الفهرس 
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Abstract
Soil phosphorus is often the potential limiting nutrient in many arid and semiarid regions. Its level is varying as the results of differences in weathering intensity and composition of parent materials. Total soil P is often a useful parameter of soil phosphate capacity. Available phosphorus that is the fraction which is adsorbed on soil surfaces represents readily extractable P held on the soil surface components. In generally, soil phosphorus occurs in several chemical forms as organic, inorganic and soil solution phosphorus. The availability of P for plant utilization is not a function of its concentration in soil, but rather on the rate of its release from soil surface into soil solution.
Soil P fractionation is a technique which uses a series of chemical extractants to sequentially remove a various chemical P forms. Many different soil P fractions have different solubility and the amount of each one depends on several soil characteristics, such as soil chemical, physical properties and geneses. Adopting appropriate soil phosphorus fractionation scheme to study the forms, transformation, and availability of soil phosphorous is critical for understanding soil phosphorus supply and its losses.
The study area located in El-Dakhla oasis between longitudes 28o 30’ 00 ” and 29o 47’ 00” E and latitudes 25o 20` 00 ” and 26o 00’ 00” N. It is situated on both sides of the road that extends from El-Kharga city to the southeast El-Frafra city in the northwest with about 150 km length and a width of about 20 to 40 km. The total study area is about 4500 km2. Thirty one transects, were designated across the main road of El-Kharga to El-Frafra. The distance between two consecutive transects differed from 5.0 to 7.0 km. Sixty sites were selected and localized depending upon the length distance of each transect on both sides of the road using the global positioning system (GPS). The distance between two consecutive soil sites on the same transect changed from 1 to 1.5 km. Twenty hundred soil samples were collected from the surface (0-30 cm) and subsurface (30-60 cm) layers of all sites using an auger and packed in nylon bags. Some soil physical and chemical properties as well as the available phosphorus were determined in these soil samples. Soil phosphorus forms were sequentially fractionated (resin-P, NaHCO3-Pi, NaHCO3-Po, NaOH-Pi, NaOH-Po, HCl-P and residual-P) according to the modified Hedley method.
The current study aims to investigate the physical and chemical properties, of El-Dakhla oasis soils, assess their phosphorus forms and examine the relationship between these phosphorus forms and the soil properties. The obtained results could be summarized as follow:
1. Soil Physical and Chemical characteristics
• Most of the soil samples were fine to medium in texture since 52.50% of the total soil samples had a clay content that was > 30% with irregular trends in the different transects or sites. The data also showed that 4.17, 15.83, 10.0, 0.83, 0.83, 25.83, 7.50, 9.17 and 25.83 % of the total soil samples had sand, loamy sand, sandy loam, loam, silt loam, sandy clay loam, clay loam, sandy clay and clay textures, respectively.
• The saturation percentage (SP) of the soil differed from 19 to 140% in an irregular trend with soil depth. Twenty nine percent of the total soil samples revealed SP values of higher than 80%.
• There was a wide variation in the CaCO3 content of the studied soils. It varied from 3.8 to 575.7 g/kg with an average value of 98.4 g/kg without a regular trend with soil depth. According to the classification proposed by FAO (2006), 5.83% of the total soil samples were considered as slightly calcareous (CaCO3 content between 0 and 20g/kg), 70% were moderately calcareous (CaCO3 content between 20 and 100g/kg), 19.17% were strongly calcareous (CaCO3 content between 100 to 250 g/kg) and 5% were extremely calcareous (CaCO3 content higher than 250 g/kg).
• As the soils of arid and semiarid regions, these soils are poor in organic matter (OM) due to the scarce and the high rate of organic matter decomposition. Their organic matter (OM) content ranged from 1.1 to 27.0 g/kg with an average value of 9.0 g/kg and it decreased with soil depth.
• The pH values of these soils varied from 6.38 to 8.44 and it increased with soil depth. About 3.33% of the total soil samples had pH values of higher than 8. According to Brady and Weil (1999), 35% of the total soil samples were neutral (pH< 7.50), 40% of them were mildly alkaline (pH 7.5 to 7.8) and 24% of them were moderately alkaline (pH 7.81 to 8.4) and only one sample was strongly alkaline (pH>8.4).
