Crops grown in all media started to show a sharp increment in plant height after 85 days after sowing (Fig. 2). In most treatments, the growth pattern appeared to be sigmoidal. The growing media did not affect the height of the roselle plants (P > 0.05). These plants reached the average height of 803 (± 46.2) mm after 127 days after sowing in all treatments.
Food waste (F) medium was highly aerated and provided more porous segments (Ruggieri et al. 2009), which might not have been suitable for long-term plant growth, despite being an essential characteristic for seed germination. Optimum macroporosity could influence the water retention potential to the medium compactness, resulting in an improved plant growth performance (Drewry et al. 2008). Thus, soil (S) may provide a better porosity compared with food compost alone. Potting mixed soil contained air porosity of 25–30% and water holding capacity of ~ 36% (Beardsell et al. 1979). Crops cultivated in soil and food waste compost (SF) showed an exponential growth after 99 days and began to outperform the plants grown in F, SM, and SFM media.
The total number of leaves did not differ among the treatments (P > 0.05). Across all treatments, the crop produced 27 ± 1.4 leaves after 127 days after sowing (Fig. 3). The production of leaves was indicative of the plant development and it is independent of plant growth. The appearance of leaves started to increase exponentially after 99 days for crops cultivated in SF media and 106 days for those grown in food waste compost (F). It is believed that during this time, the plants have produced secondary leaves and have begun to expand their canopy, where they will vigorously compete for light and nutrient. Therefore, the rate of leaf production would determine its success in establishment. In these treatments (SF and F media), plants that formed secondary leaves earlier than plants in other media have the advantage of capturing more incoming radiation through the expansion of the canopy.
Total leaf area
The total leaf area was measured following harvest at 127 days after sowing (Fig. 4). Treatments showed significant differences between SF and SM crops at P < 0.05 (Fig. 4). As expected, SF crops with total leaves of 34 ± 2.5 cm2 (Fig. 3) gave the largest leaf area of 1212 ± 160.0 cm2 (Fig. 4). In contrast, SM crops have a canopy size of 460 ± 34.4 cm2 because they only produced 23 ± 1.3 cm2 leaves at the time of harvest. Thus, the addition of FW compost into the soil at 1:1 volume ratio has elevated the growth performance of roselle crop. The FW compost supplied additional macronutrients to the crop, which promoted both vegetative and reproductive growth. In addition, the results from the analyses of nitrogen, phosphorus, and potassium contents showed that these macronutrients were present in large amounts in FW compost (F) (refer to Fig. 6).
Number of fruits
The final numbers of fruits produced were varied among the growing media (P < 0.01) (Fig. 5). The first fruit was initiated after 92 days for crops cultivated in soil (S), and soil and food waste compost (SF). At 106 days after sowing, crops sown in SF media began to produce fruits exponentially and outnumbered the production of crops in other treatments. As a result, crops in SF media produced the highest number of fruits (11 ± 1.1 fruits) at 127 days of growth. The ability of SF crops to produce more fruits could be related to its early canopy establishment (Fig. 3). SF crops produced secondary leaves earlier (after 99 days) compared with crops from other treatments. The production of secondary leaves led to the formation of branches within the plants. Consequently, more growing points were available for fruit initiation. Roselle crops grown in SM media showed the slowest progress in fruit production. Its productivity started only after 99 days after sowing, and the rate of fruit production was only one fruit per week. Thus, following the observation period of 4 weeks, this treatment has only yielded up to 4 ± 0.8 fruits at day 127 and recorded the lowest number of fruits among all treatments.
The dry matter yield of fruits and vegetative components of these roselle plants were different (P < 0.05) among all treatments (Table 3). For fruit production, crops cultivated under S, F, and SFM media produced an average biomass of 1.78 g/plant. The mixture of both soil and FW compost was able to supply additional nutrients and improve the porosity of the soil. These conditions, in turn, increased the water infiltration into the soil, root penetration, crop nutrient uptake, and water uptake. In contrast, the growing media based on food waste compost alone (F) and soil + magnetite (SM) were inadequate to support the growth of roselle crops (Figs. 2, 3, 4 and 5), and consequently, giving the lowest fruit yield (~ 0.46 g/plant). Total dry matter production for crops cultivated in S and SF media gave the highest aboveground biomass of ~ 11.37 g/plant, which was contributed by the heaviest weight of fruits, leaves, and stems (Table 3).
