Organic plant nutrients in combination with inorganic sources of nutrients influence the productivity, quality, soil microbes, and nutrient use efficiency of bitter gourd (Momordica charantia L.)

The investigation was carried out at the experimental field of the Soil and Water Management Section under the Horticulture Research Centre (HRC), Bangladesh Agricultural Research Institute (BARI), Gazipur. The duration of the investigation was two consecutive seasons: 2020 and 2021. The experimental field is situated at 23°59′N and 90°24′E and 8.4 m above sea level. The area experiences a typical tropical and subtropical continental monsoon climate, characterised by a subhumid climate (Huq and Shoaib, 2013). Meteorological data, including average minimum and maximum temperatures, humidity, sunshine hours, and rainfall, were recorded throughout the crop growing period at the research location (Table 1).

In 2021, the monthly mean minimum and maximum temperatures were relatively higher than those in 2020. In addition, there was considerably more rainfall in 2021, except for the months of April and May (Table 1).

The soil under investigation is classified as grey terrace soil according to the USDA Soil Taxonomy (USDA, 1975), specifically belonging to the Chhiata soil series within the Madhupur Tract (agroecological zone 28). Initial soil samples for the study were collected from a depth of 0–15 cm, and their physicochemical properties were analysed via standard procedures (Keeney and Nelson 1982) as detailed in Table 2.

Healthy seeds of bitter gourd (variety BARI Korola-3) were collected from the Olericulture Division under the Horticulture Research Centre of the Bangladesh Agricultural Research Institute (BARI). The seeds were sown manually into polythene bags (10 cm × 10 cm) filled with fine-textured clay loam soil on April 9, 2020 and April 10, 2021. Watering was then performed immediately after sowing. The emerging seedlings were irrigated twice a week and secured from diseases and insects via the application of Autostin^® 50 WDG (carbendazim fungicide powder) fungicide and Sevin^® insecticide. During the seedling growth period, the main research plots were prepared methodically via a tractordriven chisel plough with four passes and were carefully levelled with a tractor-driven rotavator. The weeds and other debris were manually eradicated.

The treatment plots were manually prepared with a size of 1.8 m × 2 m. The investigation was scheduled with nine treatments: T₁: control (no use of fertiliser), T₂: recommended dose of inorganic N-P-K-S-Zn-B at 120-40-85-20-3-2 kg · ha⁻¹ (Ahmmed et al., 2018), T₃: 5 t · ha⁻¹ VC + 50% of NPKSZnB, T₄: 2.5 t · ha⁻¹ PM + 50% of NPKSZnB, T₅: 5 t · ha⁻¹ VC + 75% of NPKSZnB, T₆: 2.5 t · ha⁻¹ PM + 75% of NPKSZnB, T₇: 5 t · ha⁻¹ CD + 75% of NPKSZnB, T₈: 5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 50% of NPKSZnB, and T₉: 5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 25% of NPKSZnB. Detailed descriptions of the test treatments are presented in Table 3.

The VC, PM, and CD used in the experiment was obtained from a local businessperson in the Gazipur district of Bangladesh. The elemental nutrient compositions of VC, PM, and CD were determined using standard methods (Page et al., 1982), and the results are presented in Table 4. VC and PM outperformed CD in terms of key nutrients. VC, with its N-P-K-S-Zn-B composition of 12.0-12.4-9.9-5.0-0.17-0.14 g · kg⁻¹, and PM, at 12.6-10.0-9.0-3.4-0.19-0.12 g · kg⁻¹, demonstrated higher levels of essential elements compared to CD’s 11.0-4.5-4.8-2.9-0.18-0.17 g · kg⁻¹ (Table 4). The treatments used in this study were replicated three times via a randomised complete block design. The plots were spaced 1 m apart, and the replications were also separated by 1 m. The sources of nitrogen (N), phosphorus (P), potassium (K), sulphur (S), zinc (Z), and boron (B) were organic manures, such as VC, PM, and CD, and inorganic fertilisers, such as urea (46% N), triple superphosphate (20% P, 1.3% S), muriate of potash (50% K), gypsum (18% S and 20% Ca), zinc sulphate monohydrate (36% Zn and 18% S), and boric acid (17% B). Half (50%) of the organic manure was applied per treatment during plot preparation. During the final pit preparation, the full amounts of triple superphosphate (TSP), gypsum, zinc sulphate monohydrate, boric acid, the remaining 50% of organic combination, and 50% of muriate of potash (MoP) were applied to the pit based on the treatment and mixed thoroughly with the soil manually. Twenty-five-day-old seedlings were transplanted on May 3, 2020 and May 4, 2021 with a spacing of 1.8 m × 1.2 m. Only one seedling was planted into a pit, and irrigation was applied immediately.

