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Article

A Greener Approach to Spinach Farming: Drip Nutrigation with Biogas Slurry Digestate

by
Rashmi Yadav
1,
Susama Sudhishri
1,*,
Manoj Khanna
1,
Khajanchi Lal
1,
Anchal Dass
2,
Hari Lal Kushwaha
3,
Kalikinkar Bandyopadhyay
4,
Archna Suman
5,
Anupama Singh
6,
Rajiv Kumar Singh
2 and
Raj Singh
2
1
Water Technology Centre, ICAR—Indian Agricultural Research Institute, Pusa, New Delhi 110-012, India
2
Division of Agronomy, ICAR—Indian Agricultural Research Institute, Pusa, New Delhi 110-012, India
3
Division of Agricultural Engineering, ICAR—Indian Agricultural Research Institute, Pusa, New Delhi 110-012, India
4
Division of Agricultural Physics, ICAR—Indian Agricultural Research Institute, Pusa, New Delhi 110-012, India
5
Division of Microbiology, ICAR—Indian Agricultural Research Institute, Pusa, New Delhi 110-012, India
6
ICAR—Indian Agricultural Research Institute, Pusa, New Delhi 110-012, India
*
Author to whom correspondence should be addressed.
Agronomy 2024, 14(4), 681; https://doi.org/10.3390/agronomy14040681
Submission received: 22 June 2023 / Revised: 15 July 2023 / Accepted: 18 July 2023 / Published: 27 March 2024
(This article belongs to the Special Issue Constraints and Balances among Soil Properties, Microorganisms, and Agronomic Techniques)
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Abstract

:
To achieve higher crop yields and maintain environmental conservation, it is imperative to adopt novel agricultural methods that improve both the quantity and quality of produce. The following study focused on investigating the effectiveness of integrated use of biogas slurry and inorganic nutrigation on spinach growth and nutrient uptake through drip irrigation. A field experiment was conducted using a split-plot design; spinach was cultivated with seven different treatments: biogas slurry nutrigation (BSN) (T1), integrated inorganic + organic nutrigation: 40% Recommended dose of fertilizer (RDF) + BSN (T2), 60% RDF + BSN (T3), 80% RDF + BSN (T4), 100% RDF (T5), slurry broadcasting (SB) (T6), and control (T7). The results showed that spinach grown with (T4) 80% RDF + BSN exhibited optimum plant height and leaf count compared to spinach under other treatments and was at par with T5 100% RDF for crop parameters. T5 treated plants demonstrated the longest roots, followed by T4 treated plants. The highest recorded yield of 5.72 tons ha−1 was achieved in treatment T5 with 100% RDF, followed closely by T4 with 80% RDF + BSN at 5.52 tons ha−1 and T3 with 60% RDF + BSN at 5.36 tons ha−1. These three treatments exhibited comparable yields, showcasing the effectiveness of incorporating biogas slurry nutrigation in conjunction with inorganic fertilizer for achieving high spinach yields. In terms of macronutrient content in spinach leaves, significant differences were found for nitrogen (N), phosphorus (P), and potassium (K) contents. Incorporating biogas slurry into the soil modified microbial enzyme activities, specifically dehydrogenase and phosphatase. Normally, alkaline phosphatase shows greater activity than acidic phosphatase, but the addition of biogas slurry equalized the enzymatic activity of both, establishing a harmonized enzymatic profile. Our results suggest that fertilizing spinach with integrated biogas slurry nutrigation (if properly filtered) + inorganic nutrigation through drip nutrigation is a viable strategy for improving the morphological parameters and productivity of spinach while also contributing to environmental preservation and a reduction in production costs.

