Potential of Biochar-Based Organic Fertilizers on Increasing Soil Fertility, Available Nutrients, and Okra Productivity in Slightly Acidic Sandy Loam Soil

: Reducing chemical fertilizers is critical for maintaining soil health and minimizing environmental damage. Biochar-based organic fertilizers reduce fertilizer inputs, improve soil fertility, increase crop productivity, and reduce environmental risks. In this study, a pot experiment was conducted in a greenhouse to assess the potential of biochar-based organic and inorganic fertilizers to improve soil fertility and Okra yield. Seven treatments with three replicates were arranged in a completely randomized design (CRD). Three treatments included biochar-blended formulations (i) biochar mixed with mineral NPK fertilizer (BF), (ii) biochar mixed with vermicompost (BV), and (iii) biochar mixed with goat manure (BM); two treatments included biochar enrichment formulations (iv) biochar enriched with cow urine (BCU) and (v) biochar enriched with mineral NPK fertilizer in aqueous solution (BFW), and the remaining two included control treatments; (vi) control (CK: no biochar and no fertilizers) and (vii) fertilized control (F: only recommended NPK fertilizer and no biochar). Mineral NPK fertilizers in BF, BFW, and F were applied at the recommended rate as urea, di-ammonium phosphate (DAP), and muriate of potash (MOP). Organic fertilizers in BV, BM, and BCU treatments were applied in equal quantities. All biochar-amended treatments showed improved soil chemical properties with higher pH, organic carbon, total N, and available P and K compared to the two non-biochar control plots (CK and F). Biochar blended with goat manure (BM) showed the highest effect on soil fertility and fruit yield. BM (51.8 t ha − 1 ) increased fruit yield by 89% over CK (27.4 t ha − 1 ) and by 88% over F (27 t ha − 1 ). Similarly, cow urine-enriched biochar (BCU) (35 t ha − 1 ) increased fruit yield by 29% and 28% compared to CK and F, respectively. Soil pH, OC, and nutrient availability (total N, available P, and available K) showed a signiﬁcantly positive relationship with fruit yield. The study suggests that using biochar-based organic fertilizers, such as BCU and BM, could outperform recommended mineral fertilizers (F) and produce higher yields and healthy soils, thereby contributing to mitigating the current food security and environmental concerns of the country.


Introduction
Okra (Abelmoschus esculentus), a dicotyledonous summer-season plant (Malvaceae family) grows well in a wide range of soil types (ranging from sandy loam to loamy soil) and temperatures ranging from 21 to 30 • C [1,2]. Okra is one of the most important vegetable crops in Nepal and is cultivated in the terai (lowland) and mid-hill (upland) districts [3]. The total production of okra was 284,926 metric tons with the total productivity of 13.95 metric tons ha −1 , under 20,424 ha area across the country in the year 2021 [4]. Areas under okra production increased by 12.2% within four years (from 2011 to 2015). However, the yield decreased by 0.2% during this period [5].
in Nepal, the potential of biochar-based organic fertilizers: biochar mixed or enriched with organic fertilizers (cattle urine, vermicompost, and goat manure), and mineral NPK fertilizers were assessed on okra productivity under a controlled greenhouse experiment. The objective of the study was to assess the potential of biochar (mixed or enriched with organic and inorganic fertilizers) in reducing soil acidity, increasing soil organic carbon (OC), and available nutrients (N, P, and K), thereby increasing okra yield in a moderately acidic Nepalese soil. We hypothesized that the biochar-based fertilizers will improve soil fertility (pH, OC, N, P, and K) and increase okra yield compared with both the fertilized control (applying recommended mineral fertilizers without biochar) and non-fertilized control (without fertilizers and biochar).

Experimental Site
A pot trial was conducted in the greenhouse at National Soil Science Research Center (NSSRC), NARC (Nepal Agriculture Research Council), Khumaltar, Lalitpur (27 • 39 17 north latitude, 85 • 20 44 east longitude, and 1335 m above mean sea level). The experiment was conducted over three months, from May to July 2022. The average maximum and minimum temperatures were 28.2 • C (ranging from 28 to 28.5 • C) and 19 • C (ranging from 16.5 to 20.5 • C), respectively, during the experimental period (Table 1). Cumulative rainfall during the growing season (May to June 2022) was 300.3 mm (Table 1).

