3.4. Characteristics of SBE-based bio organic fertilizer
The novel composting method has the ability to modify the morphology of the clay structure in SBE, in remedying and improving other chemical attributes besides eliminating the acidic and hydrophobic nature of the earth. The resulting SBE has been transformed into an effective bio organic material with improved organic carbon (OC) content from 7.1 to 16.5%, the CEC from 8.0 to 33.8 cmol/kg, the water-holding capacity from 6.1 to 16.5 ml/100 g and the C:N ratio from 290 to 9–21. The OC increased due to the residual oil in SBE and the high OC content of the co-composted materials. Most of the degradable organic matter was decomposed and replenished. An increase in OC after composting would have contributed to the observed increase in CEC, thereby enhancing the nutrient supplying capacity of the bio organic fertilizer made. Surprisingly, the C:N ratio improved tremendously after composting. This showed that the microorganisms present in SBE, 8000 colonial form unit (CFU) in 10 mL of diluted SBE supernatant, had utilized the residual oil and the organic matter readily available in SBE as carbon source to manipulate and transform SBE into a suitable base material facilitating microbial activities.
When SBE is associated with soil, the CEC of soil will be improved (Table 4) by weakly binding the exchangeable cations onto the negatively charged soil surface via electrostatic forces. The CEC of the mineral soil mixed with composted SBE at SBE:soil ratio of 50:50 has increased from its original 8 cmol per kg to 32–39 cmol per kg. This is indicative of an increase in organic matter (source of negative electrostatic sites), thus an increase in ability of the soil to exchange, attract and retain nutrient elements from SBE in a loosely bound bonding. This will prevent nutrient loss via leaching by allowing plants to extract them from the soil via ‘swapping’ them with H+.