1. Introduction

Modern agricultural practices prioritize enhancing both the quantity and the quality of crop yields, all while ensuring environmental preservation is taken into careful consideration. In order to meet the growing demands of a rapidly increasing population, it is essential to implement innovative farming techniques that simultaneously boost crop yields and enhance environmental conservation. These methods ought to ensure an increase in food production for the growing population in a sustainable manner [1]. The majority of the growth in energy consumption is occurring in rapidly developing economies, with China and India collectively contributing to half of the global energy demand increase. The fastest-growing energy source is renewable energy, which is responsible for 40% of the rise in primary energy [2]. By 2040, the world’s energy mix is anticipated to be the most diverse it has ever been, reflecting a wide range of energy sources. In order to achieve this objective, there is a need for rapid growth in the utilization of various forms of existing renewable energy sources. To accomplish this goal, it is essential to accelerate the adoption and utilization of diverse renewable energy sources in order to facilitate rapid progress in achieving sustainable agriculture practices. The anaerobic digestion of agricultural and livestock waste is a proven and dependable method for generating bio-energy and recovering valuable resources. As sustainable energy systems gain traction in developing nations, it is anticipated that the production of byproducts will increase concurrently. In India, efforts encompassing various renewable energy sources have been undertaken, including the National Biogas and Manure Management Program (NBMMP) during the twelfth five-year plan (2012–2017). The Indian government aims to establish 6.5 lakh biogas plants nationwide with a budget of Rs. 650 crore, focusing on enhancing biogas production and managing organic waste efficiently. The byproduct generated from the input feedstock of the biogas production process ranges from 5 to 80%, depending on the type of feedstock used. This leaves a significant amount of residue that needs to be effectively managed to achieve both financial and ecological benefits in the biogas production process [3]. Currently, inadequate application and storage methods, such as utilizing unlined earthen drains and pits, create potential pathways for nutrient loss. These result in the suboptimal utilization of digestate, hindering its effective use [4]. Biogas digestate is highly concentrated in organic matter and plant nutrients, making it an excellent soil amendment. Biogas digest liquid can be used as an alternative source of nutrients in place of chemical fertilizer as it increases the nitrogen-use efficiency and does not have adverse environmental effects. DBGS (digestate biogas slurry) is a high-quality liquid organic fertilizer produced by anaerobic fermentation [3,5,6]. It contains valuable nutrients such as nitrogen, phosphorus, and potassium [7,8]. Thus, LFBD (liquefied fraction of biogas digestate) can be an environmentally sustainable alternative to chemical fertilizers [9]. Previous studies have indicated that digestate was primarily utilized in furrow irrigation and flood irrigation practices, resulting in both wastage of digestate and environmental pollution [10]. Several research gaps exist in this field. The United Nations Economic Commission for Europe encourages the utilization of fertigation as a recommended application technique for the liquid fraction of digestate [11]. Limited information is available on drip nutrigation using the liquefied fraction of biogas digestate (LFBD). The nutrient-enriched biogas digestate liquid is often discarded as waste instead of being utilized. The liquid fraction has a high rate of readily available nitrogen [12]. Utilizing this liquid as a fertilizer can increase production and address the problem of secondary pollution caused by the low utilization rate of biogas slurry. Given the current emphasis on organic farming in India, the utilization of biogas digestate aligns with government priorities. Slurry distribution can cause nitrogen losses, leading to environmental pollution and health risks [13]. Factors such as soil conditions, fertilization, and water drainage affect the extent of these losses [14]. Effective management reduces nitrogen leaching [15] and its negative impacts. To minimize the impact, maximizing nitrogen crop interception is crucial. This can be achieved by accurately applying fertilizers while synchronizing with crop demand. Fertigation aligns with this goal, effectively reducing nitrogen losses and mitigating environmental footprint. Implementing slurry application through fertigation also yields positive outcomes, reducing ammonia emissions, groundwater seepage, and fertilizer usage [16,17]. Applying biogas slurry by drip irrigation has emerged as a significant engineering method for biogas slurry recycling. This establishes a novel approach for integrating water and fertilizer technologies, wherein injecting biogas slurry in drip irrigation system facilitates the extensive adoption of organic fertilizer over chemical fertilizers. As a result, this system achieves the dual objectives of effectively utilizing biogas slurry resources and promoting the widespread replacement of chemical fertilizers with organic alternatives [18]. Dry bio-slurry application has significant environmental impacts, including ammonia and greenhouse gas emissions, as well as nutrients through surface runoff. Surface application of bio-slurry can result in nitrogen losses through ammonia volatilization. Offensive odor from livestock farming or organic farming is the other major concern. Nutrigation, an integration of nutrient and irrigation delivered simultaneously in precise combination and timing, is an effective approach ensuring that plants receive the necessary elements for growth and development uniformly. Liquid fraction of a biogas digestate used as nutrigation effectively meets the criteria of applying the right nutrient, at the right rate, at the right time, and in the right place [17,19]. This practice significantly improves the overall sustainability of the system. Drip nutrigation is a sophisticated and efficient method of applying fertilizers through irrigation systems and shows great promise [20]. By integrating biogas slurry with irrigation water, drip nutrigation harmonizes the application of water [18] and plant nutrients. This approach ensures that an adequate supply of nutrients and water reaches the plant root zone, satisfying plant demands during various growth stages. Drip irrigation, when combined with measured quantities of irrigation water and managed fertilizer concentrations, can maximize crop yield and quality while minimizing nutrient leaching beyond the rooting zone. Food security encompasses both food availability and quality, with nutritious foods, such as leafy vegetables, supplying essential carbohydrates, protein, and vitamins for a healthy lifestyle. In India, spinach beet (Beta vulgaris L. var. bengalensis) is extensively cultivated and recognized as one of the most popular leafy vegetables. Farmers have a tendency to use more nitrogenous fertilizers in an effort to improve the output of greens; doing so may cause anti nutrient components to build up in the greens beyond an acceptable limit. Utilizing biogas slurry as an alternative nutrient and irrigation source can improve nitrogen use efficiency while minimizing damage to the environment. Developing agro-technologies and implementing efficient water use practices such as micro irrigation and incorporating drip nutrigation can enhance crop productivity and sustainability. The utilization of biogas digestate liquid as a fertilizer and the design of filtration systems to address clogging challenges are crucial areas of research. Therefore, in order to increase the production of greens and improve their edible quality, it is necessary to replace the inorganic nutritional requirements with organic nutrient sources. The current study was carried out to determine the feasibility of using biogas slurry through drip irrigation vis-à-vis the effect of the integrated use of biogas slurry (BS) and inorganic nutrigation on spinach growth and nutrient uptake under drip nutrigation.

2. Materials and Methods

2.1. Experiment Location

The field experiment was conducted in Rabi (non-monsoon) season of 2022 in the experimental farm of Water Technology Center, ICAR–Indian Agricultural Research Institute, New Delhi (Latitude 28°38′14.4″ and Longitude 77°9′1.8″). The area of the field was 441 m2. The biogas slurry sample was collected from the biogas plant installed in IFS (integrated farming system) by the division of Agronomy in ICAR–Indian Agricultural Research Institute (IARI), New Delhi. Samples of the biogas slurry were obtained using a sampler. The biogas slurry was then analyzed in the Soil and Plant Quality Laboratory in Water Technology Center for the following parameters using the standard methods (Table 1).

2.2. Weather Conditions

Throughout the duration of the experiment, the average minimum temperatures ranged from 5 °C to 26.2 °C, while the average maximum temperatures varied between 23 °C and 36 °C. The highest recorded rainfall, measuring 95.7 mm, occurred during the second week after sowing. During the study, an evaporative demand ranging from 1.4 mm to 5.1 mm was observed, surpassing the average amount of rainfall received. This higher evaporative demand necessitated supplementary irrigation to support crop growth (Figure 1 and Figure 2).

