Chemical and Biological Characteristics of Organic Amendments Produced from Selected Agro-Wastes with Potential for Sustaining Soil Health: A Laboratory Assessment

: Sustaining soil health cannot be divorced from sustainable crop production. Organic, or natural, farming is being promoted as a good sustainable agriculture practice. One aspect of organic farming that could signiﬁcantly enhance and sustain soil health, soil quality, and crop productivity is the use of high-quality soil conditioners or organic amendments produced from agro-wastes. Thus, the objective of this study was to characterize the chemical and biological properties of selected agro-wastes with potential for use as organic amendments in sustaining soil health. Standard procedures were used to produce and characterize the soil conditioners, namely fermented plant juice (FPJ), fermented fruit juice (FFJ), palm kernel shell (PKS) biochar, and kitchen waste (KW) compost. The fermented juices (FPJ and FFJ), PKS biochar, and KW compost exhibited chemical and biological properties with good potential as soil conditioners or organic amendments to sustain soil health. The fermented juices contained important microbes that can solubilize P and K in soil for crop use. The high pH and C content of the biochar and compost and the high cation exchange capacity of the biochar are good indicators of the potential of these materials to sustain soil health in terms of the liming effect of acid soils, nutrient and water retention, nutrient reserves, and a suitable habitat for microbial life. Moreover, the organic amendments contain reasonable amounts of macro- and micro-nutrients, which could be released to increase soil fertility. Despite these potential beneﬁts, ﬁeld application of these organic amendments is necessary to evaluate their effects on soil health and crop production in both the short and long term.


Introduction
Agricultural wastes, or agro-wastes, are residues and left-overs from the growing, harvesting, and processing of raw agricultural products from crop and animal farming. Organic farming (OF), or natural farming (NF), is an environmentally friendly farming Sustainability 2021, 13, 4919 2 of 15 system because it uses organic amendments that are produced from waste materials such as agricultural wastes that are free of toxic chemicals. It has been reported that NF can contribute to a holistic production management system that promotes and enhances agroecosystem health, including biodiversity, biological cycles, and soil biological activity [1][2][3]. Maintaining soil health, which includes the physical, chemical, and biological components, is a very crucial aspect in humanity's quest for sustainable agriculture, particularly crop production [4].
Organic soil amendments produced from agro-wastes have been shown to play various vital roles in sustaining soil health and crop production [5][6][7][8]. Despite these known benefits, not all agro-wastes possess good chemical and biological properties to sustain soil health due to the differences in the waste or residual materials. Some may even be deleterious to crop growth and soil health [5]. Therefore, to make informed decisions about their potential, it is important to characterize and assess agro-wastes before they are used as soil amendments. Ready availability and abundance of the agro-wastes is another important consideration in convincing farmers to recycle these residues and left-overs for use as soil amendments. This could considerably reduce their reliance on chemical fertilizers and the attendant environmental problems caused by either the excessive use of chemical fertilizers or the poor disposal or non-recycling of agro-wastes. It is worth emphasizing that the adoption of organic soil conditioners or amendments in crop cultivation must be based on specific and precise standards of production that aim at achieving optimal agro-ecosystems that are socially, ecologically, and economically sustainable [9]. When these standards of production are adhered to, farmers can fetch higher prices for their produce on the market due to the growing consumer preference for such types of certified organic agricultural products. Due to the growing proliferation of organic agricultural products on the market, many countries are developing their own standards and schemes for certifying these organic products. In Malaysia, this standard is called the Malaysian Organic Certificate Scheme [10,11]. The standard includes the use of permitted organic materials for land and soil management. In Malaysia, farm wastes such as the leaves of tapioca, bamboo, vegetables, and banana plants, grass chippings, discarded fruits, rice straw, rice hulls, sago bagasse, and corn stalks are some of the materials used to produce organic soil conditioners or amendments [10,12]. The conversion of crop agro-wastes into fermented juices, composts, and biochar are easy and acceptable methods for local farmers to utilize their agro-wastes for recycling in their farms as organic soil amendments It is against this background that this study was carried out with the sole objective to characterize the chemical and biological properties of recycled agro-wastes for potential use as organic amendments for sustaining soil health.