The research field was regularly monitored to ensure the health of the plants. As per routine work, any injured, dead or weak seedlings were replaced with vigorous new seedlings within 10 days. As per treatment, urea fertiliser was applied in three equal instalments. The first 1/3 instalment was applied via the ring method under moist soil conditions at 15 days after transplanting, and it was mixed carefully into the soil. Half of the MoP and the second 1/3 instalment of urea were top-dressed properly via the ring method at the flower initiation stage. Treatment wise, the remaining 1/3 of the urea was applied through ring method in moist soil conditions at the fruiting stage. Weeding was performed at 25, 45 and 65 days after transplanting. Imitaf^® 20 SL insecticide was applied twice at a concentration of 0.5 mL ·L⁻¹ water during the fruiting stage, with a 10-day interval, to reduce fruit fly and white fly infestations. Pheromone traps have also been employed for insect management. The base of the bitter gourd plants was carefully earthed up. Bamboo poles were used to create horizontal pandals at a height of 1.5 m when the vines started to grow. Bamboo stakes were placed vertically to support the climbing vines and lateral stems of bitter gourd, with horizontal stakes connecting all beds at the top. Strings (kunchi) were used to secure adjoining stakes. However, the vines of the test crop climbed and reached the top according to their respective treatments. Young fruits of bitter gourd were harvested 10 times, starting from the first week of June until the second week of August, in accordance with the treatments conducted in 2020 and 2021.

The amount of irrigation water used was estimated according to the procedure of Mridha (1993), who used the following equation: 1d=FC−MCi100×AS×D{\rm{d}} = {{{\rm{FC}} - {\rm{M}}{{\rm{C}}_{\rm{i}}}} \over {100}} \times {{\rm{A}}_{\rm{S}}} \times {\rm{D}} where d = depth of irrigation, mm; FC = field capacity of the soil,%; MC_i = moisture content of the soil at the time of irrigation, %; As = apparent specific gravity of the soil; and D = root zone depth, mm.

The effective rainfall was calculated on a daily basis during the growing period following the procedures adopted by Mridha (1993).

The water productivity index (WPI) was used to measure the effectiveness of the irrigation system in terms of gross bitter gourd fresh fruit yield and the total volume of water applied. The WPI is expressed in the following formula outlined by Kamal et al. (2012): 2WPI=YQWPI = {Y \over Q} where WPI = the water productivity index, kg ·m⁻³; Y = the yield (kg · ha⁻¹) for the season in the specific area; and Q = the total supply of water, including rainfall per ha for the season in the specific area, m³ · ha⁻¹.

The marketable fresh fruit yield of bitter gourd was calculated on a whole-plot basis and converted into t · ha⁻¹; whereas, the dry plant yield was also recorded on a whole-plot basis as kg · ha⁻¹. For each treatment, 20 dried fruit samples were weighed and converted into kg · ha⁻¹ for dry fruit yield. Vine length was determined by measuring two plants per treatment using a measuring tape after final harvest. The lengths were then averaged. Fruit counts were recorded for each plant at every harvest. Subsequently, five fruits from each treatment were randomly selected. Fruit length was measured using a scale, while diameter was determined using vernier callipers. Individual fruit weights were recorded using a weighing machine. Finally, after the last harvest, mean fruit count per plant, mean fruit length, mean fruit diameter, and mean individual fruit weight were calculated for each treatment. Fresh bitter gourd fruit samples of second harvest (1 kg) were collected from each treatment plot and transported to the laboratory of the Postharvest Technology Section at the Horticulture Research Centre of BARI, where they were preserved in a refrigerator at –30°C for assessment of fruit quality. The total soluble solids (TSSs) content was determined by placing a drop of juice on the prism of a hand refractometer (Atago Ltd., PAL-1, Tokyo, Japan), and the results were expressed in °Brix according to an accepted method (Anonymous 1994). Each sample’s juice was extracted from ground bitter gourd fruit. Titratable acidity was determined by diluting a 2 mL aliquot of juice to 10 mL with 8 mL of distilled water and two drops of phenolphthalein, with the pH adjusted to 8.2 via 0.1 N (w/v) NaOH, following the procedure of Ranganna (1986). The vitamin C (ascorbic acid) and β-carotene contents of the bitter gourd were determined according to the procedure of Ranganna (1986). The pH of the juice was measured via a digital pH meter (HANNA Instrument Inc., pH-211; Microprocessor, pH Meter, Italy). The total sugar and reducing sugar contents were determined according to the methods of Ranganna (1986). A fruit texture analyser (GUSS, model GS-25, Western Cape, South Africa) was used to determine fruit firmness in fresh samples, with an 8 mm diameter flat end probe inserted to a depth of 3 mm into the bitter gourd fruit (same position of each sample) at a speed of 5 mm · s. The maximum penetration force was recorded, and it was used as the firmness value. Three samples from each treatment were examined and averaged. The moisture content of the bitter gourd was measured via the following formula: 3Moisture content (%)= Wet weight − Dry weight Wet weight ×100{\rm{ Moisture content }}(\% ) = {{{\rm{ Wet weight }} - {\rm{ Dry weight }}} \over {{\rm{ Wet weight }}}} \times 100