2.3. Experimental Design

Existing micro irrigation filters are inefficient for organic fertigation due to their inability to effectively filter out fine debris present in organic manure solutions, leading to frequent clogging and hindered fertigation. The developed cascade filter system consists of three polyethylene tanks with a surface area of 1.96 m2 and a volume of 200 L each. These tanks are positioned at different heights and filled with a mixture of pebbles, gravel, and sand in a ratio of 3:1:1.5. In the laboratory setup, the tanks were connected using 25 mm CPVC pipes and an outlet valve was installed on the last tank, which was filled with sand media. A nylon mesh with a size of 400 openings per linear inch was placed before the outlet valve to ensure effective filtration. The diluted biogas digestate and water ratio at 1:2 was applied to the above filter system. The filter system was subjected to three consecutive batch operations with a loading rate of 0.06 m3; day−1 m−2. The filtrate of the digestate was then applied as nutrigation in the field through drip irrigation installed in the field. The experiment was conducted using a split-plot design, with a total of seven numbers of treatments viz. (T1) Biogas slurry nutrigation (BSN); (T2) 40% RDF + BSN; (T3) 60% RDF + BSN; (T4) 80% RDF + BSN; (T5) 100% RDF; (T6) slurry broadcasting (SB); and (T7) control with three replications. Treatment T1 involved twice-weekly nutrigation with 9.14 L of filtered slurry per plot, while treatments T2, T3, and T4 utilized nutrigation with 4.57 L of filtered slurry per plot at 10 day intervals. In treatment T6, slurry was applied at a rate of 1.52 L per square meter as a single broadcast application during the entire crop growing season. The crop under investigation was spinach (Pusa All Green variety). The plot size for each treatment was 7 m × 1.25 m (8.75 m2). The irrigation was carried out using a drip lateral to lateral spacing of 50 cm, and the emitters were spaced at 30 cm intervals along the lines. The recommended dose of fertilizer for the experiment was 120 kg ha−1 of nitrogen (N) was supplied through urea, 60 kg ha−1 of phosphorus (P) sourced from diammonium phosphate, and 50 kg ha−1 of potassium (K) from muriate of potash and was delivered to the crop by injecting the RDF of fertilizer into the irrigation water through drip irrigation. The application of digestate nutrigation as both a base dressing and side dressing was carried out in the fields at 1.772 L/m2 through drip irrigation prior to sowing, between the 28 and 29 September 2022. Treatment T1 (BSN) received weekly nutrigation with digestate twice between 4 October and 1 November, prior to the first harvesting on 7 November. Treatments T2 40% RDF + BSN, T3 60% RDF + BSN, T4 80% RDF + BSN, and T5 100% RDF were nutrigated once a week with inorganic fertilizer and biogas slurry using the drip irrigation system. The second harvesting of the crop took place on 21 November, with three rounds of biogas digestate nutrigation for T1 BSN and two rounds for treatments T2, T3, T4, and T5 between the first and second harvesting. The same procedure was repeated between the second and third harvesting on 4 December. The fourth and final harvesting occurred on 15 December. The irrigation was provided with a weekly water application of 25–30 mm per hectare a week.

2.4. Sampling and Analysis

The samples of biogas slurry were collected from the biogas plant. The biogas slurry samples were analyzed in Soil and Plant Quality Laboratory, Water Technology Center, ICAR–IARI, New Delhi for electrical conductivity (EC), pH, total nitrogen, total phosphorous, total potassium, and turbidity before and after passing through the developed filter. To analyze the soil properties before and after experimentation, plot-wise soil samples were drawn from the 0–15 cm layer and analyzed for bulk density and porosity apart from macronutrients. Conductivity and pH of biogas slurry were analyzed within two hours of sampling by digital conductivity and pH meter, respectively. Turbidity measurements were taken using a turbidity meter to assess the turbidity levels of the biogas slurry prior to and following the filtration procedure. Afterwards, the slurry was oven-dried at 60 °C for 48 h, smashed to roughly below 2 mm size and analyzed for nitrogen, phosphorus, and potassium contents by Kjeldahl’s method, vanadomolybdate phosphoric yellow color method, and flame photometer method, respectively [21]. Each analysis was done in three replications and the mean values were considered for reporting. Soil samples were analyzed for organic carbon, available nitrogen, phosphorus, potassium by Walkley and Black’s wet oxidation method [22], alkaline permanganate oxidation method Subbaiah and Asija 1956 [21], Olsen’s method using spectrophotometer and flame photometry, respectively [22]. To maintain the accuracy of the analytical data, careful standardization, measurements of procedural blanks, and analysis of duplicate samples were implemented.

2.5. Agronomical Analysis

Throughout the experimental period, the height of the plant, the area of leaves, and the count of leaves were recorded. For each experimental unit (plot), four plants from two spots in the sampling area were earmarked for recording the observations on plant height and number of leaves. The height of the plant was measured from the crop base to the apex of the last two leaves. Leaf area was determined using a leaf area meter. At the end of the experiment, leaves from all treatment plots were collected for nutrient analysis. The harvested leaves were analyzed for macro nutrients, nitrogen (N) by Kjeldahl’s method, phosphorus (P) by the vanadomolybdate yellow color method, and potassium (K) by the flame photometer method. For the above-ground biomass assessment, the stands of spinach per replication were harvested and dried manually under sunlight for 2 days and then oven-dried at 60 °C until the constant dry weight was obtained. Additionally, the roots of the spinach plants were drawn from a depth of 15 cm washed, measured, and recorded to evaluate the comparative effects of the treatments [23].

2.6. Microbiological Analysis

The soil samples from a depth of 0 to 15 cm were subjected to standard methods for analyzing soil phosphatase activity, soil dehydrogenase activity, and soil microbial biomass carbon (SMBC). The determination of soil phosphatase activity was performed using the modified molybdate-blue method, also known as the p-nitrophenyl phosphatase disodium (0.1 M) as substrate [24]. The standard method was used to estimate dehydrogenase activity [25]. To estimate SMBC, the chloroform fumigation extraction method (CFE) was employed, which is also referred to as the CFE method [26].

