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Although silicon is not an essential nutrient, its application is beneficial for …

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Materials and Methods
- Silicon sources for rice crop

Experiment was set up in a greenhouse in a potted rice crop (5 kg dry soil per pot), with subsurface samples of a Quartzipsamment with low soluble Si content in acetic acid 0.5 mol dm-3. The chemical attributes of the soil used in the experiment were: pH (CaCl2 0.01 mol L-1) 4.4; P 56 mg dm-3 (extracted by H2SO4 0.025 molc dm-3 + HCl 0.05 molc dm-3); Si 3.3 mg dm-3; Al 10 mmolc dm-3; Ca 2 mmolc dm-3; Mg 1 mmolc dm-3; sum of bases 4 mmolc dm-3; effective CEC 14 mmolc dm-3; CEC 52 mmolc dm-3; V (%) 7; m (%) 71; and organic matter 15 g kg-1.

The experiment was set up in a completely randomized design and the sources used were defined as a function of their potential for agricultural use and Si-supplying capacity. Materials were characterized with regard to their origin, total Si, Ca, and Mg contents, and Neutralizing Power (NP) (Table 1). Wollastonite a product with high degree of purity used worldwide in studies involving Si was used as standard source for comparisons. The slag samples were dried and sifted through a 50-mesh screen.

Treatments were applied as presented in Table 1. Additional treatments with 250, 375, and 500 kg ha-1 Si were applied using the standard (Wollastonite) in order to obtain the Si absorption curves by the plants. The soil was moistened up to 70% of field capacity. The incubation period lasted 40 days, and by the end of January, 2001, the rice crop (Formoso variety) was planted.

Plants nutritional requirements were met by 100 mL of a nutritive solution containing 1.43 g dm-3 urea, 1 g dm-3 (NH4)2SO4, 0.15 cm3 dm-3 H3PO4, 1.56 g dm-3 KCl, 4 cm3 dm-3 of a micronutrient solution (2.86 g dm-3 H3BO3, 1.67 g dm-3 MnSO4.H2O, 0.40 g dm-3 ZnSO4.7H2O, 0.10 g dm-3 CuSO4.5H2O, and 0.04 g dm-3 NH4MoO4.2H2O), and 4 cm3 dm-3 of EDTA iron solution, applied weekly to each pot.

After the third leaf was formed, pots were inundated with water (1 cm) and thinning was performed to set 20 plants per pot. At 150 days, the above-ground part of plants was harvested. Dry matter and grain yield, silicon uptake (dry matter and grain) were determined according to Elliott & Snyder (1991), and soil silicon extracted by acetic acid 0.5 mol dm-3 (Korndörfer et al., 1999) and CaCl2 0.0025 mol dm-3 (Raij & Camargo, 1973). Si determination in the different soil extracts was performed by beta molybdosilicic complex formation (Kilmer, 1965).

Data were submitted to analyses of variance (F test) and means comparison test (Tukey test; a = 0.05). A polynomial regression was used to determine effects of Wollastonite doses. The equivalent dose of the standard source for each treatment was determined by applying the equation obtained from Si accumulation in the above-ground part of plants as a function of Wollastonite doses.

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