Soil samples were collected at a depth of 0–15 cm during the flowering stage of bitter gourd via a standard technique. Phosphate-solubilising bacteria (PSB), Rhizobia, total bacteria, fungi, and actinomycetes were cultured on different prepared media. The isolation of these microbes was performed following the procedure outlined by Pikovskaya (1948), as described briefly in the published research paper by Quddus et al. (2024a).

Postharvest soil samples were collected from four locations in each treatment at depths ranging from 0 cm to 15 cm. These samples were then transferred to the laboratory and spread on brown paper for air drying. Once dried, the soil samples were ground, passed through a 2 mm sieve and stored in a labelled plastic container for chemical analysis via standard procedures. The physicochemical properties of the initial soil and the chemical properties of the postharvest soils were analysed as follows: the soil texture was determined via the hydrometer method (Keeney and Nelson, 1982), and the pH was measured with a glass electrode pH meter at a soil–water ratio of 1:2.5 (Keeney and Nelson, 1982). The soil moisture content was measured via the gravimetric method, and the field capacity was determined via the gravimetric water content. The particle density was measured via the volumetric flask technique; whereas, the bulk density was determined via the core sampler method (Black, 1965). Soil organic carbon levels were assessed through the wet oxidation procedure (Keeney and Nelson, 1982), and the soil organic matter content was calculated by multiplying the content (%) of organic carbon by the Van Bemmelen factor of 1.73. The electrical conductivity (EC) of the soil was determined with a Groline Direct Soil Conductivity Tester (HANNA Instruments HI98331, Romania). The total N content was estimated via the micro-Kjeldahl process (Keeney and Nelson, 1982). Exchangeable calcium (Ca) and magnesium (Mg) were extracted with a solution of 1 M NH₄OAc following the technique of Gupta (2007). The contents of Ca and Mg in the extract were determined via an atomic absorption spectrophotometer (Varian, Model SpectrAA 55B, Sydney, Australia). The exchangeable K was measured following the 1 N NH₄OAc procedure (Jackson 1973). Available P was assessed according to the methods of Bray and Kurtz (1945). Available S was estimated through turbidity via the BaCl₂ method (Fox et al., 1964). Available Zn was assessed via the diethylenetriamine pentaacetic acid (DTPA) technique (Lindsay and Norvell, 1978). The azomethine-H method was used to determine the available B content (Keeney and Nelson, 1982).

The aboveground parts of the bitter gourd plants in each plot were collected for sun drying and then dried in a digital convection oven (Human Lab Instrument Co., model Co150, Seoul, Korea) at 68°C for 48 h. The dried plants were ground to pass through a 1 mm sieve. Fresh samples of bitter gourd from each treatment were washed and sliced into round or triangular shapes for sun drying and then dried in a digital convection oven. The dried fruits were also ground to pass through a 1 mm sieve. Each sample of dried fruit and plant material was preserved in labelled polythene bags. The ground dry plant and dry fruit samples were digested with a diacid mixture (HNO₃–HClO₄: 5:1) according to the procedure of Piper (1964). The N content was measured via the micro-Kjeldahl technique; the P content, via a spectrophotometric procedure; the Ca, Mg and K contents, via an atomic absorption spectrophotometer system; the S content, via a turbidity procedure with BaCl₂ via a spectrophotometer; and the B content, via a spectrophotometer following the azomethine-H method. The zinc content in the digest was directly determined via an atomic absorption spectrophotometer (Varian, SpectrAA 55B, Sydney, Australia).