2.7. Statistical Analysis

Analysis of variance was carried out to examine the effect of biogas slurry nutrigation on the yield and quality of spinach growth parameters. The data were statistically examined using the F-test [27]. C.D. values were calculated for the parameters that exhibited significant differences. The treatment means were compared at 5% level of significance.

3. Results

The physico-chemical composition of biogas slurry before and after filtration is presented in Table 1. The parameters measured included electrical conductivity (EC), pH, turbidity, total nitrogen, total phosphorus, and total potassium. Before filtration, the EC of the biogas slurry was 1.84 ds cm−1, which decreased to 1.46 ds cm−1 after filtration. The pH value decreased from 8.52 to 7.62 after filtration. Turbidity, measured in nephelometric turbidity units (NTU), decreased significantly from 639 NTU before filtration to 199 NTU after filtration. Table 2 provides information on the physico-chemical composition of the soil before the experiment. The particle size distribution of the soil indicates that it is predominantly composed of sand (71%), with lesser proportions of silt (14%) and clay (15%). The textural class of the soil is classified as sandy loam. The bulk density of the soil is 1.52 g cc−1. Regarding the chemical properties, the soil has available nitrogen content of 125.7 kg/ha, available phosphorus content of 26.5 kg/ha, and available potassium content of 281.4 kg/ha. The organic carbon content is 0.43%. The pH value of the soil is 7.21, and the electrical conductivity (EC) is measured at 0.29.

3.1. Spinach Growth

3.1.1. Plant Height

A glance through the data revealed a clear trend; as the crop age increased, the height of the plants also exhibited a corresponding increase. Figure 3 illustrates the mean plant height (cm) under different treatments over time. Initially, the growth rate was rapid until 30 days after sowing (DAS) to 60 DAS, after which it gradually slowed down until harvest. The results demonstrated significant variations in plant height based on the different nutrigation treatments (Table 3). During the first harvest at 30 DAS, the height of spinach plants ranged from 17.5 ± 0.13 cm to 23.58 ± 0.51 cm. By the end of the experiment, the heights ranged from 20.65 ± 0.35 cm to 25.65 ± 0.93 cm. Within the initial 30 DAS, there were notable differences in plant height between spinach cultivated with biogas slurry nutrigation (T1) and those grown with integrated inorganic + organic nutrigation (T2, T3, T4, T5 representing 40%, 60%, 80%, and 100% RDF, respectively, with BSN). The mean values recorded revealed plants treated with BSN (T1) exhibited a mean height of 22.68 cm, while those treated with 40% RDF + BSN (T2) and 60% RDF + BSN (T3) displayed slightly taller plants at 24.14 cm and 24.74 cm, respectively. A significant increase in height was observed for plants treated with 80% RDF + BSN (T4), with a mean height of 25.43 cm. However, the tallest plants were recorded in the 100% RDF treatment (T5) at 26.07 cm. In contrast, plants subjected to slurry broadcasting (T6) and no fertigation (T7) had lower mean heights of 20.6 cm and 19.36 cm, respectively. When comparing the mean plant heights of all the treatments using the C.D. values (1.344), there were no statistically significant differences observed between T2 and T3, T3 and T4, T4 and T5, or T6 and T7, suggesting that these treatments had similar effects on plant height. In terms of subsequent harvests, the mean value of tallest plant height was recorded during the third harvest at 24.32 cm, followed by fourth the harvest at 23.96 cm, while the second and first harvest measured 23.40 cm and 20.89 cm. Treatment T5 (100% RDF) consistently recorded the tallest plant height, followed by T4 (80% RDF + BSN) across all harvests. These findings highlight the differential impact of nutrigation treatments on plant height during the early growth stage, with the highest plant height achieved under the 100% RDF treatment (T5) 100% RDF and (T4) 80% RDF + BSN, relatively lower heights observed in the slurry broadcasting (T6) and (T7) Control.

3.1.2. Leaf Count

The number of leaves per plant was significantly affected by different nitrogen sources (Figure 4 and Table 3). Treatment T3, 60% RDF + BSN (16.35), was found to be on par with treatment (T4) 80% RDF + BSN (15.75). The highest leaf counts were recorded for T5 100% RDF (17) followed by (T4) 80% RDF + BSN (15.75) and (T3) 60% RDF + BSN. Additionally, there was a significant difference between treatment (T6) slurry broadcasting and (T7) control. Treatment T7 had the lowest mean leaf count (7.75), followed by T6 (9.0). The number of leaves increased with each successive cutting and had a positive correlation with plant height. Harvesting frequency increased the leaf count; the third harvest recorded the maximum leaf count with a mean value of 15.72, followed by the fourth and second harvests with respective mean counts of 13.90 and 13. The lowest leaf count was found in the first harvest (11.14).

3.1.3. Leaf Area

The leaf area of plants was significantly affected by different nutrigation sources (Table 3). The highest leaf area was observed in treatment (T5) 100% RDF, measuring 54.84 cm2, followed by (T3) 60% RDF + BSN at 50.91 cm2 and (T2) 40% RDF + BSN at 48.47 cm2; and (T4) 80% RDF + BSN (45.11 cm2) was observed to be on par with treatment (T1) BSN (46 cm2) (Figure 5). The treatments (T6) BS and (T7) Control exhibited the lowest leaf area, and there was no significant difference between them. In terms of the harvesting stages, the leaf area influence by the number of harvests was found to be significant. The third harvest exhibited the highest mean leaf area, measuring 79.70 cm2. This was followed by the fourth harvest with a mean leaf area of 47.14 cm2, and the second harvest with a mean leaf area of 35.02 cm2.