• The cation exchangeable capacity (CEC) of the soil samples differed from 3.72 to 57.49 cmolc/kg with a mean value of 24.24 cmolc/kg and it increased with soil depth. The highest CEC value was recorded for a soil sample having a clay texture and an organic matter content of 12.3 g/kg.
• The electrical conductivity of the saturated soil-past extract (ECe) varied from 1.04 to 164.00 dS/m with an average value of 13.90 dS/m and it increased with soil depth reflecting the effect of irrigation on leaching the soluble salts from the surface layer downward. According to Abrol et al. (1988), 12.5 and 20.0% of the total soil samples were non-saline (ECe< 2 dS/m) and slightly saline (2< ECe >4 dS/m), respectively. However, almost, 28 and 18% of the total soil samples were considered as moderately saline (4< ECe > 8) and strongly saline soils (8< ECe >16), respectively. The rest of soil samples (21.5%) were very strong saline (ECe >16). The high salinity of these soils are also attributed to the saline nature of their parent material.
• Soluble anions could be arranged in the descending order of Cl- > SO4-2 > HCO3-. The results revealed that their concentration showed an irregular trend with soil depth. Soluble cations could be arranged in the descending order of Na+ > Ca+2 > Mg+2 > K+. In most cases, there were a random variation of distribution of these cations with depth.
• The sodium adsorption ratio of the saturated soil-past extract (SARe) values ranged from 0.09 to 77.19 with a mean value of 8.88 and it does not show any trend with soil depth. The high values of SARe were compatible with the high soil salinity. About 20.84% of the total soil samples had a SARe value of more than 13 which they are considered as sodic soils, while the rest of these soil samples (79.16%) recorded a SARe value of less than 13 which they are normal soils.
2. Soil Available (Olsen) P Content
• The Olsen-extractable P ranged from 1.18 to 65.35 mg/kg with an average value of 8.82 mg/kg in the surface layers and it varied from 0.92 to 42.49 mg/kg with an average value of 5.75 mg/kg in the subsurface layers.
• The Olsen P of the surface layers was higher than that of the subsurface one. It also increased toward the north west direction.
• The highest levels of Olsen P may be related to the high organic matter content and the fine texture of these soils. The lowest levels of available P are attributed to the P fixation by Ca ions and CaCO3 that are present in high amounts in these soils.
• According to Olsen and Sommers (1982), 69.16% of the studied soil samples were low (Olsen P < 6 mg/kg), 15% were medium (Olsen P 6-10 mg/kg) and 15.84% were high (Olsen P >10 mg/kg) in the available P content.
• The fine-textured soils almost had higher levels of Olsen P while the coarse-textured ones contained lower Olsen P levels.
• On the average basis, the Olsen P in these textured soils could be ranked in the order of clay loam > silt loam > loam > clay > sandy loam > sand clay > loamy sand > sand > sandy clay loam.
3. Distribution of Phosphorus Fractions in the Studied Soils
3.1- Effect of soil texture
• The mean level of the resin-extractable P (as a percentage of the mean total P) in the studied soil textures decreased in the order of clay loam = loam > silt loam > sand > loamy sand > sandy clay > sandy clay loam > clay > sandy loam. It varied from 0.20 to 0.38% of the mean total P in these textured soils.
• The mean P level (as a percentage of the mean total P) of the NaHCO3-Pi fraction decreased in the soil textures in the order of loam > silt loam > clay loam > sand > clay > sandy clay loam > sandy clay > sandy loam=loamy sand. This P fraction represented 0.31 to 0.88% of the mean total P in these textured soils.
• The mean percentage of P in the NaHCO3-Po fraction of the investigated soil textures had the descending order of sand > loamy sand > clay loam > sandy clay > sandy clay loam > loam > sandy loam > clay > silt loam. This fraction ranged from 0.10 to 0.41% of the mean total P in these soils.
• The mean level of the labile mean P ( sum of resin-P, NaHCO3-Pi and Po) represented 1.25, 1.00, 0.79, 1.54, 1.29, 0.95, 1.39, 0.74 and 0.82% in the sand, loamy sand, sandy loam, loam, silt loam, sandy clay loam, clay loam, sandy clay and clay textures of these soils, respectively. Therefore, it could be arranged in these textures in the descending order of loam > clay loam > silt loam > sand > loamy sand > sandy clay loam > clay > sandy loam > sandy clay.