The protein content in the dry fruit of bitter gourd was measured by multiplying the N content by the constant food factor of 6.25 delineated by Hiller et al. (1948).

The nutrient uptake by dry fruit and dry bitter gourd plants was assessed according to the following formula (Sharma et al., 2012): 4Nutrient uptake (kg·ha−1)=( Nutrient content (%)× Dry fruit/plant yield ( kg per ha ))/100{\rm{ Nutrient uptake }}\left( {{\rm{kg}}\cdot{\rm{h}}{{\rm{a}}^{ - 1}}} \right) = \left( \matrix{ {\rm{ Nutrient content }}(\% ) \cr \times {\rm{ Dry fruit/plant yield }}({\rm{ kg per ha }}) \cr} \right)/100

The agronomic efficiency (AE; kg dry fruit yield increase kg⁻¹ applied nutrient) was measured according to the following formula (Baligar et al., 2001): 5AE= Dry fruit yield (kg·ha−1) with applied nutrient − Dry fruit yield (kg·ha−1) without nutrient Rate of applied nutrient (kg·ha−1){\rm{AE}} = {{{\rm{ Dry fruit yield }}\left( {{\rm{kg}}\cdot{\rm{h}}{{\rm{a}}^{ - 1}}} \right){\rm{ with applied nutrient }} - {\rm{ Dry fruit yield }}\left( {{\rm{kg}}\cdot{\rm{h}}{{\rm{a}}^{ - 1}}} \right){\rm{ without nutrient }}} \over {{\rm{ Rate of applied nutrient }}\left( {{\rm{kg}}\cdot{\rm{h}}{{\rm{a}}^{ - 1}}} \right)}}

The physiological efficiency (PE) of nutrients was calculated following the formula of Paul et al. (2014): 6PE=Y−Y0U−U0PE = {{Y - {Y_0}} \over {U - {U_0}}} where Y is the dry fruit yield receiving fertiliser, Y₀ is the dry fruit yield receiving no fertiliser, U is the total nutrient uptake by plants receiving fertiliser and U₀ is the total nutrient uptake by plants receiving no fertiliser.

The removal efficiency (RE) of nutrient removal/uptake by the harvested portion of the test crop per unit nutrient applied was measured via the following formula: 7RE= Total nutrient removal or uptake (kg·ha−1) Applied nutrient (kg·ha−1)×100{\rm{RE}} = {{{\rm{ Total nutrient removal or uptake }}\left( {{\rm{kg}}\cdot{\rm{h}}{{\rm{a}}^{ - 1}}} \right)} \over {{\rm{ Applied nutrient }}\left( {{\rm{kg}}\cdot{\rm{h}}{{\rm{a}}^{ - 1}}} \right)}} \times 100

In the present investigation, the benefit–cost ratio (BCR) was documented for a hectare of land. The treatment wise operational cost of the study was determined on the basis of the cost of worker wages, ploughing and the cost of inputs, including fertiliser, seeds, and pesticides. The fresh fruit yield of bitter gourd was converted into t · ha⁻¹. This yield was used to compute the gross return. The rental of land was not measured in this experiment. The gross return of every treatment was computed by multiplying the fresh fruit yield by the current unit price (farm gate) of bitter gourd, such as gross return = fresh fruit yield × unit price of product. The gross margin was calculated by the deduction of the operational (variable) cost from the gross return. The BCR was measured via the following formula: 8BCR=GR÷TVC{\rm{BCR}} = {\rm{GR}} \div {\rm{TVC}} where GR = gross return and TVC = total variable cost.

The collected data were subjected to two-way analysis of variance (ANOVA) using the Statistix 10 software (www.statistix.com; accessed on January 27, 2025). The mean separation was performed by the least significant difference (LSD) test at a significance level of p ≤ 0.05 or p ≤ 0.01 (Statistix-10, 1985).

