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

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Results and Discussion
- Silicon sources for rice crop

There was a linear effect of Wollastonite application in the above-ground part of plants and grain, i.e., the higher the Si dose applied, the higher the concentration in the tissues (Figure 1).

When sources were compared, differences were observed between them. The phosphorus slag elicited the highest Si concentration in the above-ground part of plants, followed by Wollastonite and the electric furnace slag, which did not differ between themselves, for both above-ground part and grain. The stainless steel slag, LD3 and AOD only differed regarding Si content in the above-ground part. The source showing the smallest Si content was silicate clay, which did not differ from the control treatment, followed by AF2, AF1, schist ash, schist, and LD4 (Table 2).

Steel slags (LD, AOD, electric, and stainless steel furnaces) had higher Si availability than blast furnace slag, and those slags also showed solubility differences among themselves, depending on the type of steel produced and type of furnace used to produce steel (Table 2). Similar results were also obtained by Kato & Owa (1997).

The soil Si content determined by both extractors also increased with Wollastonite doses (Figure 2), but extraction in acetic acid was higher. With regard to results of soil Si and Si accumulated by plants, no significant correlations were found (Figure 3).

The AF2 slag was the source with the highest Si release by acetic acid (Table 3), but it was not, however, the treatment with the greatest uptake by the plant; actually, the opposite occurred. This happened due to Si solubility in the source itself by acetic acid. A similar fact occurs with phosphorus extracted by double acid in soils where natural phosphate is applied. Results overestimate this element because of the insoluble phosphorus solubilization in the soil by the acid (Raij, 1991). On the contrary, high correlation between soil Si extracted in acetic acid and plant-absorbed Si was observed when Wollastonite was applied, since it is practically insoluble in weak acid (Pereira et al., 2003). In this case, Si determined in the soil corresponds to the amount of Si actually released by the source.

CaCl2, the Si in the soil, extracted by showed better correlation when compared to the acetic acid, contradicting results of Korndörfer et al. (1999) who, working with a single Si source, concluded that acetic acid is superior to calcium chloride. Some Si sources show solubility in acetic acid but not in CaCl2. Therefore, acetic acid could solubilize the material applied to the soil, which is not available for plants, overestimating Si availability. This was demonstrated in another paper by Korndörfer & Gascho (1999). When different doses of Wollastonite and phosphorus slag were applied, the authors observed greater Si accumulation in rice with Wollastonite, while the source with the highest Si release in the soil was phosphorus slag.

With respect to dry matter yield, Wollastonite showed linear increase with increasing Si doses (Figure 4). This reinforces the idea that Si is indeed beneficial to rice. However, the only difference between sources occurred for grain yield between the stainless steel (highest yield) and silicate clay treatments (lowest yield) (Table 4). Maybe the 125 kg ha-1 dose was not sufficient to reveal more expressive differences between sources. In the case of Wollastonite, the response was linear and positive with increasing doses, with the application of 500 kg ha-1 Si showing the highest yield (Figure 4).

With regard to Si extraction by plants, Wollastonite also showed linear increases with increasing doses (Figure 5). Between sources, P slag also accumulated the most Si, followed by Wollastonite, which differed in Si accumulation in the grain, and by electric furnace slag, which differed in Si accumulation in dry matter. These two sources, however, did not differ regarding total Si uptake (Table 5).

Stainless steel slag was the source allowing the higher Si acumulation in grain due to its higher productivity, and was the source ranked fourth regarding total Si uptake, followed by LD1, LD3, AOD, LD2, and LD4. The sources extracting the smallest amount of Si were, again, silicate clay and AF2, which also did not differ from the control, followed by schist, schist ash, and AF1 (Table 5).

When the efficiency of the sources with regard to total Si accumulation was considered in comparison to the standard, P slag was the only source superior to the standard. Stainless steel slag and electric furnace slag were seemingly as efficient as the standard, while the other iron metallurgy slags (LD1, LD3, AOD, and LD2) showed a behavior where reduced doses of the standard, between 10 and 20%, would be enough to provide the same amount of Si as the 125 kg ha-1 Si doses provided by those sources. The other sources showed equivalent doses well below that value; the least efficient was again silicate clay, where the supplying of only 7 kg ha-1 Si provided by the standard would be sufficient to show the same effect as 125 kg ha-1 Si provided by this source.

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