Experimental Set Up and Cultivation Practices
The experimental design included seven treatments with three replicates in a completely randomized design (CRD). Three treatments included mixed biochar formulations (i) biochar mixed with mineral NPK fertilizer (BF), (ii) biochar mixed with vermicompost (BV), and (iii) biochar mixed with goat manure (GM); two treatments included biochar enrichment formulations (iv) biochar enriched with cow urine (BCU) and (v) biochar enriched with mineral NPK fertilizer (BFW) in aqueous solution and remaining two included nonbiochar control treatments; (vi) non-fertilized control (CK: plot receiving neither biochar nor fertilizers) and fertilized control (F: plot receiving recommended NPK fertilizers without biochar) ( Table 2). The chemical fertilizers NPK were applied either on their own or in combination with biochar in their dry (BF) or dissolved form (BFW), based on the recommended rate of NPK for okra in Nepal (200:180:80 kg ha −1 ) [31]. Mineral fertilizers N, P, and K were applied as urea (46% N), di-ammonium phosphate (DAP: 46% P and 18% N)), and muriate of potash (MOP; 60% K), respectively. The application of other organic amendments i.e., vermicompost, goat manure, and biochar were in equal quantity at the rate of 10 t ha −1 , which is equivalent to N content in mineral fertilizer, considering 39% of the total N in mineral form in the cattle compost [22].
Grows bags with a length of 30 cm, a width of 20 cm, and a height of 30 cm of 12 L were used for the experiment. The distance between the treatments was 30 cm. Small pebbles, stones, and weeds were separated from the soil and filled into the pots at a rate of 10 kg of soil per pot. Biochar and organic and inorganic fertilizers were calculated and applied to each pot as described in Table 2. BFW and BCU were prepared one day before seed sowing. In a 2 L beaker, 44.6 g of biochar, 1.25 g of urea, 1.74 g of DAP, and 0.59 g of MOP, were added and dissolved into 360 mL of water to prepare NPK fertilizer-enriched biochar (BFW) [24]. Likewise, in another 2 L beaker, 44.6 g of biochar was added to 360 mL of cow urine to prepare a urine-enriched biochar slurry (BCU) [24,26]. All organic and inorganic inputs with and without biochar ( Table 2) were mixed thoroughly and deeply with soil in the pot during planting. For the recommended NPK fertilizer treatment (F), a full dose of DAP and MOP and half a dose of urea were applied during soil preparation. The remaining urea was top dressed in two equal splits at 30 and 45 days after sowing (DAS).
The seeds of Arka Anamika, a variety resistant to yellow vein mosaic virus, with a potential yield of 20 t ha −1 were used in the experiment. The seeds were soaked overnight and sown manually through holes of 3 cm depth. Three holes were made in the soil with 4-5 seeds in each hole, separated by a distance of 5 cm. Thinning was performed after 15 DAS, to maintain three healthy and robust plants. The pots were watered on alternate days in the early morning and evening to ensure sufficient moisture during growing periods. The first weeding was performed after 15 DAS and subsequent weeding was carried out manually at 5 day intervals. Harvesting of okra fruit pods was done manually after maturity and subsequent multiple harvests were done at an interval of 2-3 days up to 90 DAS.

Agronomic and Yield Parameters
Stem girth and plant height in each pot were recorded at 15 DAS, 30 DAS, 45 DAS, 60 DAS, 75 DAS, and 90 DAS. A digital vernier caliper and measuring tape were used to measure stem girth and plant height.
Days to first harvest were counted for each plant in the pot, and its arithmetic mean was taken for all the treatments. The harvesting period is the total duration of the harvest from the first day to the final day of harvest. The length of the fruit was measured using measuring tape from the cap section to the distal end. The diameter of the fruit was measured at the center of the fruit pod using a digital vernier caliper. The weight of the individual harvested pods was measured to record the average fruit weight. The total number of fruits, harvested up to 90 DAS, in each pot was counted and the weight was measured to record the total fruit yield.

