Effect of Triple Superphosphate Integration with Leonardite on Some Chemical Characteristics and Microbial Population of an Alkaline Sandy Loam Soil and Concentrations of Nitrogen, Phosphorus, and Potassium in Leaves of Nigella sativa

Document Type : Research Article

Authors

1 Department of Plant Ecophysiology, Faculty of Agriculture, University of Tabriz, Tabriz, Iran.

2 Department of Soil and Water Research, Agricultural and Natural Resources Research and Education Center of East Azerbaijan Province, Agricultural Research, Education, and Extension Organization (AREEO), Tabriz, Iran.

10.22034/sps.2026.70938.1029

Abstract

Background and Objectives
Improvement of soil productivity and nutrient use efficiency in medicinal plants, especially in light-textured soils, requires integrated nutrient management strategies. Light soils, such as sandy loam, often have low nutrient retention capacity, high nitrogen and potassium leaching potential, and limited microbial activity, leading to reduced soil fertility and plant nutrient uptake. Humic substances, such as leonardite, are recognized for their ability to improve soil chemical and biological properties, enhance microbial activity, and stabilize nutrients. Phosphate fertilizers, particularly triple superphosphate, provide essential phosphorus for plant growth, but their efficiency is often limited by soil fixation and low mobility in light soils. This study aimed to evaluate the effects of leonardite (rich in humic acids) and triple superphosphate on soil chemical and biological properties (total N, available- P and K, organic matter, electrical conductivity, and microbial populations) and nutrient uptake in Nigella sativa L. grown in a sandy-loam soil under rainfed conditions.
 
Materials and Methods
The experiment was conducted in the Parchin region, Germi County, Ardabil province, Iran (39°08'47" N, 48°23'60" E). The field trial was established in a randomized complete blocks design with 9 treatments and 3 replications. Treatments included: 1- Control (no fertilizer), 2- Triple superphosphate 50 kg ha⁻¹ (P50), 3- Triple superphosphate 100 kg ha⁻¹ (P100), 4- Leonardite 100 kg ha⁻¹ (H100), 5- Leonardite 200 kg ha⁻¹ (H200), 6- P50 + H100, 7- P25 + H150, 8- P75 + H50, and 9- P25 + H50. Each experimental plot consisted of 2 m×2 m, with 25 cm row spacing, 10 cm plant spacing, and a density of 40 plants per m². The soil preparation was done with a chisel plow, and all fertilizers were applied at planting. The trial was conducted under rainfed conditions without irrigation. The soil samples were collected from 0–30 cm depth after harvest. The soil organic carbon was determined using the Walkley–Black method. Total nitrogen of soil and plant samples was measured by the Kjeldahl method. Available phosphorus was extracted by the Olsen method, and available potassium by ammonium acetate. Leaf samples were dried and analyzed for total nitrogen (Kjeldahl), phosphorus (molybdenum–vanadate colorimetry), and potassium (flame photometry). Microbial populations in the rhizosphere and non-rhizosphere soils were counted using standard plating techniques. Data were analyzed using SPSS software, means comparison was performed by Duncan’s multiple range test at p ≤ 0.05, and correlation coefficients among traits were also determined.
 
Results
The results of this study indicated that the combined application of leonardite and triple superphosphate significantly influenced the soil chemical and biological properties as well as nutrient uptake in Nigella sativa. The highest increase in soil organic matter was observed in the P25H150 (14.1 g/kg) and H200 (13.7 g/kg) treatments, representing 7% and 5% increases compared to the control (13.1 g/kg), respectively. The soil total nitrogen concentration was highest in P25H150 (0.83 g/kg), a 57% increase over the control (0.53 g/kg) and its value in H200 treatment was 0.77 g/kg (45% increase relative to the control). The soil available phosphorus reached its maximum (7 mg/kg) in P50H100 treatment, representing a 63% increase relative to the control (4.3 mg/kg). The effects of treatments were not significant on soil available potassium and electrical conductivity (EC). Rhizosphere and non-rhizosphere microbial populations were highest in P25H150 treatment and lowest in the control. Increases in microbial populations were positively and significantly correlated with soil organic matter and leaf nitrogen concentration. Leaf nitrogen concentration was highest (27.8 mg/g) in P25H150 treatment. Leaf phosphorus concentration (3.77 mg/g) and leaf potassium concentration (25.7 mg/g) peaked in P50H100. Correlation analysis revealed positive and significant relationships between soil organic matter and total soil nitrogen (r = 0.59**) as well as rhizosphere microbial population (r = 0.62**). Leaf nitrogen concentration was positively correlated with soil nitrogen (r = 0.50**), leaf phosphorus (r = 0.68**), and leaf potassium (r = 0.53**). Leaf potassium concentration and soil available phosphorus positively correlated with microbial populations in rhizosphere and non-rhizosphere. Also, leaf phosphorus concentration showed a strong positive correlation with soil available phosphorus (r = 0.75**).
 
Conclusion
Overall, integrated application of leonardite and triple superphosphate, particularly at 25 kg P + 150 kg leonardite/ha, significantly improved soil chemical and biological properties and enhanced macronutrient uptake in Nigella sativa plant in a sandy-loam soil under rainfed conditions. Also, leonardite alone at 200 kg/ha positively influenced soil fertility and leaf macronutrient concentrations. Enhanced nitrogen, phosphorus, and potassium concentrations in leaves, together with improved soil organic matter and microbial populations, demonstrated the effectiveness of the combined organic–mineral strategies in promoting nutrient stability and bioavailability. These findings highlight that optimized applications of leonardite and triple superphosphate can serve as an efficient approach for sustainable soil fertility management and improving productivity of medicinal plants, especially in low-organic matter soils and semi-arid regions.
Author Contributions
Conceptualization and methodology, M.A. & J.S.K.; Performing experiments and measurements, M.A.; formal analysis and data curation, M.A. & J.S.K.; writing-original draft preparation, M.A., writing- review and editing, M.A. & J.S.K.; supervision, J.S.K.; project administration, J.S.K.; All authors have read and agreed to the published version of the manuscript.
Data Availability Statement
Data is available on reasonable request from the authors.
Acknowledgements
This paper is published as a part of a Ph.D. thesis supported by the Vice Chancellor for Research and Technology of the University of Tabriz, Tabriz, Iran. The authors are thankful to the University of Tabriz for financial supports.
Conflict of interest
The authors declare no conflict of interest.
Ethical considerations
The authors avoided data fabrication, falsification, plagiarism, and misconduct.

Keywords

Main Subjects

References

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Journal of Soil and Plant Science
Volume 36, Issue 1
December 2026
Pages 19-41

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