Evaluation of biogas slurry along with vermicompost as a primary nutrient source for okra (Abelmoschus esculentus) in the arid climate of North Gujarat

Introduction

Biogas slurry is a by-product of biogas production, which is generated from anaerobic digestion of animal waste and crop residue. It is generally considered a waste material. However, its application can improve soil health, crop yield, and profitability of the farmers as it is a rich source of organic matter and plant nutrients. Utilizing biogas slurry and vermicompost instead of chemical fertilizers is able to significantly mitigate these issues. We established fertilization treatments to investigate the impact of biogas slurry substitution okra growth and soil properties in field experiments with varying proportions of biogas slurry and vermicompost. Utilizing small amounts of biogas slurry and vermicompost as a replacement for chemical fertilizers was able to maintain soil nutrient levels while increasing okra yield (from 140 kg/500 sqm² to 250 kg/500sqm²), stabilize urease and catalase activities, and improve microbial communities. Additionally, using a minor amount of biogas slurry and vermicompost as a substitute for chemical fertilizer resulted in an increase in the population of potentially beneficial bacteria. Structural equation modelling showed that the replacement of chemical fertilizers by biogas slurry and vermicompost directly and indirectly increased crop yield by altering soil characteristics and the microbial community. These results improve our comprehension of the impact of various biogas slurry and vermicompost substitutes and hold significant theoretical importance for advancing the utilization of biogas slurry in the future.

Materials and Methodology

The experimental site was situated in the Ranpur village, Danta block, Banaskantha. The region features an arid climate, with temperature of 38 • C to 48 • C and total annual precipitation of 400 mm recorded in 2025, respectively. The soil is classified as red stony soil. The initial chemical properties of the soil were recorded as follows: pH at 7.57, EC 0.39 (dS/m), soil organic carbon (OC) at 0.42 g/kg, available nitrogen (AN) at 175.53 kg/ha, available phosphorus (AP) at 77.53 kg/ha, and available potassium (AK) at 348 kg/ha. The biogas slurry (moisture content:96% -98%) was subjected to preliminary filtration using a horizontal spiral centrifuge (filtration precision: 370μm) and secondary filtration through a filter concentrator centrifuge (filtration precision: 150μm). The filtered biogas slurry has an organic carbon content of about 4.31g/L, total nitrogen (ammonium nitrogen) of approximately 3.5g/L, total phosphorus of about 91 mg/L, total potassium of about 3.07 g/L, chemical oxygen demand of about 36 g/L, pH of approximately 8.7, and EC of around 13.7 mS/cm.

Experimental design

The experiments used locally recommended okra varieties and were conducted on June 11, 2025, and August 14, 2025. Each experimental plot measured 1010 m 2 and was sown using the flat seeding method, with equal row spacing. The okra rows were spaced 60 cm apart with a plant spacing of 30 cm. Six treatment groups were established, including a control group, with each treatment comprising three replicates. All treatments used a combined base and topdressing fertilization method. For base fertilization, compound fertilizer (N-P 2 O 5 -K 2 O) was applied at a rate of 100-50-50 kg/ha for each treatment. The experiments were designed based on an equivalent nitrogen application rate of 100 kg/ha. Additional topdressings were applied according to different ratios of BS and vermicompost substitution for chemical nitrogen, including a mixture of water-BS and urea. The application of total nitrogen was split into two stages, representing 40% and 60% of the total nitrogen application, respectively. The specific topdressing treatments were as follows:

1. No topdressing and irrigation 2. 100% vermicompost 3. 75% vermicompost + 25% BS 4. 50% vermicompost + 50% BS 5. 25% vermicompost + 75% BS 6. 100% BS

Topdressing system operation

For our 0.05 ha okra plot apply 500-750 liters of biogas slurry diluted with irrigation water at a 1:3 ratio during the vegetative stage. Deliver the mixture through furrows in the early morning to prevent nutrient evaporation and ensure deep root penetration in the red stony soil. This top-dressing provides essential nitrogen and organic matter, which improves moisture retention and protects the plants from the intense heat. Plants were harvested at maturity stage and following parameters were studied number of fruits, consistent shape, size and weight.