Production of the Fermented Juices
The two fermented juices produced in this study were fermented plant juice (FPJ) and fermented fruit juice (FFJ). Both FPJ and FFJ were produced using agricultural wastes from pepper gardens and kitchen wastes. The juice production was based on the method of Zamora and Calub [12]. Fermented plant juice (FPJ) is associated with the enzymes in leaves that enhance plant growth and the promotion of photosynthesis [12]. The FPJ consisted of fresh, juicy, and succulent plant parts including water spinach, Chinese mustards, banana stems, tapioca, bamboo leaves, and green grasses. To produce FPJ, 1 kg of plant parts were cut into pieces using multipurpose kitchen scissors (Elianware, Penang, Malaysia) after which they were mixed with 1 kg of brown sugar in a clay jar. The brown sugar provided a ready source of carbon or energy for the microbes during the fermentation process. The clay jar was covered with a clean sheet of paper and tied with string. The clay jar was then kept in a cool and shaded place for the contents to ferment for seven days. Thereafter, the mixture was filtered to separate sludge from the juice. This process yielded approximately 1 L of juice. The steps to produce the fermented plant juice are shown in Figure 1. tion process. The clay jar was covered with a clean sheet of paper and tied with string clay jar was then kept in a cool and shaded place for the contents to ferment for s days. Thereafter, the mixture was filtered to separate sludge from the juice. This pro yielded approximately 1 L of juice. The steps to produce the fermented plant juic shown in Figure 1. Fermented fruit juice (FFJ) improves the development of plant fruits by suppl them with K [12]. The FFJ in this study was produced from discarded fruits inclu banana, papaya, pumpkin, and pepper berries ( Figure 2). As was the case for the pro tion of FPJ, 1 kg of the fruits were cut into pieces after which they were mixed with of brown sugar in a clay jar. The FFJ was prepared in the evening to prevent interfer from flying insects. The clay jar was covered with a clean sheet of paper and tied string. Thereafter, the clay jar was kept in a cool and shaded place for the contents to ment for seven days. Afterwards, the mixture was filtered to separate sludge from juice. This process yielded approximately 1 L of juice. Fermented fruit juice (FFJ) improves the development of plant fruits by supplying them with K [12]. The FFJ in this study was produced from discarded fruits including banana, papaya, pumpkin, and pepper berries ( Figure 2). As was the case for the production of FPJ, 1 kg of the fruits were cut into pieces after which they were mixed with 1 kg of brown sugar in a clay jar. The FFJ was prepared in the evening to prevent interference from flying insects. The clay jar was covered with a clean sheet of paper and tied with string. Thereafter, the clay jar was kept in a cool and shaded place for the contents to ferment for seven days. Afterwards, the mixture was filtered to separate sludge from the juice. This process yielded approximately 1 L of juice.

Chemical Analysis of the Fermented Juices
To determine the chemical composition of the fermented juices, batches of the produced FPJ and FFJ were analyzed for N, P, K, Mg, and B using standard laboratory procedures. Total N was determined using the combustion or modified Dumas method [13]. Total P, Mg, and B were determined using inductively coupled plasma optical emission spectrophotometry (ICP-OES, Hitachi, Tokyo, Japan) [14]. Using the method of Kimura and Araya [15], the total K was determined by flame atomic absorption spectrometry (AAS AA-7700, Shimadzu Corporation, Kyoto, Japan).

Identification of the Microorganisms and Screening for Phosphate and Potassium Solubilizers in the Fermented Juices
The identification of fungal and bacterial microorganisms based on the method described by Raja et al. [16] was carried out in concocted batches of FPJ and FFJ. Pikovskaya's and Aleksandrow agar media were used to detect phosphate and potassium solubilizers, respectively, according to the method described by Himedia [17]. All the observations were carried out in triplicate.

Chemical Analysis of the Fermented Juices
To determine the chemical composition of the fermented juices, batches of the produced FPJ and FFJ were analyzed for N, P, K, Mg, and B using standard laboratory procedures. Total N was determined using the combustion or modified Dumas method [13]. Total P, Mg, and B were determined using inductively coupled plasma optical emission spectrophotometry (ICP-OES, Hitachi, Tokyo, Japan) [14]. Using the method of Kimura and Araya [15], the total K was determined by flame atomic absorption spectrometry (AAS AA-7700, Shimadzu Corporation, Kyoto, Japan).

Identification of the Microorganisms and Screening for Phosphate and Potassium Solubilizers in the Fermented Juices
The identification of fungal and bacterial microorganisms based on the method described by Raja et al. [16] was carried out in concocted batches of FPJ and FFJ. Pikovskaya's and Aleksandrow agar media were used to detect phosphate and potassium solubilizers, respectively, according to the method described by Himedia [17]. All the observations were carried out in triplicate.

