الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Due to chemical similarity between silicon (Si) and phosphorus (P), silicate and phosphate anion can compete for the adsorption sites on the surface of soil particles. Their competitive reactions can enhance transformation of the unavailable P into available form to plants. Therefore, three types of experiments were performed to evaluate the behavior of Si and P in clay soil collected from Agriculture Research Centre farm, Giza, Egypt (classified as Entisol), through different ways including incubation, adsorption and pot experiments cultivated with wheat. All experiments were designed as randomized complete block with three replications.
5.1. Incubation experiments
Two series of experiments were carried out to evaluate the availability of Si and P in fine, air dried soil incubated at about 25ºC under laboratory conditions for 60 days in pots of 1kg soil capacity rewetted to field capacity twice a week. The study includes Si and P interactions. The first series of Si experiments using soil including sodium metasilicate benta hydrate {Na2SiO3.5H2O} solution at concentrations of 0, 50, 100 and 200mg Si Kg-1. While, other series of experiments were adopted involving the same mentioned Si concentrations but accompanied with P in the form of calcium super phosphate at concentrations of 7 and 10mg P Kg-1 soil (which represent 50 and 75% of the recommended dose by the Egyptian Ministry of Agriculture for wheat plants). at the end of each experiment, soil of each pot was dried, sieved (2mm) and analyzed for available silicon and phosphorus. Part of the soil from each treatment was kept to be used afterwards in the adsorption experiments.
The incubation results revealed that:
1. The availability of Si increased by increasing of the added amount up to 50mg Si Kg-1, and then decreased afterwards. Silicon adsorption and precipitation mechanism might be the reason. In addition, P availability increased with increasing of silicon application. The competitive adsorption between phosphate and silicate on soil might be the reason.
2. Phosphorus was favorable for Si availability particularly under high concentration of former, possibly attributed to competition between them which decreased silicon adsorption. Moreover, availability of P increased with increasing either Si or P concentration. The silicon concentration of 200mg Si Kg-1 soil was the best accompanied with 10mg P Kg-1 soil compared to other treatments.
5.2. Adsorption experiments
Studies of adsorption were developed to be used in an experiment dealing with the adsorption behavior of Si at different concentrations from this element and its interaction with P. Multi series experiments were performed using the pre-treated soil samples obtained at the end of incubation experiment and treated with 0, 14, 28, 42 and 56mg Si L-1 separately or in combination with 50 and 100mg P L-1.
To achieve the suggested goals, responses of adsorption rate to certain environmental conditions were studied, such conditions being the applied ion concentration in the concerned solution (Si), equilibrium concentration and interaction with other elements represented by P. Investigations dealing with mechanics were performed through evaluating the responses of ion adsorption to ion concentration in the concerned solution, kinetic studies being carried out through evaluation for both Kf (adsorptive capacity) and 1/ n (the intensity of adsorption or sorption energy constant of Freundlich isotherm model).
Results showed that:
1. The amount of adsorbed Si onto soil increased with increased either applied or equilibrium Si concentration in all incubated soil samples. This might be attributed to a multilayer Si formation at the soil surface.
2. Silicon adsorption onto soil decreased in the soil which incubated with high than with low Si concentration.
3. Application of P to the soil decreased the amount of adsorbed Si on all incubated soil.
5.3. Cultivation pot experiments
As a relatively more practical approach, pot experiments were performed for application of the results obtained from the previously mentioned incubation and adsorption experiments. This was accomplished through evaluation for the responses of wheat plant growth as well as status of Si and P in plants beside the effects of interacting ions.
Wheat plants (Giza 168) were grown in pots kept under field conditions. Twenty grains were used per pot; each pot received 5kg of fine air dried soil treated with 0, 200, 300 and 400mg Si Kg-1 soil or 0, 3.5, 7.0, 10.0 and 13.0mg P kg-1 soil (which represent 0, 25, 50, 75 and 100% of the amount recommended for wheat plants). Interaction series of experiments were performed as to involve the same previously mentioned Si and P concentrations in combination at the same concentration.
Plants were harvested at booting growth stage and divided into shoot and root and recoded both fresh and dry weight as well as moisture content. Shoot and root subjected to analyses for Si and P concentration and total content. Soil samples were also subjected to analyses for the available Si and P.
The obtained results can be summarized as follows:
1. Available Si and P in soil increased with increasing either Si or P concentrations added. Also, Si and P availability in soil increased with increasing the combined concentration of either Si or P. This means that P availability in soil as an essential element for plant growth can be enhanced by addition of Si.
2. Silicon was favorable, for plant growth, it increased with increasing of Si concentration.
3. Increasing of the applied P to soil increased the fresh and dry weights of wheat plants, but moisture content of plants either increased or decreased.
4. Interaction between Si and P applications at booting growth stage of wheat plants generally increased the growth parameters. Such responses were significant for fresh weight and dry weight.
5. The status of Si and P in grown wheat plants showed, in general, positive significant responses to Si and P concentrations in soil. These responses were dependent on the previously mentioned parameter of Si concentration in soil.
6. Silicon application improved generally the responses of status in plant with application Si concentrations.
7. The status P, in grown wheat plants, increased with increased P application.
Accordingly recommendations may be introduced as to select the suitable macroelements (e.g. P) fertilization design, including time of fertilization that goes along with values of available Si in the environment. In addition, Si fertilization is very important for wheat plant growth.