الفهرس 
Introduction
Soils of El-Dakhla OasisOnly 14 pages are availabe for public view
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Abstract
Potassium is one of the three most essential nutrient elements that are usually included in fertilizer formulation. It plays significant role in plant physiological processes. Soil K occurs in different forms of solution, exchangeable, non-exchangeable or fixed and mineral or structural K forms which are in dynamic equilibrium. Solution K is readily taken up by plants and microbes or lost via leaching. Exchangeable K represents the portion electrostatically bound to the outer surface of clay minerals and humic substances and is readily exchangeable with soil cations and is available to plants. Non-exchangeable or fixed K constitutes the fraction held between adjacent tetrahedral layers of dioctahedral and trioctahedral micas, vermiculite and intergrade minerals and is sparingly or moderately available to plants while mineral or structural K includes the fraction that is bound within crystal structures of soil mineral particles.
The study area is located in El-Dakhla oasis between longitudes of 29 º 21’ 39” and 28 º 19’ 30 ” E and latitudes of 25º 27` 04 ” and 25 º 52’ 11” N latitude. It is situated on both sides of the road that extends from El-Kharga city to in the southeast to El-Frafra city in the northwest with about 150 km length and a width of about 20 to 40 km. It lies 490 km far from Assiut city. The total study area was about 4500 km2. Thirty one transects were a located across the main road starting from Tenedah in the east to Gharb El-Mawhob in the west to cover the study area. Sixty soil sites (locations) were selected in the study area depending upon the width distance between the road and the limestone plateau .The distance between two consecutive transects varied from 5 to 7 km and the distance between two consecutive sites differed from 1 to 1.5 km. All sites were localized using the global position system (GPS). from each site, two soil samples were collected from the surface (0-30 cm) and subsurface (30-60 cm) layers with a total number of 120 soil samples taken from 60 soil sites.
This study aimed to investigate some of the physical and chemical properties of El-Dakhla oasis soils and evaluate the available and forms of K in these soils as well as the relationship between them and the soil properties. The obtained results could be summarized as follow:
1. Soil Physical and Chemical characteristics
The fine texture was dominant in the studied samples and its percentage increased in subsurface layers compared to the surface ones. Generally, 6.67, 10.00, 11.67, 13.33, 15.00, 23.33 and 20.00 % of the total surface (0-30 cm) soil samples as well as 6.67,15.00, 20.00, 1.67, 16.66, 13.33 and 26.67 % of the subsurface (30-60 cm) soil samples showed sand, sandy clay, sandy clay loam, clay loam, sandy loam, loamy sand and clay textures, respectively.
The saturation percentage (SP) values of the investigated soil samples differed from 24.00 to 138.00% without a regular trend with soil depth. About 27.50% of these samples showed SP values of higher than 75% while 26.66% of them exhibited SP values of less than 40%. In addition, about 7.50 % of the soil samples had a SP value above 100% reflecting high contents of smectite clay minerals.
The investigated soil samples displayed a wide variation of calcium carbonate (CaCO3) content that extended from 3.06 to 592.40 with an average value of 97.24 g/kg. About 3.34 % of the surface soil samples were slightly calcareous (0 and 20 g/kg), 70.00% were moderately calcareous (21 to 100 g/kg), 23.33 % were strongly calcareous (101 to 250 g/kg) and 3.33% were extremely calcareous (> 250 g/kg). However, in the subsurface soil samples, 8.33% of them were slightly calcareous, 66.67% were moderately calcareous, 20.00% were strongly calcareous and only 5.00% were extremely calcareous. The calcium carbonate content tended to increase towards the limestone plateau in the north-west direction.
The organic matter (OM) content of these soil samples varied from 0.10 to 46.40 g/kg with an average value of 10.70 g/kg and decreased with soil depth. About 43.33% of the surface soil samples had an OM content of less than 10 g/kg and 56.67% of them showed an OM level of higher than 10g/kg. However, 76.67% of the subsurface samples exhibited a soil OM content of less than 10g/kg and 23.33% of them had an OM content of greater than 10g/kg.