Biochar Production and Chemical Analysis
The method of Wahi et al. [18] was used to produce biochar from palm kernel shell (PKS), which was obtained from Kota Samarahan, Sarawak, Malaysia. The pH of the mixture of biochar and water (1:10, w/v ratio) was measured with a portable pH meter (Starter 300, Ohaus, NJ, USA). The determination of ash content was conducted according to the American Society for Testing and Materials (ASTM) D1752-84, which is a method recognized by the International Biochar Initiative [19]. The cation exchange capacity (CEC) of the biochar was determined using the ammonium acetate method [20], whereas the anion exchange capacity (AEC) was measured using bromide as an index anion based on the method of Lawrinenko [21]. The total organic carbon (TOC) of the biochar was determined using the loss on ignition method [22]. The total N of the biochar was determined using the micro-Kjeldahl method [23]. The other macro-and micro-nutrients (P, K, Ca, S, Al, B, Steps in producing the fermented fruit juice: (1) fruits cut into pieces; (2) 1 kg of cut fruits weighed; (3) cut fruits placed into a clay jar; (4) 1 kg of brown sugar added; (5) all components mixed well; (6) covered and sealed jars left to ferment for 7 days; (7) fermented mixture of cut fruits and brown sugar after 7 days; (8) fermented mixture filtered to separate sludge from the juice; (9) 1 L of the fermented fruit juice placed in a container.

Biochar Production and Chemical Analysis
The method of Wahi et al. [18] was used to produce biochar from palm kernel shell (PKS), which was obtained from Kota Samarahan, Sarawak, Malaysia. The pH of the mixture of biochar and water (1:10, w/v ratio) was measured with a portable pH meter (Starter 300, Ohaus, NJ, USA). The determination of ash content was conducted according to the American Society for Testing and Materials (ASTM) D1752-84, which is a method recognized by the International Biochar Initiative [19]. The cation exchange capacity (CEC) of the biochar was determined using the ammonium acetate method [20], whereas the anion exchange capacity (AEC) was measured using bromide as an index anion based on the method of Lawrinenko [21]. The total organic carbon (TOC) of the biochar was determined using the loss on ignition method [22]. The total N of the biochar was determined using the micro-Kjeldahl method [23]. The other macro-and micro-nutrients (P, K, Ca, S, Al, B, As, Cd, Cu, Pb, Hg, Ni, Zn, Co, Mn, Cr, and Fe) were extracted and determined using the dry ashing method [24] and inductively coupled plasma optical emission spectrophotometry (ICP-OES, Hitachi, Tokyo, Japan) [14], respectively.

Compost Production and Chemical Analysis
The raw materials used in producing the compost were leaves from tapioca, bamboo, vegetables, and banana plants, grass chippings, discarded fruits, rice straw, rice hulls, and corn stalks. The raw materials were piled in a foam box with an inner size of 43 cm (width) × 56 cm (long) × 30 cm (height) and a volumetric capacity of approximately 72 L. To maintain adequate oxygen (O 2 ) levels, the piles were turned weekly using a small hoe. A sample comprising 100 mL of FPJ in 1 L of dechlorinated water was prepared. The FPJ was applied to the compost pile to keep the moisture content at not less than 50% and at the same time accelerate the composting process through the actions or functions of beneficial microorganisms from the fermented juice [12]. Samples were taken at the end of the composting process to determine the chemical properties of the compost. The pH, TOC, and total N, P, K, and Ca were determined using similar methods used for the biochar analysis. The moisture content and electrical conductivity (EC) of the compost were determined using a moisture/EC sensor (SMEC 300 Waterscout, Spectrum Technologies, Aurora, IL, USA).

Experimental Design and Statistical Analysis
For the two types of fermented juice, the data were analyzed using independent t-tests to ascertain any significant differences between nutrient content means. Ten replications were used for measuring or characterizing the various chemical parameters of the organic amendments. The statistical software used for all data analysis was SPSS, Chicago, IL, USA (version 15).

