8.5. CEC values in the wet sabkhas range between 50.1 and 67.8 cmol,./kg soil; these high values may be due to high values of both organic materials and fine fractions. CEC in dry sabkhas ranges between 1.5 and 8.9 cmoljkg soil; these low values reflect the low content of clay fractions. Results of fertility status revealed that available nitrogen content ranges between 9.4 and 48.2 mg/kg. Phosphorus content is high as it
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ranges between 12.10 and 19.20 mg/kg. Potassium content ranges between 189 and 750 mg/kg.
4)-Overflow basins (OB), which contains many sub-units that represented by high overflow basins, moderately high overflow basins, and low overflow basins. The particle size distribution results showed that, texture classes are sand, loamy sand, sandy loam, and loam. Calcium carbonate content ranges between 5.6 and 35.7 %. The high value may reflect the presence of residuals of limestone and shell fragments. Gypsum content is very low and ranges between traces to 0.07 %; there is no or little evidence of gypsum forms in these soils. Organic matter content ranges between 0.2 and 3.4 %. The high value was found in a cultivated soil and may be due to accumulations of organic residuals and continuous addition of organic amendments to such a soil. EC values range between 1.3 and 16.6 dS/m. pH values range between 7.4 and 8.2. CEC values ranges between 6.6 and 32.0 cmolc/kg soil, these values reflect the colloidal mineral and organic materials. Results of fertility status revealed that available nitrogen content is very low as they range between 2.8 and 19.8 mg/kg. Phosphorus content is low to high as it ranges between 1.6 and 94.0 mg/kg. Potassium content is moderate to high as it ranges between 94.5 and 855 mg/kg.
5)-Decantation basins (DB), which contains four sub-units of high decantation basins, moderately high decantation basins, moderate decantation basins, and low decantation basins. The particle size distribution results showed that, texture classes are five: sand, loamy sand, sandy loam, loam, and silty loam. Calcium carbonate content ranges between 0.9 and 72.2 %. The high value may be reflecting the presence of transported
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limestone residuals by wind action; calcic horizon was identified in high decantation basins. Gypsum content ranges between 0.001 and 6.3 %; gypsic horizon could be identified in high decantation basins. Organic matter content ranges between 0.1 and 2.0 %. EC values are very low to very high as range between 0.5 and 150.0 dS/m. pH values range between 7.6 and 8.5. CEC values range between 2.5 and 35.7 cmolc/kg soil. Results of fertility status revealed that available nitrogen content ranges between 2.8 and 31.8 mg/kg. Phosphorus content ranges between 5.8 and 64.8 mg/kg. Potassium content ranges between 131.5 and 773 mg/kg.
6)-Mountain footslopes (MF). The particle size distribution results showed that, texture class is sand. Calcium carbonate content ranges between 15.0 and 52.7 %. These high values reflect the native of the original geological formation. Gypsum content is very low in these sandy soils as it ranges between 0.1 and 0.3 %. Organic matter content ranges between 0.1 and 0.9 %. EC is low in the surface layer and increases sharply to very high values in the subsurface one; and it ranges between 3.4 and 143.0 dS/m showing that, the surface layer may have been transported as a soil mantle. pH values vary between 7.2 and 7.9. CEC varies between 3.6 and 4.2 cmole/kg soil. The data of macronutrients revealed that, available nitrogen content ranges between 22.7 and 25.5 mg/kg. Available phosphorus content ranges between 4.6 and 5.0 mg/kg. Available potassium content is high as it varies between 441 and 725 mg/kg.
7)-Hill footslopes (HF). The particle size distribution results showed that, texture classes are sand and loamy sand. Calcium carbonate contents range between 12.9 and 27.7 % reflecting the
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limestone nature of the original geological formation. Gypsum content varies between 0.1 and 0.4 %. Organic matter content ranges between 0.2 and 0.4 %. EC is very high as it ranges between 43 and 108 dS/m meeting with the other parameters for the requirements of a salic horizon. pH is 8.4 for the two layers of the representative soil profile. CEC ranges between 4.1 and 9.2 cmoljkg soil reflecting the poor fertility status of such soils. The data of macronutrients revealed that, available nitrogen content ranges between 12.0 and 16.1 mg/kg. Available phosphorus content ranges between 16.0 and 17.7 mg/kg. Available potassium content ranges between 931 and 943 mg/kg.