The soil pH value ranged from 7.09 to 8.10 The soil pH values of non-cultivated areas were higher than those measured in the cultivated ones with no regular trends with soil depth. About 36.67 % of the surface soil samples were neutral (pH value 7.5) and 55.00% of them were midly alkaline (pH value 7.5-7.8) and 8.33% were moderately alkaline (pH value 7.8-8.4). In the subsurface soil samples, about 26.67% of them were neutral, 60.00% were midly alkaline and 13.33% were moderately alkaline.
The CEC of the investigated soil samples varied from 4.00 to 87.33 cmol+/kg with an average value of 25.52 cmol+/kg. The highest CEC value was recorded for a clay loam textured soil sample while the lowest value was found in a sandy-textured soil sample. About 13.33% of the surface soil samples had low CEC values (6-12 cmol+/kg), 48.33% showed moderate CEC values (12-25 cmol+/kg), 20.00% exhibited high CEC values (25-40 cmol+/kg) and 18.33% were very high values (> 40 cmol+/kg). On the other hand, only 1.67% of the subsurface soil samples showed very low CEC values, 13.33% of them had low CEC, 36.67% displayed moderate CEC, 33.33% exhibited high CEC and 15.00% were of very high CEC values.
The electrical conductivity of the saturated soil paste extract (ECe) of these samples ranged from 0.59 to 172.70 with an average value of 15.39 dS/m. About 23.33, 21.67 and 20.00% of the surface soil samples were non-saline (< 2 dS/m), slightly saline (2-4 dS/m) and moderately saline (4-8 dS/m), respectively, while, 16.67 and 18.33% of them were strongly saline and very strongly saline, respectively. On the other hand, 18.33, 23.33, 25.00, 11.67 and 21.67% of the subsurface soil samples were non-saline, slightly saline, moderately saline, strongly and very strongly saline, respectively.
The mean value of the soluble cations in the saturated soil-paste extract of the studied samples could be arranged in the decreasing order of Na+ > Ca2+ > Mg2+ > K+ with irregular trends with soil depth in most cases. On the other hand, the soluble anions in these samples could be ranked in the decreasing order of Cl- > SO42- > HCO3-.
The sodium adsorption ratio of the saturated soil-past extract (SARe) values varied from 0.13 to 167.97 with an average value of 11.64. About 80% of the surface soil samples of this area were non-sodic (SARe<13) while, 20.00% of them were sodic (SARe >13). Concerning the subsurface soil samples, 68.33% of them were non-sodic and 31.67% were sodic. In most cases the SARe increased with soil depth.
2. Soil Available K content.
The available K varied from 10.10 to 3333.80 mg/kg with an average value of 816.39 mg/kg in the surface soil samples and from 90.40 to 2910.40 mg/kg with an average value of 784.51 mg/kg in the subsurface ones.
In most cases, the available K values in surface samples were lower than that of the subsurface ones and increased toward the south-east direction. About 1.66% of surface soil samples were very low (<86 mg/kg), 1.66% were low (86-150 mg/kg), 16.66% were moderate (151-250 mg/kg), 21.68% were high (251-450 mg/kg) and 58.34% were very high (> 450 mg/kg) in the available K. However, about 8.33, 15.00, 13.33 and 63.34% of the subsurface soil samples showed low, moderate, high and very high available K content, respectively.
The course-textured soils exhibited lower levels of available K while the fine-textured soils had higher available K contents. On the average basis, the soil texture classes could be arranged depending upon their available K contents in the descending order of clay> clay loam> sandy clay loam> sandy clay> sandy loam> loamy sand> sand.
3. Distribution of Potassium Forms in the studied soils
Distribution of these K forms (water soluble, exchangeable, non-exchangeable and mineral K) in soils depends upon parent material, degree of weathering, soil texture, K added through manures and fertilizers and losses due to the removal by crops and/or leaching as well as soil erosion. Soils having the same content of the total K may vary widely in potassium availability and plant uptake depending upon the distribution of these different K forms. Thus, it is necessary to study the distribution and characterization of different forms of K as affected by some properties of the studied soils.