Chemical Characteristics of the Fermented Juices and Potential Benefits to Soil Health
Selected chemical properties of the fermented juices are presented in Table 1. The nutrient content in the two fermented juices was not significantly different. The juices also had some trace elements. The relatively low concentrations of the macro-and micro-nutrients suggest that the fermented juices produced in this study do not qualify as chemical fertilizers with the required essential nutrient amounts to complete growth, development, and the reproductive cycle of plants. This observation is consistent with that of Kimpinski et al. [25] and Zuraihah et al. [26] who reported that soil organic amendments are primarily used as soil conditioners and not as chemical fertilizers because they are rather high in organic content (90% to 95%) but generally low in macro-and micro-nutrients compared with commercial chemical fertilizers. However, it is worth noting that the nutrient content of the juices could serve as supplements in crop cultivation. Zuraihah et al. [26] reported that fermented juices are comparable to other green-waste-based soil amendments in their conditioning abilities. Zamora and Calub [12] described liquid fertilizers from plant waste fermentation as mostly consisting of a variety of lactic acid bacteria, yeasts, and phototrophic bacteria but not high in essential macro-and micro-nutrients. Although liquid fertilizers may not be high in essential nutrients, Yadav et al. [27] reported that liquid fertilizers can provide important bioresources for agriculture because their beneficial microbes can enhance plant growth and nutrient uptake through solubilization of P, K, and Zn as well as N fixation. Hence, it is possible that the activities of these beneficial microbes can increase crop yield, remove contaminants, inhibit the activity of pathogens, and produce fixed N or other plant growth and development substances.
Yadav et al. [27] reported that growth stimulation by plant microbiomes can be a consequence of biological N fixation, production of plant growth regulators such as indole acetic acids (IAAs), gibberellic acids, cytokines, and biocontrol of phytopathogens. This is achieved through the production of antibiotics, antifungals, or antibacterial agents, Fechelating compounds, nutrient competition, the induction of acquired host resistance, or enhancing the bioavailability of minerals.
These microbes can also play a role in nutrient cycling and increasing soil biodiversity, which are important components of soil health [5]. These potential positive effects on soil biological health need to be evaluated in the field through soil amendments in cropping systems.

Identification of Microorganisms in the Fermented Juices
The microorganisms identified in the fermented juices using the polymerase chain reaction (PCR) method [16] are presented in Table 2. Seventeen microorganisms were detected, isolated, and identified; nine were identified in the FPJ and eight in the FFJ. These microorganisms were classified into eight genera, namely Lactobacillus (two isolates in FPJ), Bacillus (four isolates in FPJ; one isolate in FFJ), Sporosarcina (one isolate in FPJ), Anaerobacillus (one isolate in FPJ), Aspergillus (one isolate in FPJ; four isolates in FFJ), Trichococcus (one isolate in FFJ), Talaromyces (one isolate in FFJ), and Penicillium (one isolate in FFJ). Out of the eight genera, five genera, namely Lactobacillus, Bacillus, Sporosarcina, Anaerobacillus, and Trichococcus were categorized as bacteria, whereas Aspergillus, Talaromyces, and Penicillium were categorized as fungi. Higa and Wididana [2] stated that biofertilizers contain species of microorganisms that are predominantly lactic acid bacteria and fungi. The less dominant microbes were photosynthetic bacteria, actinomycetes, among others [2]. However, the aforementioned microbes are mutually compatible and can coexist in liquid cultures. This co-existence speeds up natural compost production [28]. The PCR amplification for each sample produced BLAST ® search results with E-values, or expectation values, smaller than 0.0, i.e., ranging from (2 × 10 −2 ) to (4 × 10 −149 ) ( Table 2).
The findings suggest that the BLAST ® hits for every sample can be considered a good match for homology matches and the matches of every datum in the database were of high quality [29]. It has also been reported that the lower the E-value, the more significant the match. A value close to zero means that the result would practically expect no unrelated sequence to score as high as the query sequence [29,30]. Fogg and Kovats [30] documented that the E-value is the estimate of how many times ("counts") the results would be expected (for example, a score in a sequence comparison), at least as extreme as the one observed occurring by chance.
The percentage identity searched against the National Center for Biotechnology Information (NCBI) non-redundant database showed an identity of 96% to 100% among the isolated microorganisms (Table 2). Fogg and Kovats [30] mentioned that the percentage identity is a number that describes how similar the query sequence is to the target sequence or how many characters in each sequence are identical. Therefore, the higher the percentage identity, the more significant the match.