3.1.4. Root Length and Biomass

An illustration of the fresh mass and dry mass of spinach under different nutrigation methods is depicted in Figure 6 The statistical analysis showed significant differences in dry mass (d.f. = 6, p <0.01, C.D. = 1.938), fresh mass (d.f. = 6, p < 0.01, C.D. = 0.309), and root length (d.f. = 6, p < 0.01, C.D. = 0.551) among the treatments. The determination of spinach’s economic yields was done by the measurement of fresh leaf and total mass. The yield was assessed based on the total above-ground fresh mass achieved 30 days after sowing. The control (T7) plants had the lowest fresh mass measuring 34.78± 0.558 g. Regarding root length, as depicted in Figure 7, the mean root length was recorded longer for treatment (T5) 100% RDF (16.53 cm) followed by (T4) 80% RDF + BSN with 15.75 cm and (T3) 60% RDF + BSN with 14.4 cm. Treatments (T3) 60% RDF + BSN and (T2) 40%RDF + BSN did not differ significantly with each other. The control showed the shortest root length at 8.94 cm. In Figure 6 the BM/RL ratio provides insight into the relationship between the shoot biomass that is the above-ground part of the plant and root length for each treatment. The biomass-to-root length (BM/RL) ratio was not significantly different between treatment (T5) 100% RDF (0.51), (T4) 80% RDF + BSN (0.50), and (T3) 60% RDF + BSN (0.53). The control treatment (T7) and slurry broadcasting treatment (T6) exhibited the lowest BM/RL ratios, measuring 0.41 and 0.42, respectively. Treatment (T2) 40% RDF + BSN (0.47) and treatment (T1) BSN (0.48) were found to be comparable for BM/RL ratio. Significant variations were observed among the treatments in terms of yield, measured in ton ha−1 (Figure 8). The highest recorded yield was observed in treatment (T5) 100% RDF, reaching 5.72 tons ha−1, followed by (T4) 80% RDF + BSN at 5.52 tons ha−1, and (T3) 60% RDF + BSN at 5.36 tons ha−1. Treatments (T4) 80% RDF + BSN, (T5) 100% RDF, and (T3) 60% RDF + BSN exhibited comparable yields. Obviously, the lowest yield was observed in the control plots (1.87 tons ha−1).

3.2. Spinach Nutrient Uptake

An illustration of accumulation of macronutrients (N, P, and K) in spinach leaves under the treatments in the present study is shown in Figure 9, indicating significant variations in N uptake among the treatments. The treatment (T4) 80% RDF + BSN exhibited the highest N uptake of 31.54 kg ha−1. Following closely was (T5) 100% RDF with an uptake of 30.18 kg ha−1. On the other hand, the lowest N uptake was observed in the control (T7) with 15.44 kg ha−1, followed by (T6) slurry broadcasting, with an uptake of 16.67 kg ha−1. These findings suggest that the choice of treatment significantly influences N uptake in the study. Higher application rates of RDF combined with BSN (T2, T3, and T4) resulted in increased P uptake, while the T1 (BSN) recorded comparatively lower P uptake in comparison to other treatments in the study. Treatment (T7) and the use of slurry broadcasting (T6) did not differ significantly and were recorded to have lower P uptake. T5 (100% RDF) exhibited the highest P uptake among all treatments. Treatment (T3) 60% RDF + BSN exhibited 5.78 kg ha−1 and was observed to be on par with the treatment (T2) 40% RDF + BSN with 5.89 kg ha−1. The comparison of potassium (K) uptake among the treatments, using a critical difference (C.D.) value of 0.37, reveals several significant differences. The treatment (T5) 100% RDF (14.12 kg ha−1) had the maximum K uptake followed by (T4) 80% RDF + BSN (13.04 kg ha−1) and (T3) 60% RDF + BSN (12.11 kg ha−1), and treatment (T2) 40% RDF + BSN (9.92 kg ha−1) was on par with (T1) BSN (10.17 kg ha−1).

3.3. Effect on Soil

The NPK uptake in the soil showed no significant difference between the treatments during the experiment (Figure 10). However, there was a slight movement towards the neutral and higher range in the soil pH. In the experiment, the inorganic nutrigation (T5) 100%RDF treated soil recorded the maximum NPK uptake as compared to the (T1) BSN.

3.4. Microbiological Parameters

An illustration of microbiological analysis is given in Figure 11. The differences in treatments and variations in nutrigation practices led to a range of dehydrogenase (DHA) activity levels in the soil, ranging from 0.71 to 0.80 g TPF per gram of soil per hour. The treatments that incorporated BSN demonstrated higher DHA activity compared to the treatment with 100% RDF nutrigation, the soil’s DHA activity was recorded as 0.61 g TPF per gram of soil per hour (Table 4). Among the treatments, T7 (Control) exhibited the lowest DHA activity at 0.71, while all the treatments involving BSN demonstrated favorable dehydrogenase activity. In terms of phosphatase enzymes, alkaline phosphatase was found to be more dominant than acid phosphatase in the phosphatase community. The alkaline phosphatase activity ranged from 62.73 to 86.45 μg of p-nitro phenol (PNP) formed per gram of soil per hour, while the acid phosphatase activity ranged from 44.38 to 68.53 μg PNP formed per gram of soil per hour. Similar to the trend observed for DHA, both alkaline and acid phosphatase were higher in the BSN nutrigation treatments compared to the 100% RDF treatment. The application of BSN positively influenced alkaline phosphatase activity. The levels of soil microbial biomass carbon (SMBC) varied between 2.23 and 3.02 mg of CO2-C per kilogram of soil per day, while soil basal respiration ranged from 2.95 to 3.78 mg of CO2-C per kilogram of soil per day. T1 (BSN) exhibited higher SMBC levels with lower soil basal respiration and a low metabolic quotient, indicating shifts in microbial populations.

