# Malaysian Virgin Soil Extracts as Natural Growth Enhancer for Targeted Green Microalgae Species
## 3 Results and Discussion  
### Nutrient Recovery from Different Extraction Methods  
The selection for different extraction methods was based on autoclave methods from previous studies which used specific temperatures such as 105 \({}^{\circ}\)C [22] and 121 \({}^{\circ}\)C [23] to sterilize the sludge samples. Soil samples were used for this study due to the presence of copious amounts of humic substances. Humic substances are well recognized to aid natural plant growth as a result of the occurrence of rich bioactive organic carbon sources. Therefore, the activity of numerous antioxidant enzymes can be effectively improved to promote the physiological metabolisms of plant cells [24]. Studies carried out by [PERSON] [24] and [PERSON] [25] observed that the supplementation of humic acid alleviated the growth of _Euglena piciformis_ AEW501 and _Scenedensmus Capricornus_, respectively. When the culture medium was improved with 50 mg/L humic acids, maximum biomass productivity of 35 mg/L/d and greatest high-value product (lipid content) accumulation of 60% were achieved with _Euglena piciformis_ AEW501 [24]. Humic substances below the concentration of 2 mgC/L had a positive effect on the growth of _Scenedensmus Capricornus_, which was simultaneously accompanied by increased synthesis of chlorophyll and macromolecules in the microalgae [25]. The retrieval of dissolved organic carbon (DOC), total dissolved nitrogen (TDN), and total dissolved phosphorus (TDP) is summarized in Table 3. On the whole, the recovery of TDP was substantially lower (\(p<0.05\)) compared to that of DOC and TDN irrespective of the treatment methods used. This may be attributable to dissimilar rates of organic matter mineralization in soil [26]. Significantly lower extractable nutrients (\(p<0.05\)) were achieved with soil extracts from the 24-h natural extraction method carried out at room temperature in comparison to autoclaved soil extracts. The use of autoclave heat shock supported the release of labile nutrients into the soil extracts with minimum alterations to the physical properties of the soil extracts [27], contributing to increased concentrations of DOC, TDN, and TDP when weighed against the natural extraction process. It was interesting to note that the utilization of autoclave 121 \({}^{\circ}\)C twice treatment method yielded the greatest recovery of DOC of 336.56 mg/L, which was 474% and 108% higher than natural extraction dan autoclave 105 \({}^{\circ}\)C twice treatment method, respectively. The recovery of TDN and TDP observed a similar trend when the autoclave 121 \({}^{\circ}\)C twice treatment method was exploited to obtain the soil extracts, whereby 164% and 26% higher recovery of TDN and 75% and 40% higher recovery of TDP were recorded compared to that of natural extraction dan autoclave 105 \({}^{\circ}\)C twice treatment methods, respectively. Autoclave methods allowed more extraction of organic matter than natural extraction at 24 h which may be due to the disintegration of polysaccharide structures, alteration processes that influence the soil surface, and the occurrence of unknown reactions or nutrients within the soil matrix [28]. In addition, the collapse of chemical bonds could only occur when the activation energy was exceeded to disrupt the soil materials into carbon, nitrogen, and phosphorus compounds released in the soil extracts [29]. Hence, the autoclave methods assisted the supply of surplus energy which enabled the release of higher concentrations of DOC, TDN, and TDP in the soil extracts in comparison to natural extractions which occurred at a lower temperature, hampering the breakdown of any chemical bonds since the heat energy is too minimal to overcome the activation energy [19].