Screening for Phosphate and Potassium Solubilizers in the Fermented Juices
To know whether the 17 isolates in the fermented juices possessed nutrient solubilizing potential, these microorganisms were screened for P and K solubilization activities using appropriate media. The results of the microorganisms' capability as P and K solubilizers are shown in Table 3. Based on the clear halo zones formed around the isolates in Pikovskaya's media, the 17 microorganisms in the FPJ and FFJ showed signs of P solubilization. Six microorganisms from FPJ showed K solubilizing capability, whereas, for FFJ, three microorganisms demonstrated K solubilizing ability with clear halo zones formed around the isolates in the Aleksandrow media. This suggests that 9 out of the 17 isolates can potentially solubilize both P and K ( Table 3). The beneficial microorganisms that showed both P and K solubilization potential were Bacillus pseudofirmus, Sporosarcina globispora, Bacillus subtilis, Bacillus amyloliquefaciens, Bacillus licheniformis, Aspergillus niger (FPJ), Bacillus sp. PK-9, Aspergillus niger (FFJ), and Aspergillus terreus.  Studies have shown that certain microorganisms, especially bacteria, function as plant growth promoters through soil nutrient solubilizing effects [31,32]. Microorganisms that are capable of solubilizing nutrients such as P and K are important because they can convert insoluble P and K in soils into soluble P and K. This is done through the release of organic acids, chelation, and ion exchange [33]. Talaat et al. [34] and Yadav et al. [27] reported that soluble P and K, which are converted by beneficial microorganisms, can be easily taken up by plants for growth and development. As mentioned by Saied et al. [33], one of the main mechanisms responsible for releasing available forms of P to plants in soils is the production of organic acids (solubilization of insoluble inorganic phosphate compounds such as tricalcium phosphate, dicalcium phosphate, hydroxyapatite, and rock phosphate), which increases the activity of acid phosphatases (mineralization of organic phosphorous). The major mechanism of K mineral solubilization is through the production of organic acids, inorganic acids, and protons (acidolysis mechanism) [27,33], which are able to convert the insoluble K (mica, muscovite, and biotite feldspar) to soluble forms of K for plant uptake. This is an indication that these fermented juices if added to soil have the potential to improve soil health through an increase in the availability of nutrient reserves in soils.
The variations in the availability and activity indicate that there is a higher probability that the same raw materials and techniques used to produce the fermented juices may not produce similar species as the ones isolated in this study. Nevertheless, the process of producing fermented juices will still need a wide range of beneficial microorganisms to convert raw materials to an end product that can act as a soil conditioner and, perhaps, a fertilizer [1,12,26]. The fermented juices derived from plant parts are liquids containing a variety of lactic acid bacteria, yeasts, and phototrophic bacteria [2,26,35], which play a significant role in organic, or natural, crop farming to sustain soil health. If these fermented juices are added to soil, they most likely will enhance the biological health of the soil through an increase in the soil microbial population and soil nutrient reserves. The increase in nutrient reserves can happen through microbial nutrient cycling in soil.
Zimmermann [36] reported that these organisms create conditions that enable the production of useful substances such as vitamins, enzymes, hormones, amino acids, and antioxidants that create a healthy and conducive environment. Other studies reported that Sustainability 2021, 13, 4919 9 of 15 microorganisms including those from the genera Aspergillus [27,37], Lactobacillus [27,34], Bacillus [3,27], and Penicillium [27,38] act as components in biofertilizers to enhance plant growth and the enrichment of soil nutrients by inducing the availability of nutrients for plant uptake.
Thus, the use of microbes as components of biofertilizers is considered an alternative to chemical fertilizers to improve soil health and crop productivity [11]. Park and DuPonte [39] highlighted that beneficial microorganisms have considerable biopotential and are a novel tool for providing substantial benefits to agriculture, because the organisms are able to colonize roots and rhizospheres to stimulate the growth and development of plants. Yadav et al. [27] reported that plant growth promoting (PGP) microorganisms possess many attributes that are directly related to plant growth via the production of plant growth hormones and N 2 fixation and the solubilization of P, K, and Zn or indirectly by the production of 1-aminocyclopropane-1-carboxylate (ACC) deaminase, ammonia, antibiotics, hydrocyanic acid, lytic enzymes, and siderophores. Extensive work on biofertilizers has revealed that beneficial microbes have the capability of providing required nutrients to crops in amounts that are sufficient for the enhancement of the crop yield [27,40,41]. To this end, there is a strong belief that microbes with multifunctional PGP attributes can be utilized as ecofriendly biofertilizers for sustainable agriculture, particularly in sustaining soil chemical and biological health [1,27].