Biochar and Soil Analysis
The biochar used in this study was produced from Bracken and Eupatorium adenophorum (Banmara). Biochar was characterized by a pH of 9.58, total nitrogen of 0.77%, organic carbon of 13.50%, and cation exchange capacity (CEC) of 19.08 cmol c kg −1 (Table 3). For the preliminary soil information, triplicate representative soil samples were collected from the field to make a composite sample. Sampled soils were analyzed for soil texture, pH, organic matter (OM), organic carbon (OC), total nitrogen (N), available phosphorous (P), and available potassium (K). For post-harvest soil analysis, soil samples were collected from all the individual pots, stored in zip lock bags, labeled well, and analyzed for the aforementioned soil properties.
The collected soil samples were air-dried for two days and passed through a 0.2 mm sieve for OM and OC analysis. For other soil parameters (pH, total N, available P, and available K), the soil was passed through a 2 mm sieve. Soil texture was determined using the hydrometer method [32]. Soil pH was measured using a pH meter (soil: distilled water ratio of 1:2) [33]. OM and OC were determined using the Walkley-Black method [34]. Total N was measured using the Kjeldahl digestion and distillation method [33]. The modified Olsen's method was used for the available phosphorous [35] and the ammonium acetate and flame photometer were used to determine the available potassium [33] ( Table 3).
The preliminary analysis showed that the soil was sandy loam Inceptisol with low OM content, medium N content, and low available P and K content (Table 3).

Data Analysis
Data were analyzed using STAR (Statistical Tool for Agricultural Research), version 2.0.1 developed by International Rice Research Institute (IRRI)and SPSS (Statistical Package for Social Sciences), version 25 developed by IBM. One-way ANOVA was performed to assess the effect of biochar mixed or enriched with organic and inorganic amendments on soil properties and plant growth parameters. Tukey's HSD (honest significant difference) test was performed at a 5% level of significance to compare the mean between different treatments. Linear regression was performed to identify the relationship between soil chemical properties (pH, OC, total N, and available P and K) and okra production.

Soil Properties
Biochar blended or enriched with mineral NPK (BFW and BF) and organic fertilizers (BCU, BV, and BM) treatments were pooled to give an average mean value for each biocharmixed NPK (B-mineral) and organic fertilizers (B-organic). B-mineral and B-organic were compared with the average mean of non-biochar CK and F treatments (control). Soil pH increased from 5.8 (control) to 6.1 in B-mineral and up to 6.6 in B-organic pots (Table 4). OC increased from 1.5% (control) to 2% and 2.2% in B-mineral and B-organic treatments, respectively (Table 4). The total nitrogen increased by 28% in B-mineral (0.18%) and by 78% in B-organic soils (0.25%) compared with the control (0.14%). Available P increased by 33% (40.4 kg ha −1 ) in Bmineral and by 93% (58.1 kg ha −1 ) in B-organic compared with the control (30.02 kg ha −1 ). Similarly, available K increased by 119% (246.5 kg ha −1 ) in B-mineral and by 195% (331.9 kg ha −1 ) in B-organic compared with the control (112.3 kg ha −1 ).

Plant Growth Parameters
Plant height and stem girth over a growing season measured at every 15 d interval from 15 DAS to 90 DAS are shown in Figures 1 and 2
Values are means ± standard deviation, followed by the different letters in the column denoting significant differences at the 0.05 level using Tukey's HSD test.