4. Discussion

Our results demonstrated the feasibility and efficacy of utilizing biogas slurry as a drip nutrigation, showcasing its significant impact on the growth parameters of spinach (var. Pusa All Green). Notably, when biogas slurry nutrigation was combined with inorganic nutrigation, pronounced effects on plant growth were observed. This is particularly significant as the use of digestate is increasingly recognized as an important strategy to meet environmental regulations and contribute to the sustainable development goals outlined in Agenda 2030 [28]. Plant productivity, including factors such as yield, plant height, leaf count, biomass, and nutrient absorption capacity, serves as a valuable indicator of the fertilization potential of soil amendments. Biogas digestate, being an environmentally friendly organic soil amendment, contains essential plant nutrients in substantial quantities, making it a valuable resource for promoting plant growth [29]. The nutrient concentration in biogas slurry used in our study was N with 78.25 mg/L, P with 45.23 mg/L, and K 39.65 with mg/L.
Our study further demonstrates that the integration of biogas slurry nutrigation with inorganic nutrigation significantly influenced plant height across different growth stages, from 30 days after sowing (DAS) until the final harvest (75 DAS). Initially, there was a noticeable increase in plant height during the later stages of the growth period. However, after the third harvest (Figure 3), a decline in height was observed, potentially due to the cessation of nutrigation at that point. Among the treatments incorporating biogas slurry nutrigation and different chemical fertilizer ratios, the spinach plants receiving 100% RDF (T5) displayed the greatest height. Following closely, the 80% RDF + BSN (T4) treatment resulted in relatively taller plants. This indicates that a higher proportion of inorganic fertilizer played a significant role in promoting increased plant height, while the addition of biogas slurry nutrigation as an organic fertilizer supplement also positively influenced plant growth. This could be attributed to the readily available nutrients present in inorganic fertilizers. A similar finding was reported in a study involving cabbage transplants after 90 days [30]. The cutting frequency also had a significant influence on plant height at the final harvest stage. In the fourth harvest, all the treatments resulted in significantly lower plant height compared to the third harvest. This could be due to the breakdown of apical dominance caused by the cutting, which stimulated the plants to produce more side shoots and leaves. Repeated cuttings can lead to the loss of photosynthates that would otherwise be used for plant growth [31]. Researchers have also documented evidence showing a decrease in plant height with an increased number of cuttings [32]. Organic materials have the characteristic of releasing plant nutrients gradually while also holding them for an extended period [33]. The number of leaves showed an incremental pattern with each subsequent cutting and exhibited a positive correlation with plant height (Figure 4). The maximum number of leaves recorded in T5 could be attributed to the quick availability of higher nutrient levels and growth substances throughout the entire crop growth period. Similar findings regarding increased leaf number in spinach were reported with organic manure and RDF application by Jha and Jana [34], as nitrogen plays a crucial role in promoting vigorous vegetative growth, which leads to an increase in the number of leaves per plant. The lower leaf count observed in the fourth harvest could be attributed to an expansion in leaf area, which potentially resulted in higher nutrient assimilation and consequently fewer leaves being produced due to enhanced availability of nutrients from readily soluble inorganic fertilizer compared to organic sources. Organic sources are considered slow-release fertilizer as mineralization has to occur for the nutrients, especially nitrogen, to become available [35]. The maximum leaf area was found to be (T5) 100% RDF followed by treatment (T4) 80% RDF + BSN on par with treatment (T3) 60% RDF + BSN (Figure 5). The presence of readily available nitrogen in the plots treated with integrated organic and inorganic nutrigation might have contributed to the higher leaf area observed in those plants. Treatment (T5) 100% RDF exhibited an increase in leaf area due to the sufficient supply of nitrogen and other nutrients, which likely stimulated greater metabolic activity within the leaves. The observed increase in leaf number in spinach could be ascribed to the synthesis of carbohydrates and phytohormones, resulting in a corresponding expansion of leaf area [36,37]. In a study assessing the effectiveness of cattle dung biogas digestate, it was observed that the application of inorganic fertilizer resulted in a greater increase in leaf area compared to the utilization of biogas digestate in spinach [38]. Roots play a vital role in providing stability and serving as a conduit for plants to absorb essential water and nutrients required for their survival and growth. The longer root length for (T5) 100% RDF followed by (T4) 80% RDF + BSN was observed. Treatments (T3) 60% RDF + BSN, (T4) 80% RDF + BSN, and (T5) 100% RDF had relatively higher BM/RL ratios compared to other treatments, suggesting that these treatments were more efficient in producing biomass in relation to root length. Treatments (T1) BSN, (T2) 40% RDF + BSN, (T6) SB, and (T7) Control had relatively lower BM/RL ratios, indicating a lower biomass production in relation to root length compared to the other treatments. These results suggested that the application of higher RDF levels combined with BSN (biogas slurry nutrigation) (T3, T4, T5) leads to increased biomass production relative to root length compared to other treatments. An increase in the ratio of shoot biomass to the plant root length indicates a higher level of nutrient absorption and enhanced stress tolerance in plants [39]. In order to help crops cope with abiotic stresses such as water stress during different stages of plant development, the application of soil amendments, including organic fertilizers, that facilitate proper growth and development of root systems is essential [40]. The variations in yield among the treatments could be attributed to the positive influence of vegetative growth. Parameters such as the number of leaves, leaf area, and plant height exhibited significant increases, which had a direct impact on the overall yield. These results Indicate that the improved vegetative growth characteristics played a crucial role in achieving higher yields in specific treatments.
The maximum nitrogen uptake in the 80% RDF + BSN (T4) treatment might be attributed to the synergistic effects of combined nutrient availability and organic soil enhancement. The findings suggest that the choice of treatment significantly influences N uptake in the study. Plants that were fertilized with combined inorganic nutrient sources demonstrated superior nutrient uptake in terms of nitrogen (N), phosphorus (P), and potassium (K) (Figure 9). However, this can be attributed to the combined effect of increased accumulation of dry matter and nutrient content, resulting from enhanced nutrient availability through chemical fertilizers. Similar findings were reported in amaranthus [41] and in spinach [42], where increased nutrient uptake was observed with the application of chemical fertilizers. Significant differences were observed in N and P uptake, with higher rates of inorganic fertilizer combined with slurry nutrigation resulting in increased N, P, and K uptake. The findings regarding macronutrient (NPK) leaf accumulation in the present study were in alignment with those reported for maize [43]; in their study, the application of biogas digestate significantly increased the accumulation of all macronutrients in maize plants. These soil amendments play a crucial role in promoting favorable biomass growth and distribution in crops, ultimately leading to improved overall growth. Consequently, the utilization of such soil amendments, particularly organic fertilizers, would contribute to the promotion of optimal biomass growth and its effective distribution throughout crops.
The soil pH exhibited a slight shift towards the neutral to higher range. This shift is crucial as it plays a vital role in the adsorption of plant nutrients, leading to a reduction in leaching. The soil treated with inorganic nutrigation (T5) at 100% RDF demonstrated the highest uptake of nitrogen, phosphorus, and potassium (NPK) compared to the soil treated with BSN (T1) alone. Earlier studies have revealed that inorganic fertilizer leachates had considerably higher nitrogen concentrations compared to biogas digestate derived from maize and pig manure [44]. It is anticipated that the leachate from the inorganic fertilizer treatment would exhibit the highest nitrogen levels since inorganic fertilizers typically contain higher nutrient contents that are readily available, unlike the biogas digestate and control treatments [45].
Soil microbial enzymes are indicative of the biological dynamics within the soil. When extraneous substances are introduced to the soil, these enzymatic activities are expected to undergo modifications. The addition of biogas slurry at varying proportions aligns with the inherent microbial activities, as evidenced by the quantification of dehydrogenase and phosphatase actions. Normally, alkaline phosphatase exhibits higher activity compared to acidic phosphatase in such soil conditions. However, the incorporation of biogas slurry product has equalized the enzymatic activity of both alkaline and acidic phosphatase, thereby establishing a balanced enzymatic profile.