Characteristics of the Palm Kernel Shell (PKS) Biochar and Potential Benefits to Soil Health
The selected chemical properties (Table 4) of the palm kernel shell (PKS) biochar were typical of those reported in the literature [19,42,43]. The high pH of the PKS biochar suggests that it can be used to neutralize acidic soils [44]. Glaser et al. [45] suggested that biochars provide long-term benefits including better retention of all cations because they have higher cation exchange capacity (CEC) and anions [21] because of their higher anion exchange capacity (AEC). The CEC and AEC of the biochar could enable the temporary retention of both positively and negatively charged nutrients including exchangeable ammonium (NH 4 + ), nitrate (NO 3 -), and orthophosphates from being leached from soils [45]. The high pH of the biochar also relates to its ash content, which contains significant amounts of base cations, particularly Ca (Table 4). This could, to some extent, have a liming effect on mineral acid soils and P availability from Fe and Al fixation. These results are in agreement with that of Rajkovich et al. [46] who reported that ash in biochar has a liming effect because of chemical compounds such as KHCO 3 and CaCO 3 . This property makes biochar a potential soil organic amendment to improve and sustain soil buffering capacity for a long period compared with chemical limes. The solubilization of these alkaline chemical species can increase soil pH [44], decrease soil Al, Fe, and Mn ion toxicity, as well as increasing soil CEC [45].
The high C content of the PKS biochar suggests it has the potential of sequestering C into a stable form [47]. Except for Ca, Al, B, and Mn, other elements such as N, P, K, S, As, Cd, Cu, Pb, Hg, Ni, Zn, Co, Mn, Cr, and Fe were relatively low. The low Al, B, and Mn but high Ca of the PKS biochar might be because of the lower thermal degradation (390 • C) than conventional thermal degradation (500 • C to 1000 • C) as vaporization or loss of these elements did not occur during pyrolysis [18]. Some of the alkali nutrients can only volatilize at temperatures greater than 760 • C during pyrolysis [48]. Hunt et al. [49] opined that an increase in nutrient content with thermal degradation can also be explained by the loss of volatile compounds (C, H, and O) from the original material and relatively small losses of alkali nutrients.
The PKS used for the production of the biochar may contain low amounts of heavy metals that might have come from the frequent use of fertilizers, herbicides, and pesticides in oil palm plantations [49]. Heavy metals that are regarded as the most toxic and environmentally damaging include Cd, Cr, Cu, Hg, Ni, Pb, and Zn [50]. Ross [51] reported that many of these metals, especially transition metals such as Cu, Ni, and Zn, are essential for plant metabolism. Heavy metals are a group of elements with specific gravities greater than 5 g cm −3 [51] and they are both industrially and biologically important [19]. Although not a heavy metal by chemical definition, metalloid As is given the status of "risk element" or "potentially toxic element" because of its carcinogenic effect on humans and toxicity to plants [52]. According to the Malaysian Department of Environment [50], an excessive concentration of heavy metals and As, through direct or secondary exposure, causes a toxic response in biota or humans, resulting in an unacceptable level of environmental risk, and therefore heavy metals and As may be classed as pollutants. From the PKS biochar chemical analysis (Table 4), 7 out of the 19 elements it contained are categorized as toxic heavy metals and need attention whenever this type of biochar is used for agriculture or filtering water. Heavy metals such as As, Cd, Cr, Cu, Pb, Hg, Ni, and Zn of the PKS biochar were all within the non-detectable range, suggesting that this biochar is safe for use. In Malaysia, the maximum heavy metal levels allowed for As, Cd, Cr, Cu, Pb, Hg, Ni, and Zn are 20,5,250,375,150,4,125, and 700 mg kg −1 , respectively [50]. Since the PKS biochar is very low in the aforementioned toxic metals (Table 4), this organic amendment can be used as a soil conditioner for crop cultivation, albeit in a prudent manner, to ensure soil health sustainability. It has a high potential to sustain soil biological and chemical health due to its high C and Ca content and high CEC. Biochar has been shown to positively affect soil microbial populations and diversity, which is an indication of its potential to sustain soil biological health [53,54].