Effect of Fe3O4 on plant growth
The growth performance in response to magnetite application was compared between roselle crops cultivated in soil + magnetite (SM), and its control, soil (S) growing media. The results showed that crops grown merely in soil performed better without the application of Fe3O4. For example, the canopy size of S crops was bigger (795 ± 36.8 cm2) than the canopy size of SM crops (460 ± 34.4 cm2) (Fig. 4). Crops in S medium produced slightly more fruits (5 ± 0.3 fruits) than those in SM media (4 ± 0.4 fruits) (Fig. 5). The total aboveground biomass in S crops was also two times heavier (10.59 ± 0.647 g) than the SM crops (5.17 ± 0.475 g) (Table 3).
Similarly, the comparison between SFM and its control SF showed that the addition of Fe3O4 into the growing medium has reduced the yielding ability of roselle crops. For example, SFM crops yielded only 7 ± 1.7 fruits compared with SF crops (11 ± 1.1 fruits) after 127 days (Fig. 5). In addition, the total aboveground biomass produced by SFM crops was much lower, at 7.03 ± 2.541 g/plant compared with the total aboveground biomass produced by SF crops (12.15 ± 2.343 g/plant) (Table 3). Crops grown in SFM media also produced a smaller canopy size of 716 ± 226.8 cm2 compared to 1212 ± 153.0 cm2 from SF treatment (Fig. 4).
Roselle plants could be deduced to demonstrate low tolerance to direct application of Fe3O4. This could be due to the Fe3O4 powder having no attachment to the soil, and also likely due to the non-optimized amount of Fe3O4. This observation corroborates the results in a study reported by Zhu et al. (2008), whereby the addition of Fe3O4 in soil did not favor the growth of pumpkin.
Nutrients and plant tissue analysis
In all growing media, the levels of nitrogen, phosphorus, and potassium content were higher prior to crop cultivation (initial) compared with post-harvest (Fig. 6a–c). The initial nitrogen content in SF media was 342.8 mg/kg, which reduced to 290.2 mg/kg after 127 days (post-harvest) (Fig. 6a). The declining nutrient level as a function of time is expected as the loss of nutrients was accounted for from the uptake by plants and leaching. In addition, the cultivation of roselle during the wet season (total precipitation of 1848 mm throughout the growing period) is believed to be the main cause of nutrient leaching. However, the amounts of nitrogen, phosphorus, and potassium losses from the growing media via leaching and plant uptake were not assessed in this present study, and should be considered in future work.
The initial contents of all macronutrients were in abundance in the food waste compost (F) media, with 938.75 mg/kg of nitrogen, 2.09 mg/kg of phosphorus, and 78.68 mg/kg of potassium. The inclusion of banana peels, which constituted the second largest component in the food waste compost, could have contributed to the high level of potassium. Banana peels provide potassium nutrition, which is beneficial for fruit development (Kalemelawa et al. 2012). Nevertheless, the nutrient compositions for the individual components that make up the food waste, namely tea leaves, coffee grounds, banana peels, eggshells, and lemongrass leaves were not quantified. This was an oversight in this study and therefore, should be included in future study. In plant tissues, the concentrations of phosphorus and potassium were found to be the highest in the soil (S) treatment (Fig. 6b–c). The mobility of the phosphate ions in soil could have increased the phosphorus uptake by the plants through rhizosphere acidification and root proliferation, which provide enzyme-catalyzed hydrolysis to secrete the organic phosphorus (Jing et al. 2010; Shen et al. 2011).
Accumulation of heavy metals in plant tissue
The chemical properties of the top soil and FW compost are presented in Table 4. FW compost recorded a slightly higher pH than the top soil. The basic condition of FW compost could be attributed to the eggshells and banana peels, which could serve as a liming material (King’ori 2011) and an alkalinity agent with great acidic controlling properties (Barreira et al. 2008), respectively. The top soil recorded a higher concentration of metals compared to FW compost. Iron (Fe) and aluminum (Al) were found to be present in elevated concentrations in the top soil. In the case of FW compost, Al and Fe were detected to be high, but their concentrations were relatively lower than those in the top soil.