The year of cultivation employed a significant influence on bitter gourd growth and yield attributes. The test crop cultivated in the second year demonstrates significantly enhanced performance across growth and yield traits, as detailed in Table 5. A significant enhancement in bitter gourd growth and yield attributes was observed with the combined application of organic and inorganic fertilisers (Table 5). Significantly, treatment T₈ (5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 50% of NPKSZnB) resulted in the longest vines at 417 cm, while T₁ exhibited the shortest at 239 cm. The same T₈ treatment significantly boosted the number of fruits per plant, reaching 45.1, surpassing all other treatments (Table 5). The combined application of organic and inorganic fertilisers significantly influenced fruit length, diameter, and individual fruit weight (Table 5). Treatment T₈ (5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 50% of NPKSZnB) generated the most impressive results, producing fruits with the longest length (15.2 cm), largest diameter (5.83 cm), and heaviest individual weight (101 g), while the control treatment (T₁) exhibited the least result values in all these parameters (Table 5).

The year of cultivation had a significant effect on the dry plant yield of bitter gourd; whereas, the fresh fruit yield and fruit dry matter yield were not statistically different between the 2 years. The yields of bitter gourd responded significantly to the application of various organic and inorganic fertilisers (Figures 1A and 1B).

Figure 1.

(A) Fresh fruit yield in t · ha⁻¹ and (B) fruit dry matter in kg · ha⁻¹ and dry plant yield in kg · ha⁻¹ of bitter gourd affected by the combined application of organic and inorganic fertilisers. The error bars represent the ± standard error of the mean (n = 3). The mean values indicated by the uncommon letters in the bars are significantly different at the 5% level according to the LSD test. T₁: control, T₂: recommended dose of inorganic N-P-K-S-Zn-B at 120-40-85-20-3-2 kg · ha⁻¹, T₃: 5 t · ha⁻¹ VC + 50% of NPKSZnB, T₄: 2.5 t · ha⁻¹ PM + 50% of NPKSZnB, T₅: 5 t · ha⁻¹VC + 75% of NPKSZnB, T₆: 2.5 t · ha⁻¹PM + 75% of NPKSZnB, T₇: 5 t · ha⁻¹ CD + 75% of NPKSZnB, T₈: 5 t · ha⁻¹VC + 5 t · ha⁻¹ CD + 50% of NPKSZnB, and T₉: 5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 25% of NPKSZnB. CD, cow dung; LSD, least significant difference; PM, poultry manure; VC, vermicompost.

The combined application of 5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD along with 50% of the NPKSZnB fertiliser (T₈) resulted in the maximum fresh fruit yield (13.1 t · ha⁻¹) (Figure 1A), fruit dry matter yield (1299 kg · ha⁻¹), and dry plant yield (3909 kg · ha⁻¹) (Figure 1B) of bitter gourd. Treatment T₅ (5 t · ha⁻¹ VC + 75% NPKSZnB) showed statistically similar results, with 12.5 t · ha⁻¹ fresh fruit, 1255 kg · ha⁻¹ fruit dry matter, and 3702 kg · ha⁻¹ dry plant yield. The lowest yields of bitter gourd were found in the control treatment (T₁) (Figures 1A and 1B).

Table 6 shows the seasonal water use and WPI throughout the crop seasons of 2020 and 2021. The overall yield in the second year was significantly greater due to more efficient water management during the vegetative stage of bitter gourd cultivation (Table 6). In the first year, the crop received 12 irrigations; whereas in the second year, 10 irrigations were applied due to higher rainfall incidence. Specifically, during the experimental period in 2020, there was 841 mm of precipitation; whereas in 2021, it increased to 1412 mm (Table 1). The WPI was recorded higher (4.90 kg ·m⁻³) in 2021 than in 2020. In 2021, a higher WPI (4.90 kg ·m⁻³) was obtained with 10 irrigations totalling 269.20 mm of seasonal water use; whereas in 2020, a WPI of 4.14 kg ·m⁻³ was achieved with 12 irrigations totalling 311.36 mm of seasonal water use (Table 6).