Soil Properties
Using biochar in combination with organic and inorganic NPK fertilizers has significantly improved soil properties and nutrient availability, with superior effects observed for biochar-based organic fertilizers (BCU, BV, and BM) ( Table 4). In accordance with this, earlier studies have reported increased soil pH [17,22,36] and OC [22,[37][38][39] upon biochar application in combination or enriched with mineral and organic fertilizers. An increase in soil pH is due to the alkaline nature of biochar that can quickly release base cations, such as Ca 2+ , Mg 2+ , and K + into the soil solution, and reduce H + and Al/Ca ratio [17,40]. Moreover, biochar has reactive pH-dependent functional groups, such as phenolic (OH) and carboxylic acid (COOH), which could increase the soil pH [40]. Moreover, the small fraction of ash content in biochar could result in the subsequent dissolution of hydroxides and carbonates, thereby reducing soil acidity [41,42]. Soil OM was observed higher in biochar-amended soils (Table 4), which could be due to high OC content in biochar, soil microbial abundance, reduced leaching, and lower mineralization rate (higher mean residence time) due to greater carbon stability [36,[42][43][44]. On average, biochar blended with organic and mineral fertilizers increased soil OC by 70% compared to controls, which is corroborated with earlier studies that reported an 80% increase in OC in similar soils upon biochar application in combination with organic and inorganic fertilizers [22].
Soil nitrogen and available P and K were found significantly higher in both biocharamended mineral and organic fertilizers over control (Table 4), with a superior effect observed for biochar-based organic fertilizers (BCU, BV, and BM). This is corroborated by earlier findings where N, P, and K were significantly increased upon biochar application compared with non-biochar soils [22,45]. Zhang et al. [46] reported an increase in soil N by 18.5% and 32.9% over control with the application of 1% and 5% biochar-based organic fertilizer, which is in line with our findings (Table 4). Higher soil N content upon biochar amendment could be explained by the abundance of surface functional groups and highly porous structure of biochar, leading to reduced leaching, and retaining a higher amount of nitrogen, as well as fostering microbes in biochar pores, which enhance soil biological processes such as mineralization, nitrification and other mineral solubilization activities [28,47,48].
Similarly, higher soil available P upon biochar amendment (Table 4) is in line with the findings from Mensah [49], who reported an increase in available P with the addition of biochar in combination with mineral and organic fertilizers (biofertilizer, vermicompost, and peat substrate). Increased soil P upon biochar amendment could be due to reduced leaching losses, higher P retention, and the synchronization of biochar and organic fertilizers, which enhance the P biogeochemical cycles (total P, available P) in soil [42,48,50]. Moreover, higher soil available P could be related to increased soil pH upon biochar amendment (Figure 4f). Soil PO 4 is tightly bound in Fe and Al under acidic conditions; thus, the biochar amendment could increase pH towards neutral, thereby making more P available in the soil solution [51].
Increased available K in biochar-amended soil (Table 4) could be due to the direct addition of K from biochar and biochar ash per se, and also due to the higher surface area of biochar, which adsorbs K strongly and reduces leaching losses of K in soil [19]. This is corroborated by the earlier studies in which Mensah [49] reported higher available K with biochar in combination with 50% biofertilizer and 50% inorganic fertilizer compared with the control. The high concentration of K in biochar-amended soils could result from low exchangeable acidity due to the precipitation of Al as hydroxyl releases cations into the soil. Moreover, the K saturation percentage increases with biochar application due to the more significant effects of biochar on exchangeable K. Moreover, the increase in soluble K content could be attributed to the limited capacity of exchange sites and clay interlayers for K adsorption [49,52].

Plant Growth Parameters
Biochar enriched with mineral fertilizers showed higher efficiency in increasing the plant height (Figure 1), which is in line with the study by Utomo et al. [53], where plant height was significantly increased in nitrogen-enriched biochar compared with non-biochar treatments in 90 DAS. Sharma et al. [54] reported a significant increase in plant height of knolkhol with biochar applied in combination with vermicompost by 18.2%, and with cattle manure (FYM) by 15.8% compared to control, which corroborates our findings where biochar mixed with vermicompost, and goat manure showed a superior effect on plant height (Figure 1). The increase in plant height could be attributed to the improved soil pH, EC (electrical conductivity), and soil fertility [49], and also due to plant-promoting effects and nutrient loadings, stimulated by biochar [30].
The application of vermicompost and cow urine in combination with biochar has shown the maximum stem girth (Figure 2). A significant increase in the knob diameter of knolkhol with the application of biochar blended with vermicompost, reported by Sharma et al. [54], corroborates our findings. The ability to retain nutrients due to the low bulk density and high-water holding capacity of biochar could explain the improvement in crop performance, increasing the stem girth of the okra plant as observed in our findings [55].

