The control treatment with the 100% hydroponic fertilizer had significantly the highest concentrations of phosphorus and nitrogen . The subsequent substitution of the biogas slurry with the hydroponic fertilizer from 20% to 60% resulted in a direct increase in the concentration of P and N. Relative to the control treatment, the 20% substituted treatment showed a 5.6 and 4.8 times less P and N, respectively. Unlike for P and N, the concentrations of the cations Na, Ca, Mg and K were significantly higher in the treatments with biogas slurry, with the control having the lowest concentrations. The concentration of Ca and K in the 20% substituted treatment was 107.7% and13.9% more relative to the control treatment. However, for Mg the highest concentration was observed in the 60% substituted treatment, whilst for Na, the highest concentration was observed in the 40% substituted treatment. Several trace metals which include As, Cd, Cr, Ni, Zn and Pb were measured in this study. However, it was noteworthy that the concentrations of most of these metals were below detectable limits except for Pb and Zn only . Of these two metals, there were no significant differences observed between the treatments on concentrations of Zn, whilst significant differences were observed for Pb. The concentration of Pb ranged from 0 to 0.0015 mg/L whilst that of Zn ranged from 0.08 to 0.15 mg/L. The pH in the different treatments was not significantly different, with all values being neutral ranging from 7.69 to 7.91. However, pH was slightly higher in the treatments with biogas slurry, resulting in an average 2.4% difference relative to the control. The electrical conductivity was also significantly higher in the treatments with biogas slurry whilst it was lowest in the control treatment. On average, EC within the treatments with biogas slurry was 23.8% higher compared to the EC in the control treatment. Across all the treatments, EC ranged from 275.7 to 356.7 μS/cm.

The chlorophyll content of a plant is a general indicator of nutrient status and nutrient response of the plant. In our study, chlorophyll content was expressed as an index ranging from 1 to 45, with 1 being the lowest. There were significant differences observed between treatments on chlorophyll content index , with the control having the highest values whilst the 20% substituted treatment had the lowest index . Relative to the control treatment, the 20%, 40% and 60% substituted treatments resulted in 39.4%; 22.8% and 8.7% significantly lower CCI. The marketable fresh fruit yield was significantly different among all treatments, with a clear trend where the higher the fertilizer substitution, the higher the tomato yield. Relative to the control, the 20%; 40% and 60% treatments resulted in 14.4; 4.6 and 0.8 times less fresh fruit yield, respectively. It was important to note that substitution of the biogas slurry even with 60% of the hydroponic nft channel fertilizer did not result in fruit yields comparable to those in the control treatment. On another note, the sugar content which was measured in Brix units, increased with a decrease in nutrient substitution of the different treatments . There were statistically significant differences in sugar content with the 20% substituted treatment having the highest sugar content whilst the control had the lowest content. Relative to the control treatment, the 20%, 40% and 60% resulted in 80.3%; 46.5% and 30.6% more sugar content, respectively. The last parameters measured to determine yield were the number of fruits and flowers between the treatments. Similar to fresh yield results, there were significant differences observed among treatments, with the control having the highest number of fruits and flowers, whilst the 20% substituted treatment had the lowest numbers . Compared to the control treatment, the 20%; 40% and 60% treatments resulted in 7.4; 2.6 and 1.1 times less number of fruits, respectively. In a similar trend, relative to the control, the 20%; 40% and 60% treatments resulted in 4.8; 1.1 and 0.9 times less number of flowers, respectively .For the cations Ca, Mg, K and Na in the tomato plant biomass, the 20%substituted treatment resulted in significantly the highest uptake of Ca and Mg . For Ca uptake, the 20% substituted treatment resulted in 102.7% more Ca, whilst the 40% and 60% substituted treatments resulted in an average 2.9% more Ca only, relative to the control. For K concentration, as the concentration of the hydroponic fertilizer substitution increased, the uptake of K by the plant decreased.

The K in the tomato plant biomass under the 20% substituted treatment was 20.4% more than that of the control treatment. The lowest concentration of Mg in the plant biomass was observed in the control treatment, which was 122% less than that observed in the 20% substituted treatment. However, there were no significant differences observed between the 40% and 60% substituted treatments for Mg concentration. It was interesting to note that Na concentration was significantly the highest in the control treatment, whilst the biogas digestate based treatments did not record significantly different concentrations . On average, the Na concentration in the control treatment was 116.3% more relative to all the three biogas digestate based treatments. The concentration of most of the measured trace metals in the tomato plant biomass was below detectable limits except for Pb and Zn only . The concentration of Pb in the plant biomass showed an unusual trend across the treatments, with the 60% substituted treatment having the highest concentration whilst the 20% substituted treatment had the lowest concentration. The concentration of Pb in the 40% and 60% substituted treatments was 22.7% and 96% higher, respectively, compared to the control treatment, whilst the 20% substituted treatment recorded a 78.6% less Pb, compared to the control. On the contrary, the concentration of Zn in the plant biomass significantly decreased with an increase in the level of substitution of the hydroponic fertilizer . The 60% substituted treatment recorded plants with the lowest concentration of Zn followed by the control. The concentration of Zn in plants within the 20% substituted treatment was 57.8% higher, relative to the control.Biogas digestates have been reported to have high levels of biological oxygen demand, suspended solids and low dissolved oxygen, whilst being rich in nutrients such as N and P . However, studies that have evaluated the fertilizer potential of these digestates in hydroponics crop production are scarce . In our study, the raw biogas digestate diluted at 20% and 40% resulted in crop phytotoxicity as indicated by the low germination indices. Germination index is a very sensitive parameter used for evaluating the phytotoxicity of organic materials and GI indices below 50% have been reported to indicate materials unsuitable for use in crop production . These lower GI have been attributed to heavy metals, ammonia and low molecular weight compounds that reduce seed germination and root elongation . Though heavy metals were significantly low in the raw biogas digestates , it is possible that the presence of higher levels of ammonia versus nitrates and other organic compounds in the digestates used in our study could have resulted in elevated phytotoxicity at higher concentrations of biogas digestates.