The quality traits of bitter gourd, viz. the TSSs, vitamin C, beta carotene, moisture, fruit firmness, and reducing sugar, total sugar, and protein contents significantly differed due to the combined application of organic and inorganic fertilisers; however, the year of cultivation and their interaction (year × treatment) revealed non-significant effects (Table 7). The highest TSS content (⁰Brix 4.00) in bitter gourd was found with the application of 5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD, along with 50% of the NPKSZnB fertiliser (T₈), which was similar to the T₂ (recommended dose of N-P-K-S-Zn-B at 120-40-85-203-2 kg · ha⁻¹) treatment. The vitamin C content alternated between 70.9 mg ·100 g⁻¹ and 77.6 mg ·100 g⁻¹ across the treatments; the increase in the content of vitamin C (77.6 mg ·100 g⁻¹) in T₈ was not statistically different from most of the treatments; whereas, the decrease in the value of vitamin C was similar to that in the control (T₁) treatment. The highest content of β-carotene (122 mcg ·100 g⁻¹) was associated with the application of 5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD along with 50% of the NPKSZnB fertiliser (T₈), which was on par with the application of 5 t · ha⁻¹ VC with 50% of the NPKSZnB fertiliser (T₅), although a lower content of β-carotene was detected in the control treatment (T₁). The maximum moisture content (93.4%) of bitter gourd was recorded in the T₈ treatment, which was not significantly different from that of most of the other treatments. The fruit firmness of bitter gourd was also highest (2.26 kgf) in T₈, which was comparable with that in most of the treatments; whereas, the firmness was lowest in the control treatment (Table 7). The values of reducing sugars and total sugars were inconsistent among the treatments. The increase in protein content (18.1%) in the T₈ treatment was similar to that in the T₅ treatment; whereas, a decrease in protein content (14.5%) was detected in the control (T₁) treatment (Table 7).

The cultivation year significantly affected the total uptake of N, P, K, S, Zn, and B by bitter gourd. The highest total uptake of N, P, K, S, Zn, and B was observed in the second year (2021); whereas, the lowest uptake occurred in 2020 (data not shown). All the treatments containing combined organic and inorganic fertilisers had a significant effect on the total nutrient uptake by bitter gourd. The total uptake value of nitrogen (167 kg · ha⁻¹) significantly increased with the application of 5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD along with 50% of the NPKSZnB fertiliser (T₈), which was statistically similar to the T₅ (5 t · ha⁻¹ VC + 75% NPKSZnB fertiliser) treatment; whereas the lowest total uptake value of N was obtained in the control treatment. Compared with the control (T₁), the T₈ treatment led to a significant increase in total P uptake (22.8 kg · ha⁻¹) (Figure 2A).

Figure 2.

(A) Total uptake of N, P, K, and S by bitter gourd and (B) total uptake of Zn and B by bitter gourd as affected by the combined application of organic and inorganic fertilisers. The error bars represent the ± standard error of the mean (n = 3). The mean values indicated by the uncommon letters in the bars are significantly different at the 5% level according to the LSD test. T₁: control, T₂: recommended dose of inorganic N-P-K-S-Zn-B at 120-40-85-20-3-2 kg · ha⁻¹, T₃: 5 t · ha⁻¹ VC + 50% of NPKSZnB, T₄: 2.5 t · ha⁻¹ PM + 50% of NPKSZnB, T₅: 5 t · ha⁻¹ VC + 75% of NPKSZnB, T₆: 2.5 t · ha⁻¹ PM + 75% of NPKSZnB, T₇: 5 t · ha⁻¹ CD + 75% of NPKSZnB, T₈: 5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 50% of NPKSZnB, and T₉: 5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 25% of NPKSZnB. CD, cow dung; LSD, least significant difference; PM, poultry manure; VC, vermicompost.

In this study, the maximum total K uptake (96.5 kg · ha⁻¹) by the test crop was obtained from the T₈ treatment followed by the T₅ treatment, although the minimum total uptake of K was from the T₁ (control) treatment. Similarly, the highest total S uptake by bitter gourd (18.0 kg · ha⁻¹) occurred in the T₈ treatment; whereas, the lowest total S uptake (4.65 kg · ha⁻¹) occurred in the T₁ (control) treatment (Figure 1A). Furthermore, the T₈ treatment resulted in the highest total Zn uptake (0.249 kg · ha⁻¹) and total B uptake (0.191 kg · ha⁻¹) by bitter gourd; whereas, both were lowest in the T₁ (control) treatment (Figure 2B). These results underscore the significant influence of fertilisation strategies on nutrient uptake by bitter gourd, emphasising the effectiveness of organic amendments in combination with reduced inorganic inputs.

The application rates of organic and inorganic fertilisers positively influence the NUE of bitter gourd. Various indicators, such as AE, PE, and RE, were employed to assess NUE. The AE of N, P, K, S, Zn, and B responded significantly to the application of combined organic and inorganic fertilisers to the soil (Table 8).