The soil classification using the Soil taxonomy system (USDA, 1999) is applied up to the level of the sub-great group for mapping units, while to family level for the profile description. Results are as follow:
1-Sand sheet (S).
- High sand sheet (S1).
Profile 6: Typic Torripsamments.
Profile 9: Typic Torripsamments.
- Moderately high sand sheet (S2).
Profile 18: Typic Torripsamments.
- Low sand sheet (S3).
Profile 10: Typic Torripsamments.
2-Hummocky area (II).
- High hummock (H1).
Profile 5: Typic Torripsamments
- Moderately high hummock (H2).
Profile 7: Typic Haplosalids.
- Low hummock (H3).
Profile 11: Typic Torripsamments.
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3-Alkali flat (A). - Wet sabkhas (Al). - Dry sabkhas (A2).
Profile 13: Calcic Aquisalids. Profile 14: Calcic Aquisalids.
4-Overflow basins (0).
- High overflow basins (01).
- Moderately high overflow basins (02).
- Low overflow basin (03).
rofile 1: Typic Torripsamments. rofile 17: Typic Torripsamments. rofile 2: Typic Torripsamments.
rofile 3: Typic Psammaquents.
5-Decantation basins (D).
- High decantation basins (D1).
- Moderately high decantation basins (D2).
- Moderate decantation basins (D3). - Low decantation basins (D4).
rofile 12: Gypsic Haplosalids. rofile 16: Typic Haplocalcids. rofile 15: Typic Haplocalcids. rofile 22: Typic Torripsarnments.
ofile 4: Typic Torripsamments. rofile 21: Typic Aquisalids. rofile 8: Typic Aquisalids.
rofile 23: Typic Psammaquents.
6-Mountain footslope (M).
rofile 20: Lithic Haplocalcids.
7-Hill footslope (Hi).
rofile 19: Duric Haplosalids.
According to the classification of USDA (1954), surface and ground water of Siwa could be assessed as follows:
(1)- The surface water of lakes have the order of very high salinity and very high sodium hazards (C4-S4). EC ranges between 249.0 and 475.8 dS/m, and SAR ranges between 69.2 and 2201.0, meanwhile water of the field canals are medium or very high salinity hazards; and low to very high sodium hazards
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(C2-S1, C4-S2, C4-S3, C4-S3, and C4-S4), EC ranges between 0.4 and 6.5 dS/m, and SAR ranges between 1.4 and 11.4. The dominant salt in these water seem to be sodium chloride.
(2)- The total soluble salts of ground water reveal that the water of wells are medium or high salinity hazards; and low to very high sodium hazards (C2-S1, C4-S2, C4-S3, and C4-S4). EC ranges between 0.4 and 4.4 dS/m, and SAR ranges between 1.5 and 14.5, meanwhile the water of springs are very high salinity as well as sodium hazards (C4-S4), EC ranges between 4.9 and 15.4 dS/m, and SAR ranges between 12.4 and 42.0. The dominant salt in these water seem to be sodium chloride.
The final goal of this study is attempting to mach soil characteristics or qualities of the different mapping units with the requirements of some selected crops to identify the land suitability for these selected crops (Alfalfa, barley, beans, green paper, maize, olives, onion, soya, sugarcane, tomato, and wheat). The conclusions reached in this respect are a wide variety in mapping units due to some limiting factors. The most important limiting factors which governed the land suitability for most crops are as follow:
Physical characteristics (S)
- Texture / Structure
- Soil depth (cm)
- CaCO3 (%)
Fertility (F)
- CEC (cmolc/kg soil) c
- pH (in soil suspension) p - Organic carbon 0.0 (%) o