Moreover, the observed phytotoxicity cannot be explained by the electrical conductivity or medium salt content, as these were well within the recommended values. It was also noted that different crops showed varying phytotoxicity responses to the biogas digestates, indicating the difference in their tolerance levels. Several researchers have indicated that biogas digestates are very high in phosphorus and nitrogen, as these are not utilized during the anaerobic digestion process . However, the concentration of these nutrients is highly dependent on the source of the manure. In our study, the manure was collected from animals that had been feeding on natural shrubs from the Namib Desert, whose macro nutrient content is very low. This could explain the low macro nutrient contents observed in the biogas slurry, which were not in the same range as reported by Juan et al. . The low macro nutrient concentrations therefore informed the need for substitution of the biogas slurry with the different levels of inorganic hydroponic fertilizer. Even under substitution with hydroponic fertilizer, the macro nutrient levels in the biogas digestate based treatments remained low in our study. This clearly indicates the lack of positive benefit from the cow manure based biogas digestate especially on the concentrations of macro nutrients in hydroponic nutrient solutions. However, the concentration of the cations was increased under the biogas based treatments, which is in agreement with results of Wang et al. even when only 2.6% biogas digestate were used with mineral fertilizers. The results of our study therefore indicate that the diluted biogas digestate can be an important source of micro nutrients such as Ca and Mg, though supplementation with other sources of macro nutrients is required. It was interesting to note that the concentration of heavy metals in the biogas digestate were lower than the recommended limit for irrigation water . These higher concentrations of elements like Ca and Mg as well as the lower concentration of heavy metals observed in the tomato plant biomass can be explained by the nutritional composition of the nutrient solutions used in this study.

The yield parameters which include chlorophyll content, fresh yield and number of fruits all showed a direct response to the concentration of N and P in the different solutions. These reductions in yield properties of the tomatoes could be attributed to the variability and imbalance in nutrient composition of biogas digestate based hydroponic solutions . According to Moller and Muller ,nft growing system in crops like tomatoes, conversion of ammonia to nitrates and nitrites together with supplementation of micro-nutrients may be required before biogas digestates are used in hydroponic culture. Based on the results of our study, the high nutrient demand for tomatoes suggest that other less demanding crops such as lettuce may be more suitable for cultivation using cow based biogas digestates. However, the sugar concentration was inversely related to the nutritional composition of the hydroponic growth media in our study. Similar results were reported by Wang et al. and this has been attributed to plant physiological response to high salt content under the biogas digestate based treatments.High demand has bolstered trade in medicinal plants and subsequently, creating the need for commercial cultivation of these plants. However, successful commercial cultivation of medicinal plants relies on achieving consistently high quality and quantity of medicinal materials at a reasonable cost of production. Consequently, efficient technologies and techniques are being developed. For example, greenhouse technologies circumvent the setbacks which are normally associated with open air conditions, such as variations in biotic and abiotic factors, and limited arable land . A plethora of studies have examined the effects of varying levels of specific abiotic factors on the production of secondary metabolites . On the contrary, much fewer studies have investigated the effects of biotic factors on nutraceutical and medicinal plants . Some entomopathogenic fungi are endophytic; some are easily mass-produced in vitro; some are rhizospheric; they are quite ubiquitous . An endophytic fungus forms a mutually beneficial symbiotic relationship with a plant; it lives inside a plant’s tissues without causing disease to the plant, meanwhile, boosting plant defenses and in return the plant acts as the host.

The increase in quantity and diversity of secondary metabolites in endophyte-containing plants are probably responsible for the reduction of insect herbivory on plants. The endophytic fungi-plant relationship can be explored for cultivation of targeted high value medicinal plant species with the view to optimizing medicinal properties by increasing quantity and quality of secondary metabolites in these plants. Beauveria bassiana, an entomopathogenic fungus which occurs naturally and ubiquitously in the soil , is endophytic and an interesting candidate for the evaluation of endophytic fungal colonization on the medicinal properties of plants. Plants belonging to the genus Allium including garlic, onion, and Chinese chive are known for their medicinal properties; they have proven pharmacological and nutraceutical activities. They contain organosulfur compounds such as allicin and phenolic compounds like gallic acid, quercetin, coumaric acid, and ferulic acid . Phenolic compounds are bio-active substances that show antioxidant, anticancer, anti-inflammatory, and antimicrobial activity, and prevent chronic disease .