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

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- Silicon sources for rice crop

The importance of Si fertilization in rice (Oryza sativa L.) and sugarcane (Saccharum spp.), among other crops, has already been demonstrated by Okuda & Takahashi (1964) in Japan; Ayres (1966) and Halais (1968) in Mauritius; Gascho (1978), Snyder et al. (1986), and Anderson et al. (1987) in Florida; and Korndörfer et al. (2002) in Brazil. The critical sufficiency levels of this element in the soil and plants are currently being established in Brazil (Korndörfer et al., 1999; 2002). It is therefore necessary to identify the most promising, potentially available Si sources to plants.

Plant residues, such as rice hulls and sugarcane bagasse, are sometimes used as Si sources. In addition to the fact that they are slow-release Si sources, these residues have other uses, such as the generation of steam, and are insufficient to meet the demand for Si in agriculture. On the other hand, there are Si-rich metallurgic slags which could meet this demand. The high temperatures used in iron industry release Si from crystalline form to reactive and consequently more soluble forms.

Analyzing results of 23 field experiments, during the 1992-1996 period, Korndörfer et al. (2001) observed increase in grain yield of irrigated rice - 1,007 kg ha-1 - in plots receiving Si as Ca silicate. On the other hand, accumulated Si in rice plants could reduce transpiration rate, by decreasing water intake (Marschner, 1995 and Takahashi, 1996). Results by Faria (2000) corroborate this assertion; when soil moisture was at 80% of field capacity, no increases in grain yield were observed. Conversely, under a lower soil humidity value - 60% of field capacity - grain yield increased were linearly with increasing Si rates, an indication that Si plays important role on increasing tolerance of rice plants to water stress.

The most important characteristics of a Si source for agricultural use are: high soluble Si content, suitable physical properties, easy mechanized application, ready availability for plants, low cost, balanced ratios and amounts of calcium (Ca) and magnesium (Mg), and absence of heavy metals. Many iron metallurgy slags possess these traits, and some of them are promising sources of Si (Korndörfer et al., 2002).

Considering the lack of information and the great demand for agronomically efficient and economically viable Si sources for agriculture purpose, the objective here was to evaluate several Si-rich materials (slags, silicates, thermophosphates etc.) with regard to their ability to supply this element to plants.

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