Characteristics of the Kitchen Waste Compost and Potential Benefits to Soil Health
The selected chemical properties of the compost that was produced from kitchen and farm wastes are presented in Table 5. The results suggest that the compost is typical of other composts reported in the literature [55][56][57]. The pH of the compost (Table 5) indicates that it is slightly alkaline. According to Agegnehu et al. [58], at the initial stage of composting, organic acids are formed. The acidic condition favors the growth of fungi to break down lignin and cellulose. As composting proceeds, the organic acids are neutralized, leading to the production of mature compost with a pH of 6 and 8 [58,59]. The electrical conductivity (EC) or level of salinity of the compost was 3.60 dS m −1 (Table 5), and this value is consistent with that of Huerta-Pujol et al. [60] who reported an optimum range of EC for field application as 2-4 dS m −1 . Compost with very high or very low EC is not recommended for crop cultivation, because high EC can cause root injury and burning, whereas low EC affects crop absorption and the utilization of mineral elements from the soil [3]. According to Partanen et al. [40], salts in the form of mineral ions are naturally present in all composts and normally concentrate somewhat during composting and may pose limitations for soil application, as plants have varying sensitivities. The moisture content (42.10%) of the compost (Table 5) is within the acceptable range of 40% to 60% by weight [44]. At a lower moisture level, microbial activity is limited; however, at a higher level, the composting process is likely to become anaerobic with a bad odor. Excessive moisture causes water to fill the pore spaces in composts. This impedes the diffusion of oxygen through the compost materials and results in anaerobic conditions [28].
The total C and N were 27.89% and 1.12%, respectively, (see Table 5), and the values were in agreement with that of Kimpinski et al. [25] who also found that the optimum value of total C is normally higher than 10%, whereas total N ranges between 0.99% and 2.01%. The C/N ratio (24.90) of the compost (Table 5) is considered good [61]. Fischer and Glaser [44] observed that as C gets converted to CO 2 (assuming minimal N losses) the C/N ratio decreases during the composting process with the ratio of finished compost typically close to 20/1. Sharma et al. [3] were of the opinion that to achieve an ideal compost the C/N ratio may need to be adjusted depending on the bioavailability of these elements. This is commonly an issue with high C materials, which are often derived from wood and other lignified plant materials, as lignin reduces biodegradability. Particle size is also an important factor, with smaller particles degrading faster than larger particles of the same material [62]. Bioavailability can also be a factor with N sources, where nearly instant availability can exceed the assimilative capacity of the microbial community and be lost as ammonia odors and nitrate in leachate [3,60].
The N, P, K, and Ca levels in the kitchen waste compost were 1.12%, 10.23%, 7.32%, and 0.52%, respectively, (see Table 5) and were far higher compared to the fermented juices. Variation in the nutrients was because the raw materials of the compost were leaves of tapioca, bamboo, vegetables, and banana plants, grass chippings, discarded fruits, rice straw, rice hulls, and corn stalks [10]. Zamora and Calub [12] mentioned that P in composts commonly comes from leafy wastes, whereas K comes from fruits and grains. In compost production, Zamora and Calub [12] also mentioned that kitchen waste such as bones and fish contribute high amounts of Ca. Generally, high-quality composts have a full spectrum of plant nutrients, although the exact amounts vary from sample to sample [48] and wellrotted or mature compost is rich in macro-and micro-nutrients [3]. Marcote et al. [63] opined that because composts make macro-and micro-nutrients more available to plants, they are nature's ultimate organic fertilizer and soil conditioner. The best compost in terms of nutrient elements results from a wide variety of waste materials from the yard and kitchen, with a mixture of brown (carbon-rich) and green (nitrogen-rich) waste. Their addition to soil can be a big plus to soil health and sustainable crop production. It is worth noting that a combination of the biochar, compost, and fruit juices could be more beneficial to soil health rather than the individual agro-wastes [64], which can only be ascertained through amending soils in the field.

Conclusions
The fermented juices (FPJ and FFJ), PKS biochar, and compost possess chemical and biological properties that qualify them as potential soil conditioners or organic amendments for sustaining soil health and crop production. The fermented juices also contain important microbes that can solubilize P and K for crop use. It is worth noting that the kitchen waste compost contained far higher amounts of N, P, and K compared to the fermented juices or biochar. The high C content and CEC of the biochar can have significant positive effects on soil microbial communities, which is good for soil biological and chemical health, respectively. Therefore, these organic amendments have the potential to release macro-and micro-nutrients when added to soil. However, to validate the potential of these organic amendments, field trials involving crops, soils, and various combinations of the organic amendments are necessary to evaluate their actual effects on sustaining soil health and crop growth in both the short and long term.