• The mean P level of NaOH-Pi of the total P in these soil textures showed a reduction in the order of loam > silt loam > clay loam > clay > sandy clay loam > sandy clay > sand > sandy loam > loam sand. It differed from 0.57 to 2.80% of the mean total P in these textured soils. It revealed significantly positive correlations with both silt and clay contents with r values of 0.357** and 0.309*, respectively as well as a significant, negative correlation with the sand content (-0.401**).
• Meanwhile, the mean level of the NaOH-Po of the mean total P in the studied soil textures exhibited the descending order of clay > clay loam=silt loam > loam > sandy clay > sandy clay loam > sandy loam > sand >loamy sand. It illustrated 0.32 to 1.25% of the mean total P.
• On the average basis, the P level (as a percentage of the total P) of the HCl-P fraction in the investigated soil textures had the descending order of sandy clay loam > sandy clay > sand > sandy loam > clay loam > loam > loamy sand > clay ≈ silt loam. This fraction is considered the most dominant one after the residual P fraction. It represented 12.33 to 45.46% of the mean of total P in these textured soils.
• Therefore, the mean level of non-occluded P (NaOH-P and HCl-P) of the sand, loamy sand, sandy loam, loam, silt loam, sandy clay loam, clay loam, sandy clay and clay textures of the investigated soils was 29.02, 22.80, 28.02, 26.26, 16.25, 47.88, 29.89, 45.92 and 15.94% of the mean total P, respectively.
• The mean P percentage of the residual P (occluded) in these soil textures could be ranked in the order of clay > silt loam > loamy sand > loam > sandy loam > sand > clay loam > sandy clay. It is considered the most dominant one in these soils which it illustrates 51.16 to 83.24% of the mean total P in these textured soils.
• The average value of the total P of the investigated sandy, loamy sand, sandy loam, loam, silt loam, sandy clay loam, clay loam, sandy clay and clay soil textures was 1.39, 1.52, 1.84, 1.06, 1.22, 1.72, 1.42, 2.4 and 1.99 g/kg, respectively. It could be arranged in the descending order of sandy clay > clay > sandy loam > sandy clay loam > loamy sand > clay loam > sand > silt loam > loam soils.
Generally, the distribution of P fractions in the sandy and loamy sand soil samples was ranked in the order of Residual P > HCl-P > NaOH-Pi > NaOH-Po > NaHCO3-Pi > NaHCO3-Po > Resin-P. However, it was in the descending order of of Residual P > HCl-P > NaOH-Pi > NaOH-Po > NaHCO3-Pi > NaHCO3-Po > Resin-P in the sandy loam, sandy clay loam and sandy clay soils. In addition, the P fractions of the silt loam, clay loam and clay soil samples were arranged in the descending order of Residual P > HCl-P > NaOH-Pi > NaOH-Po > NaHCO3-Pi > NaHCO3-Po > Resin-P. Meanwhile, the P level in the loam soil sample decreased in the order of Residual P > HCl-P > NaOH-Pi > NaOH-Po > NaHCO3-Pi > NaHCO3-Po > Resin-P.
3.2 Effect of soil depth
• The mean level of P in the surface layer of these soils could be ranked in the descending order of Residual P > HCl-P > NaOH-Pi > NaOH-Po > NaHCO3-Pi > Resin-P > NaHCO3-Po.However, in the subsurface layer, it decreased in the order of Residual P > HCl-P > NaOH-Pi > NaOH-Po > NaHCO3-Pi > NaHCO3-Po > Resin-P.
• The mean level of the resin-P in the surface layer is higher than that of the subsurface one which it decreased from 0.40% of the mean of the total P in the surface layer to 0.27% in the subsurface one. The average level of NaHCO3-Pi also decreased from 0.82% of the total P in the surface layer to 0.52% of the total P in the subsurface one while the NaHCO3-Po mean level increased from 030% in the surface layer to 0.36% in the subsurface layer. Moreover, the labile P decreased from 1.52% in the surface layer to 1.15% in the subsurface one.
• A reduction also occurred in the mean level of NaOH-Pi and NaOH-Po from 2.24 and 3.52% of the mean total P, respectively, in the surface layer to 1.82 and 2.44%, respectively, in the subsurface one. However, the HCl-extractable P mean level of the studied soils differed from 23.01% of the total P mean level in the surface layers to 24.68% in the subsurface ones. It did not largely change between the surface and subsurface layers. So, the non-occluded P (NaOH extractable Pi and Po as well as HCl-P) level of the surface layers showed almost the same level (26.53%) as that of the subsurface ones (27.12%).