For N, the AE values ranged from 3.83 kg · kg⁻¹ to 6.98 kg · kg⁻¹ across the treatments, with the highest AE observed in T₄ (2.5 t · ha⁻¹ PM + 50% of the NPKSZnB fertiliser), which was statistically similar to that in the T₆ treatment; whereas, the lowest AE of N was noted in T₉ (Table 8). The highest AE value for P (16.2 kg · kg⁻¹) was achieved in the T₂ treatment, followed by the T₇ and T₄ treatments; whereas, the lowest AE value was observed in the T₉ treatment. Similarly, the highest AE value of K (9.83 kg · kg⁻¹) was found in the T₄ treatment, followed by the T₇ and T₆ treatments, with the lowest AE value in the T₉ treatment (Table 8). In terms of S, the highest AE value (34.5 kg · kg⁻¹) was observed in T₄, which was statistically similar to that in the T₂ and T₆ treatments (Table 8). The maximum AE value of Zn (324 kg · kg⁻¹) was registered in the T₄ treatment, which was statistically identical to that in the T₆ treatment, with the minimum AE value in the T₂ treatment. For B, the AE value was highest (492 kg · kg⁻¹) in T₄, while the lowest AE value was in the T₉ treatment (Table 8).

The PE of P, K, S, Zn, and B in bitter gourd was significantly influenced by the combined management of organic and inorganic fertilisers, except for the PE of N (Table 9).

The PE of N was not significantly different among the treatments having the combination of organic and inorganic fertilisers. In this study, the highest PE of P (68.1 kg · kg⁻¹) in bitter gourd was noted in the T₉ treatment (5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 25% NPKSZnB fertiliser), which was similar to that in the T₃ treatment (5 t · ha⁻¹ VC + 50% of the N-P-K-S-Zn-B fertiliser); whereas, the lowest PE of P was listed in the T₈ treatment (5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 50% of the NPKSZnB fertiliser). The PEs of K (16.5 kg · kg⁻¹) and S (94.0 kg · kg⁻¹) in bitter gourd was increased significantly in T₃ (5 t · ha⁻¹ VC + 50% of the NPKSZnB fertiliser); whereas, decreases in the PEs of K and S were detected in the T₈ treatment. Greater PEs of Zn (6.29 mg · kg⁻¹) and B (8.25 mg · kg⁻¹) were detected in the T₃ treatment; whereas, both were lower in the T₈ (5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 50% of the NPKSZnB fertiliser) treatment (Table 9). The RE of N, P, K, S, Zn, and B in bitter gourd was significantly influenced by the combined application of organic and inorganic fertilisers (Table 10). The RE of N varied between 78.0% and 135% among the treatments, where the highest RE of N was observed in T₄ (2.5 t · ha⁻¹ PM + 50% of the NPKSZnB fertiliser); whereas, the lowest was in the T₉ treatment. A significant increase in the RE of P (44.2%) in bitter gourd was registered in the T₂ (recommended dose of N-P-K-S-Zn-B at 120-40-85-20-3-2 kg · ha⁻¹) treatment; whereas, a decrease in the RE of P was computed in the T₉ treatment. In our study, a significant improvement in the RE of K (107%) and the RE of S (69.7%) in bitter gourd was obtained in the T₄ (2.5 t · ha⁻¹ PM + 50% of the NPKSZnB fertiliser) treatment; whereas, a decrease in the RE of K and S was observed in the T₉ treatment. The highest RE values of Zn (9.58%) and RE values of B (10.8%) were attained in the T₃ treatment; whereas, the lowest RE value for Zn was in the T₂ treatment, and the lowest RE value for B was found in the T₉ treatment (Table 10).

Organic and inorganic fertilisation significantly affected PSB, Rhizobia, fungi, total bacteria, and actinomycetes in the soil of bitter gourd at the flowering stage (Table 11). The PSB population was significantly greater (100 × 10⁵ cfu · g⁻¹ soil) in T₈ than in the other treatments. The highest population of Rhizobia (110 × 10⁵ cfu · g⁻¹ soil) was obtained in T₈, followed by the T₅ treatment. Similarly, the maximum fungal population (100 × 10⁵ cfu · g⁻¹ soil) was recorded in the T₈ and T₄ treatments. The largest population of total bacteria (1900 × 10⁵ cfu · g⁻¹ soil) was significantly greater in the T₈ treatment than in the other treatments. The population of actinomycetes was greatest (130 × 10⁵ cfu · g⁻¹ soil) in T₈, significantly surpassing those in the other treatments. The minimum populations of PSB, Rhizobia, fungus, total bacteria, and actinomycetes were observed in the control treatment (Table 11).