3.1-Effect of soil texture
The mean level of the soluble K as a percentage of the mean total K in the studied soil textures decreased in the order of sandy loam> clay loam> clay > sandy clay loam > loamy sand > sandy clay > sand. It ranged from 1.48 to 3.46% of the mean total K in these textured soils.
The mean percentage of exchangeable K fraction of the investigated soil samples ranked in the soil textures in the decreasing order of clay loam> sandy loam> clay> loamy sand > sandy clay> sandy clay loam> sand. This respective K fraction differed from 1.48 to 9.22% of the mean total K in these textured soils.
The mean level of the non-exchangeable K fraction in the studied soil textures could be arranged in descending order of sandy loam> clay loam> sandy clay≈ clay> sand > sandy clay loam> loamy sand. Also, it varied from 10.57 to19.19 % of the mean total K in these textured soils.
The mineral K level in the studied soil textures decreased in the order of sandy clay loam> sand > loamy sand > sandy clay> clay> clay loam> sandy loam. This fraction extended from 71.44 to 84.62% of the mean total K.
The average values of the total K of the investigated ranked in the descending order of sandy clay> clay > sandy clay loam> sandy loam > sand> clay loam> loamy sand which had mean values of 17.40, 15.30, 13.20, 12.30, 11.70, 11.27, and 6.58 g/kg, respectively
Generally, all soil textures that represented the studied soils showed the same K form descending order of mineral > non-exchangeable K > exchangeable K > soluble K except the sand texture which displayed the same trend but with nearly equal levels of soluble and exchangeable K forms.
3.2-Effect of soil depth
On the average basis, the mean percentages of the soluble, exchangeable, non-exchangeable and mineral K forms in the surface layer of the studied soils were 1.82, 4.37, 13.80 and 80.01 % of the mean total K, respectively. The respective forms represented 2.35, 5.10, 16.32 and 76.23 % of the mean total K, respectively, in the subsurface layer of these soils.
The mean level of soluble K in the subsurface layer is higher than that of the surface one which decreased from 2.35 to 1.82%. However, the exchangeable K mean level in the surface layer is lower than that of the subsurface one which increases from 4.37 to 5.10%.
A reduction was also found in the mean level of the non-exchangeable K fraction in the subsurface layer compared to that of the surface one which decreased from 16.32 to 13.80%. The opposite trend was noticed for the mineral K fraction which its mean level decreased from 80.01 to 76.23% in the surface and subsurface layers, respectively.
3.3- Effect of soil CaCO3 content.
The mean K level of the soluble, exchangeable and non-exchangeable K levels increased from 1.31, 4.56 and 10.91% of the mean total K, respectively, in the soil samples of a low CaCO3 content (<100g/kg) to 5.02, 8.11 and 21.33%, respectively, in the samples having a high CaCO3 content (>100g/kg). However, the mineral K decreased from 83.22% in the soil samples that had a low level of CaCO3 to 65.54% in the samples of a high CaCO3 level.
The K forms had the same descending order of mineral K> non-exchangeable K > exchangeable K > soluble K in both low and high calcium carbonate soil samples.
3.4-Effect of soil salinity
The mean level of soluble K fraction increased with increasing the soil salinity from 1.37% in the slightly saline (ECe < 4 dS/m) to 1.92% in the moderately saline (ECe 4-8 dS/m) and 2.53% of in the highly saline (ECe > 8 dS/m) soil samples.
The exchangeable K fraction, slightly decreased from 4.88% in the slightly saline to 4.80% in the moderately saline soil samples, respectively, and then increased in the highly saline soil samples to reach 5.49% of the mean total K.
Moreover, the mean values of the non-exchangeable K fraction increased from 12.73% in the slightly saline to 18.92% in the moderately saline soil samples. Then it decreased in the highly saline soil samples to reach 13.13%.
The mean level of mineral K decreased from 81.02% in the slightly saline to 74.36% in the moderately saline soil samples and then increased in the highly saline soil samples reaching 78.85%.
Generally, all defferent saline soil samples had the same descending order of mineral K > non-exchangeable K > exchangeable K > soluble K.
3.5-Effect of soil organic matter (OM) content.
The solubl