• The residual (occluded) P fraction of the studied soil is considered the most common one in both surface and subsurface layers followed by the HCl-P fraction. Its mean level had almost the same value in both surface (71.94%) and subsurface (71.73%) layers. However, the mean value of the total P in these soils slightly decreased from 1.433 g/kg in the surface layer to 1.333 g/kg in the subsurface one.
3.2 Effect of soil CaCO3 content
• The labile P in these soils decreased with increasing the CaCO3 content. It decreased from 1.25% of the total P in the low CaCO3 (< 100g/kg) samples to 0.48% in the high CaCO3 (≥ 100g/kg) samples.
• The mean level of Pi and Po extracted with NaOH decreased from 1.96 and 1.32% of the mean level of total P, respectively, in the low CaCO3 soil samples to 0.74 and 0.36%, respectively, in the high CaCO3 ones. However, the mean P level of HCl-P increased from 29.96% of the mean total P in the soil samples of the low CaCO3 content to 40.30% of the mean total P in those of the high CaCO3 ( 100g/kg) content. The mean level of non-occluded P of these soils increased from 33.24% of the mean level of the total P in the low CaCO3 samples to 41.40% in the high CaCO3 ones.
• The mean level of the residual (occluded) P decreased from 65.51% of the total P mean level in the samples of the low CaCO3 content to 58.12% in those of the high CaCO3 content. Generally, the mean level of P fractions in both low and high CaCO3 content samples could be ranked in the descending order of Residual P > HCl-P > NaOH-Pi ≈ NaOH-Po > NaHCO3-Pi > NaHCO3-Po ≈ Resin-P.
3.4 Effect of soil organic matter content
• A slight change occurred in the resin-P level of 0.27% of the total P mean level in the relatively low (< 10 g/kg) organic matter (OM) content samples to 0.21% in the relatively high (≥ 10 g/kg) soil OM ones. So, the soil organic matter content had a slightly effect on the resin-P in these soils. Both NaHCO3-Pi and-Po slightly decreased from 0.89% in the low OM soil samples to 0.82% in the relatively high OM ones. Moreover, the labile P level decreased from 1.16% in the low OM soil samples to 1.03% in the relatively high OM samples.
• The mean level of NaOH extracted Pi and Po in these soils increased from 1.44 and 0.73% of the mean level of total P, respectively, in the low OM soil samples to 1.88 and 1.61%, respectively, in the high ones. Also, the mean level of organic P (NaHCO3-Po plus NaOH-Po) of these soils increased from 1.03% in the low OM samples to 1.99% in the high OM ones.
• The percentage of P associated with Ca and CaCO3 (HCl-P) decreased from 26.23% in the low OM samples to 24.58% in the relatively high OM samples. Increases in the NaOH-Po plus NaHCO3-Po percentage (0.96%) and decreases in HCl-P percentage (1.65%) occurred in the relatively high OM samples compared to those of the low OM ones. Meanwhile, the mean level of non-occluded P (NaOH-P and HCl-P) of these soils slightly decreased from 28.40% in the low OM soil samples to 28.07% in the relatively high OM ones.
• The mean level of the residual P increased from 70.44% in the samples of the low OM content to 70.90% in those of the relatively high OM content. The P level of both relatively low and high OM soil samples had the same descending order of Residual P > HCl-P > NaOH-Pi > NaOH-Po > NaHCO3-Pi > NaHCO3-Po ≈ Resin-P. This means that the organic matter content of these soils did not affect the P level in its fractions.
3.5 Effect of soil salinity
• The mean concentration of resin-P increased from 0.17% of the mean of total P in the slightly saline (ECe < 4 dS/m) samples to 0.27% in the moderately saline (ECe 4-8 dS/m) soil samples and 31% of in the highly saline (> 8 dS/m) samples. So, the soil salinity could result in an increase in the resin-P.
• The Pi level extracted with NaHCO3 increased from 0.37% in the slightly saline samples to 0.59 and 0.58% in the moderately and highly saline samples, respectively. However, the NaHCO3–Po increased from 0.28% in the highly saline samples to 0.36% in the moderately saline and then decreased to 0.22% in the highly saline ones, respectively. Concerning the mean of the labile P, it also increased from 0.82% in the slightly saline soil samples to 1.22 and 1.11% in the moderately and highly saline samples, respectively.