Results and Discussion

Impact on Growth and Yield Attributes

The experimental data reveals a clear positive correlation between the proportion of biogas slurry and vermicompost used and the physical development of the okra plants. The height of the okra plants increased progressively with higher BS substitution rates. The tallest plants were recorded in the 100% BS (T 6 ) treatment at 95.12 cm, compared to only 80.02 cm in the control group (T 1 ) and Fruit weight Individual fruit weight showed a similar upward trend. The 100% BS treatment yielded the heaviest fruits at 8.2 gm, which is nearly double the weight of the control group fruits (5.1 gm). The number of fruits per plant varied from 12.30 to 18.00, with a mean of 15.81 fruits per plant. The CV (12.97%) reflects moderate variability among treatments. Table 2 shows that different ways of managing nutrients affected the length, diameter and yield of okra fruits. The treatment with no fertilizers or irrigation (T₁) had the worst results, with the shortest fruit length of 9.4 cm the smallest diameter of 1.18 cm and the lowest yield of 4.55 mt/ha. Using vermicompost alone (T₂) was better than the control treatment. When vermicompost and BS were used together the fruit length, diameter and yield all increased. The more BS used the better the results were. The highest yield of 8.45 mt/ha was seen with T₆, which used 100% BS. This was closely followed by T₄, which used a mix of 50% vermicompost and 50% BS giving a yield of 8.10 mt/ha. The longest fruits (12.51 cm) and the largest diameter (1.42 cm) were found with T₄ showing that a balanced mix of vermicompost and BS is good for making okra fruits bigger. Overall using BS with vermicompost worked better, than using vermicompost alone and this helped okra plants grow and produce more yield. Table 3 shows what happens to the soil when we use combinations of biogas slurry and vermicompost. We looked at how these combinations affect the chemical properties of the soil like the soil pH, electrical conductivity and soil organic carbon. The soil pH went up a bit from 7.57 in the control to 7.84 when we used 100% biogas slurry. This means that using biogas slurry made the soil more alkaline, but not by much. The electrical conductivity was between 0.35 and 0.45 dS/m. It was highest when we used 100% biogas slurry. This shows that using biogas slurry increased the amount of soluble salts in the soil but it was still safe. Soil organic carbon is where we saw an improvement. It went up from 0.42% in the control to 0.92% when we used 100% biogas slurry. This clearly shows that biogas slurry is good for the soil and helps increase the amount of carbon in it.

The numbers we got from our tests show that using biogas slurry and vermicompost had an effect on the soil organic carbon and this effect was statistically meaningful. Overall Table 3 shows that using biogas slurry either on its own or with vermicompost made the soil healthier. It particularly helped increase the soil carbon, which is good for the soil and helps us manage it in a sustainable way. Biogas slurry is beneficial, for the soil. It helps make the soil more fertile.

Conclusion

Among all the treatments, T 6: 100% BS (100%BS) consistently demonstrated superior results. These findings underscore the potential of biogas slurry in enhancing okra growth, soil health, and nutrient availability in agricultural systems.

