Monday, October 21, 2013

Silicon content in rice husk biochar (RHB)

     Silicon oxide forms the main component (90-97%) of the rice husk ash with trace amounts of CaO, MgO, K2O and Na2O.  The melting point of SiO2 is 1410-1610°C, while that of K2O and Na2O is 350 and 1275°C respectively. It has been suggested that at higher temperatures, the low-melting oxides fuse with silica on the surface of the rice husk char and form glassy or amorphous phases, preventing the completion of reaction. (Anshu et al., 2004).

Silicon Oxide Molecule

     We realized an investigation on the effects of rice husk biochar and its silicon content on corn (Zea mays L.) growth; the analysis of the fresh rice husk used to obtain biochar showed high levels of Si, Ca and Mg (Milla et al., 2013). After pyrolysis the same elements were found to increase in the rice husk biochar. Wood biochar was found to have a higher content of Ca when compared with RHB, having a low content of Si and Mg.

     Silicon (Si) is not yet classed as an essential nutrient but it exists in all plants grown in soil and is recognized as a functional nutrient. The benefits of silicon include increasing pest and pathogen resistance, drought and heavy metal tolerance, and improved quality and yield of agricultural crops. Si is taken up at levels equal or greater than essential nutrients such as Nitrogen and Potassium in certain plants such as rice and sugarcane, for which it is considered agronomically essential for sustainable crop production (Savant et al., 1999). Si exists in all plants grown in soil (Takahashi, 1995) and its content in plant tissue ranges from 0.1 to 10% (Epstein, 1999).

     Si is considered as a nutrient of agronomic essentiality in that its absence causes imbalances of other nutrients resulting in poor growth, if not death of the plant (Savant et al., 1997). Numerous laboratory, greenhouse and field experiments have shown the benefits of silicon fertilizers for agricultural crops and the importance of silicon fertilizers as a component in sustainable agriculture (Matichenkov and Calvert, 1999).


There are two different effects on plants due to silicon fertilizers:
1. An indirect influence through soil fertility, and
2. A direct influence on the plants

The benefits of Si on plants include (Ma and Yamaji, 2006; Savant et al., 1999):
- Increased growth and fruit yields in some species.
- Tolerance to abiotic stress: frost, drought and salinity, toxicity by Al, Mn, heavy metals.
- Tolerance to biotic stress: insects and infection.
- Resistance to lodging.

Si also controls the chemical and biological properties of soil with the following benefits:
- Reduced leaching of phosphorous (P) and potassium (K) (Sadgrove, 2006).
- Reduced Aluminium (Al), Iron (Fe), Manganese (Mn) and heavy metal mobility (Maichenkov and         Calvert, 2002).
- Improved microbial activity (Matichenkov and Calvert, 2002).
- Increased stability of soil organic matter.
- Improved soil texture (Sadgrove, 2006).
- Improved water-holding capacity (Sadgrove, 2006).
- Increased stability against soil erosion (Sadgrove, 2006).
- Increased cationic exchange capacity (CEC) (Camberato, 2001).

      Therefore, even if a plant is a low Si-accumulator, it will benefit from the improved soil properties resulting from the application of Si. Silicon deficiency in crops has been recognized since the 1970’s. The optimization of silicon nutrition has been shown to have positive effects on plants. In particular, substantial research on rice and sugarcane has shown that silicon application can significantly enhance insect pest and disease resistance with consequent yield increases.
Development of leaf blast symptoms at 96 h after inoculation with Magnaporthe grisea in rice plants nonamended (-Si) or amended with (+Si) with silicon.

      Plants differ in their ability to accumulate Si (Ma and Yamaji, 2006), but in order for any plant to benefit from Si it must be able to acquire this element in high concentrations. Several reports in the literature suggest that Si nutrition has a definite role in certain type of crop cultivation, especially on weathered tropical soils such as Oxisols, Ultisols, Entisols and Histosols (Savant et al., 1999). In our tests, Si from rice husk biochar  played a significative roll in the water spinach nutrition, boosting the mass production of the plant, showing a significative difference when compared with the mass production of water spinach where wood biochar was applied.                    

                           RICE HUSK BIOCHAR                                      WOOD BIOCHAR

      Rice husks are unique within nature. They contain approximately 20% opaline silica in combination with a large amount of the phenyl propanoid structural polymer called lignin (Oliver, 2004). In our test, the Si properties of the rice husk were increased after pyrolysis.

      In a study made by Hossain et al. (2011), about the influence of pyrolysis temperature on production and nutrient properties of biochar, the researchers concluded that: pyrolysis temperature has significant effect on the chemical properties of the biochar produced, with important implications regarding their suitability as a soil amendment. In addition, the study confirmed that the yield of biochar decreases with increasing pyrolysis temperature.

     The study also shows that biochar produced at low temperatures (300°C, 400°C) is acidic. Biochars produced at lower temperatures might be suitable for alkaline soils to correct for alkalinity problems.

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