Si application alleviates influence of some abiotic stresses on crop plants. Meanwhile, scarce information is avaiable about the significance of Si for helping the plants to overcome the injuries of N deficiency and herbicides pressure. Thus, two–year 2–field experiments were carried out in wheat. Experiment I involved three Si concentrations (Si0ppm, Si250ppm and Si500ppm) and two N levels(N100% and N50%). Experiment II examined four combinations of fenoxaprop–p–ethyl and Si: fenoxaprop–p–ethyl+Si250ppm and fenoxaprop–p–ethyl+Si500ppm (each either in sequence or in tank mixture), fenoxaprop–p–ethyl alone, hand weeding and weedy check. Under N deficiency (N50%), Si500ppm increased plant height as compared to the control. No significant differences in SPAD values were detected amongst Si concentrations under each of the two tested N levels. Si can partially alleviate negative N deficiency effect on wheat yield, causing its increase to level obtained under normal N supply. Si has no effect on weed biomass when applied with fenoxaprop–p–ethyl either in sequence or in tank mixing. The most promising treatment for maximizing wheat grain yield was the application of 100 kg N ha–1 (N50%) x fenoxaprop–p–ethyl+Si250ppm in sequence", which also, in the same time, means reducing both cost of crop production and environment pollution.

Silicon (Si) is an element, not essential, but is beneficial for some plants. The Freundlich model was used to describe Si adsorption on soil samples incubated with different Si and phosphorus (P) concentrations. Soil incubated samples were: T1 soil had no Si or P (control); T2, T3 and T4 soils contained 50, 100, and 200 mg Si L-1, respectively. T5 and T6  soils contained 50 mg Si L-1 in combined with either 7 or 10 mg P L-1 along with T7 and T8 soil  contained 100 mg Si L-1 in combined with either  7  or 10 mg P L-1. In final, T9 and T10 soil contained 200 mg Si L-1 in combined with either 7 or 10 mg P L-1. A series of adsorption experiments were performed using sodium metasilicate pentahydrate (Na2O3Si.5H2O) solution prepared to have concentrations representing 0, 14, 28, 42 and 56 mg Si L-1. The supernatant of Si concentration was determined. The amount of element adsorbed was calculated as the difference between applied element concentrations and that remaining in solution after equilibration. Adsorption isotherms were determined at room temperature (25 oC±1).   Results revealed that a positive trend was generally found; increases in amount of adsorption onto soil with increasing Si concentration and equilibrium concentration in concerned solution either applied separately or applied + initial available Si concentration in soil. The Freundlich equation provides a good fit to the sorption data for all incubated soil samples and R2 values were ranged from  0.82 to 0.97. Present study indicated that adsorption capacity value (Kf) decreased from T1 to T3 soil samples then increased at T4 sample, and intensity adsorption values (1/n) gave almost an opposite trend to that of capacity adsorption (Kf) values.   Moreover, adsorption of Si onto soil incubated with different Si concentrations in combined with P2 decreased as compared to P1. Opposite trend was obtained with equilibrium Si concentration in concerned solution. Also, present study showed that the higher values of Kf obtained in P1 soils (T5, T7 and T9), compared to Kf values of P2 soils (T6, T8 and T10), and intensity adsorption values (1/n) gave almost an opposite trend to that of capacity adsorption (Kf) in both P1 and P2 soils. Finally, large Si sorption capacity and low Si affinity for the surface sites were observed in soil incubated with high Si concentration compared to soil incubated with low ones.