• Both mean values of NaOH-Pi and Po of these soils slightly increase and decreased, respectively, from 0.73 and 0.77%, respectively, in the slightly saline samples to 0.83 and 0.75%, respectively, in the moderately saline samples and then greatly increased to 2.44 and 1.56%, respectively, in the highly saline samples.
• The mean level of HCl-extractable P increased from 22.41% of the mean level of the total P in the slightly saline soil samples to 26.03% in the moderately saline ones but then decreased to 19.23% in the highly saline samples. Meanwhile, the mean of the non-occluded P of the studied soils increased from 23.91% in the slightly saline samples to 27.61% in the moderately saline ones and then decreased in the highly saline samples to 23.23%.
• The mean residual (occluded) P of the investigated soils decreased from 75.26% in the slightly saline samples to 71.17% in the moderately saline ones and then increased to 75.65% in the highly saline samples. The mean of the total P of these soils decreased from 1.621 g/kg in the slightly saline samples to 1.307 g/kg in the moderately saline ones but then slightly increased to 1.440 g/kg in the highly saline ones. Generally, the P level of the slightly and moderately saline soil samples could be ranked in the descending order of Residual P > HCl-P > NaOH-Pi ≈ NaOH-Po > NaHCO3-Pi > NaHCO3-Po > Resin-P fractions. while in the highly saline soil samples it decreased in the order of Residual P > HCl-P > NaOH-Pi > NaOH-Po > NaHCO3-Pi > Resin-P> NaHCO3-Po.
3.6 Effect of soil available P content
• The mean of the resin-P relatively increased from 0.21% in the soil samples of the low Olsen P content to 0.23 and 0.31%, respectively, in the samples of the medium and high Olsen P, respectively. So, the resin-P increased as the soil available (Olsen) P increased.
• Both NaHCO3-Pi and Po increased with increasing the Olsen P content. These respective fractions increased from 0.34 and 0.20%, respectively, in the low Olsen P samples to 0.40 and 0.29%, respectively , in the medium Olsen P samples, and 0.68 and 0.31%, respectively, in the high Olsen P samples. The mean level of labile P of these soils increased with increasing the concentration of Olsen P in the soil samples.
• The mean level of NaOH-Pi in the soil samples of low, medium and high contents of Olsen P was 1.36, 0.74 and 2.28% of the mean total P, respectively. The mean level of NaOH–extractable Po also increased with increasing the Olsen P content of these studied soils samples. Moreover, the HCl-P mean level decreased with increasing the Olsen P level of these soil samples.
• The mean level of the inorganic P pool of these soils decreased from 38.00% in the low Olsen P samples to 31.17 and 20.43% in the medium and high Olsen P samples, respectively. However, the mean level of the organic P pool increased from 0.63% in the samples of the low Olsen P level to 0.97 and 1.24% in the samples of the medium and high Olsen P, respectively. In addition, the non-occluded P decreased with increasing the Olsen P content of the soil samples. It decreased from 38.09% in the low Olsen P samples to 31.45 and 20.68% in the medium and high Olsen P ones, respectively. The mean level of the residual (occluded) P also increased from 61.15% in the low Olsen P samples to 67.64 and 78.02% in the medium and high Olsen P ones, respectively.
Generally, the mean P level of these fractions in both low and high Olsen-P soil samples had the descending order of Residual-P > HCl-P > NaOH-Pi > NaOH-Po > NaHCO3-Pi > NaHCO3-Po ≈ Resin-P. while, in the medium Olsen P ones, it showed the order of Residual-P > HCl-P > NaOH-Pi > NaOH-Po > NaHCO3-Pi > NaHCO3-Po > Resin-P
It might be recommended that
1- Additions of organic matter and manures to these soils accelerate transformations among different P fractions and reduce CaCO3 levels resulting in adequate levels of available soil P for plant uptake.
2- Considering the various fractions of phosphorus in relation to different physical and chemical properties, P management of El-Dakhla Oasis soils deserves a priority approach for sustainable agricultural.
3- The Hedely et al. (1982) method of the sequential phosphorus fractionation showed be revised to be adopted in the soils of arid regions.