5. Conclusions

Incorporating biogas slurry into drip irrigation systems offers the feasibility of applying substantial nitrogen amounts throughout the crop season, leading to significant improvements in the morphological growth response of spinach. This approach has the potential to enhance nitrogen use efficiency and reduce the need for synthetic nitrogen application, providing a cost-effective, innovative solution to the farmers while reducing dependency on chemical fertilizers. The abundant presence of essential plant nutrients in biogas slurry makes it a valuable resource for promoting plant growth. Furthermore, the incorporation of biogas slurry nutrigation with chemical fertilizers enhances various growth parameters such as plant height, leaf count, biomass, and nutrient absorption capacity. The balanced enzymatic profile observed with biogas slurry nutrigation suggests improved soil microbial dynamics. Drip irrigation systems, when properly managed and with appropriate filtration to match nozzle characteristics, are suitable for biogas slurry nutrigation. Further research is recommended to validate the results, assess any potential long-term effects on drip system clogging and soil, and ensure the sustainable implementation of biogas slurry nutrigation. Overall, the utilization of biogas slurry nutrigation as a soil amendment holds promise for sustainable agriculture, contributing to a closed-loop economy and aligning with the goals of sustainable development.

Author Contributions

Conceptualization—S.S.; execution—R.Y., S.S., A.D., K.L. and M.K.; investigation—R.Y. and S.S.; methodology—S.S., M.K., K.L., A.D., R.Y., A.S. (Archna Suman) and A.S. (Anupama Singh); validation—R.Y., S.S., K.L., A.D., A.S. (Archna Suman), M.K., K.B. and H.L.K.; formal analysis—R.Y., A.D., M.K., A.S. (Archna Suman) and A.S. (Anupama Singh); resources—K.L., M.K., K.B., H.L.K., A.S. (Archna Suman), A.S. (Anupama Singh), R.K.S. and R.S.; data curation—R.Y.; writing—R.Y., S.S. and A.D.; review and editing—S.S., A.D., K.L., M.K., H.L.K., K.B., R.K.S. and R.S.; visualization—S.S.; supervision—S.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data will be available once the PhD thesis is submitted.