The concentrations of lead (Pb), cadmium (Cd), and copper (Cu) found in roselle crops were negligible (0–0.05 mg/kg) (Fig. 7). With the exception of SFM treatment, Pb was not detected in roselle plant tissues. In contrast, a previous study reported that Pb was detected in roselle plant tissues from the application of organic manure into soil (Wuana and Mbasugh 2014). The initial sample of growing media did not indicate the presence of Cd, but after 127 days of cultivation, the presence of Cd could be associated with organic matter and carbonate. This association provided a mobility pathway for Cd to be taken up by the plant, together with iron (Fe) mineral dissolution (Muehe et al. 2013). Hence, Cd is known as a mineral-associated contaminant.
Crops grown merely on soil (S) and food waste compost (F) have extracted the least amount of nickel (Ni) (0.11 and 0.15 mg/kg, respectively) compared to crops grown in the rest of the growing media (~ 0.22 mg/kg). The highest level of manganese (Mn) (1.44 mg/kg) was found in crops cultivated in food waste compost (F), followed by SFM crops (0.74 mg/kg). The concentrations of zinc (Zn) and Fe were 0.59 and 1.62 mg/kg, respectively, across all growing media. Fe content in plant tissues was higher compared with the contents of Cu, Zn, and Pb, which are similar to the results reported by Chiroma et al. (2012). This could be due to the high possibility of Fe strategizing its translocation from the growing medium to the plant tissue, mainly on the interaction of ferric chelate reductase, which ended up mostly in the leaf chloroplast (Jeong and Connolly 2009).
Among all heavy metals detected, aluminum (Al) was present in the highest amount of 2.39 mg/kg across all growing media. However, the presence of Al in plant tissues can be considered as low. This is because Al is mainly accumulated in the roots (underground) compared to aboveground, such as in stems, leaves, and fruits due to its low ability to be transported into higher parts of the plant (Ondo et al. 2017).
The heavy metal concentrations in plant tissues for all elements did not violate the permissible limits of World Health Organization (WHO) guidelines for vegetable crops (Bigdeli and Seilsepour 2008). Thus, roselle plant could be deduced to possess low tolerance to heavy metals. The pH values of soil (7.1) and FW compost (8.5) that were between neutral and basic would have prevented these heavy metals from being optimally absorbed (Table 4). According to Zheng et al. (2005), elements such as Cu, Fe, Mn, and Pb have great solubility and provide the bioavailability for plant uptake in the region of slightly acidic medium compared to a basic condition. Thus, the low amounts of heavy metals in roselle plant tissues shows that the fruits are safe to be consumed.
Iron leaching test
Fluctuation of Fe leaching in some weeks could be due to environmental factors, such as rainfall because the pots were placed in an open area. Rain water has been exposed to the pots, which could have contributed to the fluctuating trend of Fe content every week. SFM can be seen to be more dramatically affected by the Fe3O4 introduction to the medium (Fig. 8). The leachate might contain some amount of medium erosion from the SFM pots due to the texture of FW compost, which was light, aerated, and could easily pass through the cotton cloth into the leachate. This might have caused the higher Fe content in SFM leachate, which was not entirely from the addition of Fe3O4. SM treatment showed a marginal change in Fe leaching throughout a period of 127 days. Leachate from the SFM was at 18.8 ± 8.11 mg/L on day 29 of the highest Fe concentration. Meanwhile, day 57 recorded the lowest concentration of 0.5 ± 0.43 mg/L (Fig. 8). SM treatment showed the maximum Fe concentration of 1.6 ± 1.25 mg/L on day 50, while the lowest concentration of 0.2 ± 0.13 mg/L was collected on day 78. According to the Ministry of Health in Malaysia (MOH), Fe concentration limit is 1.0 mg/L for raw water bodies, which is then considered as suitable to be treated as water supply. The Department of Environment (DOE) in Malaysia has outlined the maximum amount of 5.0 mg/L of leachate discharge as inorganic Fe (Department of Environment 2010). Based on the leaching test, the high concentration of Fe was deemed unsuitable to be introduced to agricultural areas near water bodies, or water reservoirs to avoid Fe toxicity in water. The high concentration of Fe could also be attributed to the initial Fe levels present in soil and FW compost.