Fruit Yield
Total fruit yield was found significantly higher in biochar enriched with cattle urine (BCU) and biochar blended with goat manure (BM) compared with the non-fertilized (CK) and fertilized control (F) (Figure 3). In accordance with this, Schmidt et al. [25] reported a significant increase in crop yields by 95% with cattle urine-enriched biochar compared with the control and fertilized control receiving NPK mineral fertilizer. In another study, they reported an almost two-fold increment in pumpkin yield with urine-enriched biochar compared with the control in Dhading, Nepal [24]. Similarly, Shrestha and Pandit [56] reported double pea yields with the application of urine-biochar amendments compared with the control plot receiving only NPK mineral fertilizer and only manure. Several earlier studies are well documented where urine-enriched biochar improved soil fertility and increased crop yield substantially [14,24,25]. Higher maize productivity with urineenriched biochar application could be due to the formation of organic coatings in biochar pores, which enhance the nutrient retention capacity of biochar [28]. Nutrient-enriched biochar allows penetration of available nutrients, such as N, P, and K in biochar micro-and nano-pores, and releases slowly based on the physiological requirements of the plant over the cropping season [24,26]. In addition, urine application could promote stem growth due to cell elongation by promoting hormonal activities, resulting in the faster growth of the okra plant [57].
In our study, biochar blended with goat manure (BM) showed a greater effect on fruit yield among all the treatments, including control (CK) and fertilized control (F) (Figure 3). In accordance with this, Sanchez-Monedero et al. [58] reported significantly higher fruit yield in tomatoes by 16.4% with biochar-blended sheep manure and compost, compared with the control. Increased yield could be due to improved soil fertility (higher soil pH and OC) and a higher amount of available nutrients (soil N, P, and K) observed in goat manure blended biochar (BM) over both controls and all other biochar-amended treatments (Table 4). Moreover, a higher yield could be explained by the positive relationship between soil properties (pH, OC, N, P, and K) and total fruit yield (Figure 4), which is in line with an earlier study where a strong positive correlation between soil properties (pH, P, and K) and crop yields [59] was reported after using organic biochar amendments. Improvements in soil chemical properties and nutrient availability with BM are corroborated by a study conducted by Ingold et al. [60], who reported increased soil OC and total N with the use of biochar-blended goat manures. Higher soil N in BM could be due to higher levels of N in goat manure per se, which was reported to be around 4.9% N [61], relatively higher compared with N content in cattle manure or farmyard manure. Increased crop yield and growth parameters (plant height, stem diameter) upon goat manure application is widespread [61,62]. Ojeniyi and Adegboyega [63] reported that the use of goat manure significantly increased the growth and yield of okra, Amaranthus, and Chelosia. However, the agronomic performance of goat manure mixed with biochar is less studied and is noticeably scarce. Our findings on soil properties, nutrient availability, and okra productivity as a function of biochar-blended goat manures could be highly significant to researchers and institutions at national and international levels.
Increased yields upon BM application could be due to a higher amount of N in goat manures that are retained in biochar pores for longer periods and released slowly based on the synchrony between soil nutrient supply and plant demand [24,30]. Similarly, higher soil available P and K in BM treatments could have increased the okra yield, which is corroborated by Pandit et al. [18], where biochar blended with organic fertilizers considerably increase soil P and K, which was positively correlated with maize yield in similar soils. Similarly, BM showed higher soil pH (Table 4) and a positive relationship with okra yield (Figure 4a), which could be due to liming effect of biochar (alleviating acid stress) in sandy loam acidic soils [17]. Jemal and Yakob [64] reported that the addition of 12 t ha −1 of biochar recorded the lowest exchangeable acidity (0.39 cmol c kg −1 ), thus, underscoring the potential of biochar as a liming material. Moreover, there could be a compounding effect of soil N and available P on okra yield, as pH was positively correlated with soil N and P (Figure 4f,g). The increase in soil pH enhanced the availability of soil N and other essential immobile nutrients such as P and K, which increases root/shoot growth and yield of plants [48,65]. Moreover, a 13% and 43% increase in the yield of fresh eggplant in two consecutive years with the application of biochar-based fertilizers holds close links with our results [66]. Moreover, the improvement in nutrient absorption and photosynthesis activities upon biochar-based fertilizers might have increased carbohydrate synthesis, dry matter, and subsequent accumulation of fruits, leading to high yields [67,68].

Conclusions
The results suggest that the addition of biochar-based organic fertilizers, such as biochar enriched with cattle urine (BCU) and biochar blended with goat manures (BM) could significantly improve soil chemical properties (pH, OC, total N, and available P and K) and okra fruit yield in a sandy loam Nepalese soil. Biochar blended with goat manure was found efficient among all the organic and inorganic treatments to increase soil nutrient availability such as N, P, and K, thereby increasing okra yield. This study analyzed and presented soil chemical parameters for various organic and inorganic nutrients mixed with biochar and lacks detailed studies on the organic matter transformation and nutrient bioavailability. Therefore, more detailed spectroscopic and microbiological studies are needed to mechanistically disentangle the higher effect of biochar-goat manure formulations on increasing nutrient availability in the soil. This is the first study in which the agronomic effects of biochar-based organic and inorganic fertilizers have been studied under controlled conditions in okra. Thus, further studies on different crops/vegetables, soil types, and agroecological zones of Nepal are recommended. In view of the shortage of fertilizers in Nepal, the promotion of biocharbased organic fertilizers should be encouraged through research and extension by the concerned authorities for wider adoption by farmers in their farmland.