Acknowledgments

The authors are grateful to The Director, ICAR–IARI, and New Delhi for providing fellowship to R.Y. and Project Director, Water Technology Center, ICAR–IARI, New Delhi for providing required facilities during the research program.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Temperature and rainfall during the cropping period (28 September to 15 December).
Figure 1. Temperature and rainfall during the cropping period (28 September to 15 December).
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Figure 2. Relative humidity, sunshine hours, evaporation, and average wind speed during the cropping period (28 September to 15 December).
Figure 2. Relative humidity, sunshine hours, evaporation, and average wind speed during the cropping period (28 September to 15 December).
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Figure 3. Effect of spinach plant height under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80% RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
Figure 3. Effect of spinach plant height under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80% RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
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Figure 4. Effect of spinach plant leaf count under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80%RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
Figure 4. Effect of spinach plant leaf count under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80%RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
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Figure 5. Effect of spinach plant leaf area under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80% RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
Figure 5. Effect of spinach plant leaf area under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80% RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
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Figure 6. Effect of spinach BM/RL ratio under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80%RDF + BSN, (T5) 100%RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
Figure 6. Effect of spinach BM/RL ratio under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80%RDF + BSN, (T5) 100%RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
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Figure 7. Effect of spinach plant root length under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80%RDF + BSN, (T5) 100%RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
Figure 7. Effect of spinach plant root length under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80%RDF + BSN, (T5) 100%RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
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Figure 8. Effect of spinach yield under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80%RDF + BSN, (T5) 100%RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
Figure 8. Effect of spinach yield under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80%RDF + BSN, (T5) 100%RDF, (T6) SB, and (T7) Control. (DAS: days after sowing).
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Figure 9. Effect of spinach macro nutrient uptake under different fertilization methods (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80% RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control.
Figure 9. Effect of spinach macro nutrient uptake under different fertilization methods (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80% RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control.
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Figure 10. Effect of soil nutrient uptake under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80% RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control. (The similarity of the letters indicates no significant difference).
Figure 10. Effect of soil nutrient uptake under different fertilization methods: (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80% RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control. (The similarity of the letters indicates no significant difference).
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Figure 11. Effect of soil microbial biomass carbon (mg CO2-C kg−1 day−1), soil basal respiration (mg CO2-C kg−1 day−1), and q CO2 (mg CO2-C mg−1 microbial biomass C) under different treatments (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80% RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control. (The similarity of the letters indicates no significant difference).
Figure 11. Effect of soil microbial biomass carbon (mg CO2-C kg−1 day−1), soil basal respiration (mg CO2-C kg−1 day−1), and q CO2 (mg CO2-C mg−1 microbial biomass C) under different treatments (T1) BSN, (T2) 40% RDF + BSN, (T3) 60% RDF + BSN, (T4) 80% RDF + BSN, (T5) 100% RDF, (T6) SB, and (T7) Control. (The similarity of the letters indicates no significant difference).
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Table 1. Physico-chemical composition of biogas slurry before and after filtration.
Table 1. Physico-chemical composition of biogas slurry before and after filtration.
Parameters Before FiltrationAfter Filtration
EC (ds/cm)1.841.46
pH8.527.62
Turbidity (NTU)639199
Total Nitrogen (mg/L)22,30378.25
Total Phosphorous (mg/L)983445.23
Total Potassium (mg/L)289939.65
Table 2. Physico-chemical composition of soil before the experiment.
Table 2. Physico-chemical composition of soil before the experiment.
Soil ParametersValues
Physical properties
Particle size distribution
Sand (%)71
Silt (%)14
Clay (%)15
Textural classSandy Loam
Bulk density (g/cc)1.52
Mean Weight Diameter0.98
Geometric Weight Diameter0.54
Chemical properties
Macro nutrients
Available Nitrogen (kg/ha)125.7
Available Phosphorous (kg/ha)26.5
Available Potassium (kg/ha)281.4
Organic Carbon (%)0.43
pH7.21
EC0.29
Table 3. Effect of different fertilization methods and harvesting days on the growth parameters of spinach.
Table 3. Effect of different fertilization methods and harvesting days on the growth parameters of spinach.
TreatmentPlant Height (cm)Leaf CountLeaf Area (cm2)
BSN22.6813.7546.00
40% RDF + BSN24.1314.5048.47
60% RDF + BSN24.7416.3550.91
80 RDF + BSN25.4315.7545.10
100% RDF26.0617.0054.84
Slurry Broadcasting20.609.0034.82
Control19.367.7535.72
C.D. (p = 0.05)1.3440.6131.885
S.E (m) ±1.0170.1970.605
Harvesting days
30 DAS20.8911.1418.64
45 DAS23.4013.0035.02
60 DAS24.3215.7179.70
75 DAS23.9513.9047.13
C.D. (p = 0.05)1.3830.3072.504
S.E (m) ±0.3920.0870.428
Table 4. Effect of different treatment on soil dehydrogenase activity (μg TPF formed g−1 soil h−1), alkaline phosphatase (μg PNP formed g−1 soil h−1) and acid phosphatase (μg PNP formed g−1 soil h−1).
Table 4. Effect of different treatment on soil dehydrogenase activity (μg TPF formed g−1 soil h−1), alkaline phosphatase (μg PNP formed g−1 soil h−1) and acid phosphatase (μg PNP formed g−1 soil h−1).
TreatmentMean Dehydrogenase (μg TPF Formed g −1 Soil h−1)Mean Alkaline Phosphatase (μg PNP Formed g−1 Soil h−1)Mean Acid Phosphatase (μg PNP Formed g−1 Soil h−1)
T1 (BSN)0.78068.5386.45
T2 (40% RDF + BSN)0.76058.6879.85
T3(60% RDF + BSN)0.79860.1877.35
T4(80% RDF + BSN)0.80757.8779.69
T5(100% RDF)0.61057.2278.71
T6 (Slurry Broadcasting)0.74752.7370.61
T7 (Control)0.71244.3862.73
C. D (p = 0.05)0.0342.7364.229
S.E (m)±0.0110.8781.357
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Yadav, R.; Sudhishri, S.; Khanna, M.; Lal, K.; Dass, A.; Kushwaha, H.L.; Bandyopadhyay, K.; Suman, A.; Singh, A.; Singh, R.K.; et al. A Greener Approach to Spinach Farming: Drip Nutrigation with Biogas Slurry Digestate. Agronomy 2024, 14, 681. https://doi.org/10.3390/agronomy14040681

AMA Style

Yadav R, Sudhishri S, Khanna M, Lal K, Dass A, Kushwaha HL, Bandyopadhyay K, Suman A, Singh A, Singh RK, et al. A Greener Approach to Spinach Farming: Drip Nutrigation with Biogas Slurry Digestate. Agronomy. 2024; 14(4):681. https://doi.org/10.3390/agronomy14040681

Chicago/Turabian Style

Yadav, Rashmi, Susama Sudhishri, Manoj Khanna, Khajanchi Lal, Anchal Dass, Hari Lal Kushwaha, Kalikinkar Bandyopadhyay, Archna Suman, Anupama Singh, Rajiv Kumar Singh, and et al. 2024. "A Greener Approach to Spinach Farming: Drip Nutrigation with Biogas Slurry Digestate" Agronomy 14, no. 4: 681. https://doi.org/10.3390/agronomy14040681

APA Style

Yadav, R., Sudhishri, S., Khanna, M., Lal, K., Dass, A., Kushwaha, H. L., Bandyopadhyay, K., Suman, A., Singh, A., Singh, R. K., & Singh, R. (2024). A Greener Approach to Spinach Farming: Drip Nutrigation with Biogas Slurry Digestate. Agronomy, 14(4), 681. https://doi.org/10.3390/agronomy14040681

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