References

1. Abd El-Kader AA, Shaaban SM, Abd El-Fattah MS. Effect of irrigation levels and organic compost on okra plants (Abelmoschus esculentus L.) grown in sandy calcareous soil. Agriculture and Biology Journal of North America. 2010;1(3):225-231.
2. Abraham ER, Ramachandran S, Ramalingam V. Biogas: can it be an important source of energy? Environ Sci Pollut Res Int. 2007;14(1):67-71.
3. Akanbi WB, Togun AO, Adeniran JA, Olaniyan AB. Effects of split application of organomineral fertilizer on okra growth, nutrient uptake and fruit yield. Niger J Hortic Sci. 2004;9:102-109.
4. Akubuenyi FC, Osim SE, Udo IA. Comparative impact of bio-slurry (biofertilizer) produced from anaerobic digestion of domestic waste and inorganic fertilizer on the growth and yield of okra [Abelmoschus esculentus (L.) Moench]. Int J Plant Soil Sci. 2025;37(3):177-184.
5. Chattoo MA, Ahmad N, Khan SH, Sidique SH, Hussain K. Residual effect of organic manures and inorganic fertilizers on succeeding crop pea (Pisum sativum L.) cv. Bonnevilla. Asian J Hortic. 2010;4:299-304.
6. Chowdhury FJ, Kibria MG, Alamgir M, Islam M. Effects of biogas plant residues and NPK fertilizers on growth and nutritional quality of amaranth (Amaranthus tricolor L.). Int J Bio-resour Stress Manag. 2017;8(1).
7. Eke KA, Essiens BA, Ogbu JU. Determination of optimum planting time of okra in the derived savannah. In: Proceedings of the 42 nd Conference of the Agricultural Society of Nigeria; 2008. p. 242-245.
8. Ghimire K, Gautam DM, Mishra K, Pande KR, GC A. Influence of biogas slurry and urea on yield and quality of okra (Abelmoschus esculentus L.) fruits. J Agric Environ. 2015;16:161-169.
9. Gurung JB. Review of literature on effect of slurry use on crop production. Biogas Support Programme. 1997. p. 102.
10. Islam MM. Comparative effects of biogas plant residues, poultry manure and inorganic fertilizer on growth and yield of ladies finger. IOSR J Agric Vet Sci. 2014:29-33.
11. Islam MR, Rahman SME, Rahman MM. The effects of biogas slurry on the production and quality of maize fodder. Turk J Agric For. 2009;34:91-99.
12. Karki KB. Estimation of plant nutrient loss from biogas slurry. Biogas Support Programme. 1997.
13. Kujur R, Khan HI, Lal EP, Verma LK. Impact of biofertilizers and different doses of NPK on growth and photosynthetic pigments of okra plant (Abelmoschus esculentus L. Moench). J Plant Sci Phytopathol. 2023;7:9296.
14. Maqbool S, Ul Hassan A, Javed Akhtar M, Tahir M. Integrated use of biogas slurry and chemical fertilizer to improve growth and yield of okra. Sci Lett. 2014;2(1):56-59.
15. Mikled CS, Jiraporncharoen S, Potikanond N. Fermented slurry as fertilizer for the production of forage crops. ThaiGerman Biogas Programme Application. 1994;8(3):297-306.
16. Muhmood A, Majeed A, Niaz A, Javid S, Shah SSH, Shah AH. Nutrient uptake and the yield of okra and carrot in response to biogas slurry and inorganic N fertilizers. Int J Plant Soil Sci. 2015;7(5):297-305.
17. Nasir A, Khan FH, Riaz M, Khan MA. Comparative study of biogas slurry with farmyard manure as fertilizer on maize crop. Sci Int (Lahore). 2010;22(4):297-301.
18. Parvathy S, Vaishnavy SS. Effect of biogas slurry on growth and biomass production of Sorghum vulgare and Oryza sativa. J Ecobiol. 2009;24:335-338.
19. Rautaray SK, Ghosh BC, Mitra BN. Effect of fly ash, organic wastes and chemical fertilizers on yield, nutrient uptake, heavy metal content and residual fertility in a rice-mustard cropping sequence under acid lateritic soils. Bioresour Technol. 2003;90:275-283.
20. Rekhi RS, Benbi DK, Singh B. Effect of fertilizers and organic manures on crop yields and soil properties in ricewheat cropping system. Rice-Wheat Consortium Paper Ser. 2000;6:1-6.
21. Shahbaz M, Akhtar MJ, Ahmed W, Wakeel A. Integrated effect of different N-fertilizer rates and bioslurry application on growth and N-use efficiency of okra (Hibiscus esculentus L.). Turk J Agric For. 2014;38(3):311-319.
22. Stone DM, Elioff JD. Soil properties and aspen development five years after compaction and forest floor removal. Can J Soil Sci. 1998;78(1):51-58.
23. Tiamiya RA, Ahmed HG, Muhammad AS. Effect of sources of organic manure on growth and yields of okra (Abelmoschus esculentus L.) in Sokoto, Nigeria. Niger J Basic Appl Sci. 2012;20:213-216.
24. Warnars L, Oppenoorth H. A study on bioslurry results and uses. 2014. p. 20.