الفهرس 
REVIEW OF LITERATUREOnly 14 pages are availabe for public view
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Abstract
Field capacity of the soil acquires a special practical importance due to its use in determining the maximum amount of water to be added to the soil at one time for irrigation, it is closely related to ground moisture constants and is also affected by some other factors such as soil texture, porosity, organic matter content, irrigation depth, evaporation and transpiration. In this study, a set of treatments were carried out to study the relationship among –in situ- field capacity and laboratory determinations of soil water content under different applied pressures of different soil textural classes. One hundred and sixty-eight of surface soil samples were collected. Then the collected soil samples were classified according to their textural classes based on USDA triangle. from each textural class, one soil sample was selected and characterized by determining some physical and chemical properties. Soil water characteristics data, h(θ), and soil water depletion rate θ(t) were determined using all of the collected soil samples, while pore size distribution and soil water tension at simulated field capacity were calculated using fitted equations of soil water characteristics data.
Simulated field determinations of -in situ- field capacity were done and the obtained results reveal that sand and loamy sand soils reached field capacity at about 60 mbar water tension (60 ± 6 mbar), after 33.11 and 34.22 hours from the end of adding an excess of irrigation water, respectively. While sandy loam one reached field capacity at about 86 mbar soil water tension after 39.09 hours from the end of irrigation. The soils of other textural classes reached field capacity at about 330 mbar soil water tension (± 30 mbar) where these values of soil water tension (hfc) were 314.19, 330.29, 330.23, and 365.03 mbar of sandy clay loam, sandy clay, clay loam, and clay, respectively. While, the aforementioned four soil textural classes needed 46.99, 54.59, 81.45, and 90.01 hours to reach their field capacity (tfc), respectively. Generally, increasing water holding pores and / or fine capillary pores, both soil water tension at field capacity (hfc) and elapsed time to reach it after heavy irrigation (tfc) are increased. In the case of both sand and loamy sand soils textural, the highest significant values of the correlation coefficient -0.8723 and 0.8876 respectively- were found between field capacity -in situ- and soil water content balanced with 60 mbar of applied pressure. While sandy loam soil achieves the highest significant value of correlation coefficient, 0.9077 at 100 mbar of applied pressure. The correlation coefficients among field capacity -in situ- and soil water content balanced with 330 mbar of applied pressure of sandy clay loam, sandy clay, clay loam, and clay soils have the highest significant values and equal 0.8774, 0.9549, 0.8983 and 0.9560, respectively. Generally, there is no single and predefined value of applied pressure to get field capacity using laboratory determination, but it differs according to soil texture, structure, pore size distribution, organic matter, and bulk density. A trial was done to estimate soil field capacity using fitted saturation point and deterministic coefficient of soil textural class under study. A high significant correlation (r = 0.9996) was found between actual -in situ- field capacity and the estimated one of the studied soil textural classes using the suggested coefficients. According to the statistical analysis of the differences between actual -in situ- field capacity and the estimated one using the above-mentioned suggested coefficient, are insignificant.
CONCLUSION
Under arid conditions, irrigated agriculture is the main way of agricultural production. Field capacity is a soil parameter that is widely used in calculating the amount of irrigation water. Unfortunate field methods to determine field capacity are tedious, labor and time consume, while in laboratory determinations there is no predefined value of pressure can be used for all soil samples. Therefore, this trail aimed to find the relationship among field capacity -in situ- and laboratory determinations of soil water content under different applied pressures of different soil textural classes. Results reveal that sand and loamy sand soils reached field capacity at about 60 mbar water tension (60 ± 6 mbar), after 33.11 and 34.22 hours from the end of adding an excess of irrigation water, respectively. While sandy loam one reached field capacity at about 86 mbar soil water tension after 39.09 hours from the end of irrigation. The soils of other textural classes sandy clay loam, sandy clay, clay loam, and clay, reached field capacity at about 330 mbar soil water tension
(± 30 mbar) and needed 46.99, 54.59, 81.45, and 90.01 hours to reach their field capacity, respectively.
Generally, there is no single and predefined value of applied pressure to get field capacity using laboratory determination, but it differs according to soil texture, structure, pore size distribution, organic matter, and bulk density.
The trial was done to estimate soil field capacity using fitted saturation point and a deterministic coefficient (estimation coefficient) of each soil textural class under study is successes. A high significant correlation (r = 0.9996) was found between actual in-situ field capacity and the estimated one of the studied soil textural classes.
The estimated field capacity calculated using the coefficient 0.70 for coarse and medium textured soils and 0.85 for heavy textured soils. This coefficient has obtained by dividing in-situ field capacity with saturation point of each soil textural class under study.
The differences between actual -in situ- field capacity and the estimated one using the above-mentioned suggested coefficient are statistically insignificant for all soil textural classes under study.