The year of cultivation had no significant effect on the chemical properties of the postharvest soil (Table 12). However, the combined application of organic and inorganic fertilisers affected the total N, exchangeable K, and available Zn and B; whereas, the postharvest soil pH, organic matter, and available P and S did not significantly change (Table 12). The results of the experiment revealed a slight increase or decrease in postharvest soil pH across treatments relative to the initial soil pH values. Among the treatments, higher levels of soil organic matter, total N, exchangeable K, and available P, S, Zn, and B were mostly observed in T₈; whereas, lower levels of these properties were observed in the control (no use of fertiliser) treatment (Table 12). A comparison of the chemical properties of the postharvest soil with those of the initial soil revealed that nutrient concentrations were mostly greater in the postharvest soil across all the treatments (Table 12).

The year of cultivation did not affect the gross return, gross margin or BCR of bitter gourd (data not presented). However, the gross return, gross margin and BCR of bitter gourd responded significantly to the combined application of organic and inorganic fertilisers (Figures 3A and 3B).

Figure 3.

Influence of combined use of organic and inorganic fertiliser practices on the (A) cost, gross return, gross margin, and (B) BCR of bitter gourd. The error bars represent the ± standard error of the mean (n = 3). The mean values indicated by the uncommon letters in the bars are significantly different at the 5% level according to the LSD test. T₁: control, T₂: recommended dose of inorganic N-P-K-S-Zn-B at 120-40-85-20-3-2 kg · ha⁻¹, T₃: 5 t · ha⁻¹ VC + 50% of NPKSZnB, T₄: 2.5 t · ha⁻¹ PM + 50% of NPKSZnB, T₅: 5 t · ha⁻¹ VC + 75% of NPKSZnB, T₆: 2.5 t · ha⁻¹ PM + 75% of NPKSZnB, T₇: 5 t · ha⁻¹ CD + 75% of NPKSZnB, T₈: 5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 50% of NPKSZnB, and T₉: 5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 25% of NPKSZnB. BCR, benefit–cost ratio; CD, cow dung; LSD, least significant difference; PM, poultry manure; VC, vermicompost.

Output price: Fresh fruits of bitter gourd at US$ 0.35 · kg⁻¹, and input price: urea fertiliser = US$ 0.16 · kg⁻¹, triple superphosphate = US$ 0.28 · kg⁻¹, muriate of potash = US$ 0.20 · kg⁻¹, gypsum = US$ 0.17 · kg⁻¹, zinc sulphate (lab grade) = US$ 13.9 · kg⁻¹, boric acid (lab grade) = US$ 12.8 · kg⁻¹, bamboo = US$ 3.49 · piece⁻¹, jute rope = US$ 1.40 · kg⁻¹, wage rate = US$ 6.98 · day⁻¹, tilling (single pass) = US$ 16.3 · ha⁻¹, Bavistin fungicide = US$ 2.33 · 100 g⁻¹, Thiovit 80 (water dispersible granules) = US$ 1.63 · 100 g⁻¹, Ripcord insecticide = US$ 1.51 · 100 mL⁻¹, Karate 2.5 EC = US$ 5.23 · 500 mL⁻¹, VC organic fertiliser = US$ 0.11 · kg⁻¹, poultry manure = US$ 0.10 · kg⁻¹, and cow dung = the gross returns are considered on the basis of the farm gate price (fresh fruit of bitter gourd) of the Gazipur district in Bangladesh. 1 US$ = 86 BDT. BDT is the Bangladesh currency.

The highest gross return (US$ 4552 · ha⁻¹) was computed in the T₈ (5 t · ha⁻¹ VC + 5 t · ha⁻¹ CD + 50% of the NPKSZnB fertiliser) treatment, followed closely by the T₅ treatment. In contrast, the maximum gross margin (US$ 2295 · ha⁻¹) (Figure 3A) and highest BCR (2.25) were observed in the T₆ treatment (2.5 t · ha⁻¹ PM + 75% NPKSZnB fertiliser), which was comparable to those of most of the other treatments, while both were lowest in the control treatment (T₁) (Figures 3A and 3B).