Fabrication and Comparative Evaluation of PVA-Ninhydrin and PVA-Nano ZnO- Ninhydrin Composite Films as Colourimetric Sensors for Soil Fertility Assessment
Abstract
The fabrication of a rapid, simple and cost-effective method for soil fertility assessment is crucial for sustainable agricultural practices. Since nitrogen is an essential nutrient for soil fertility, its detection in soil helps to quickly identify the quality and fertility of the soil. In this study, two different composite films comprising polyvinyl alcohol (PVA)–ninhydrin and PVA–nano zinc oxide (ZnO)–ninhydrin were prepared and evaluated for their soil fertility sensing ability. Both films were prepared by the solution-casting method and dried under sunlight. The fabricated films were well characterized and investigated for thermal stability, mechanical properties, and antibacterial activities. The simple and rapid method of soil fertility testing was carried out with different soil samples. Thermogravimetric analysis revealed that ZnO incorporation enhanced the thermal stability of the composite films by delaying thermal degradation and increasing the residual mass. Mechanical studies proved enhanced flexibility and durability of the ZnO-containing film compared to the PVA–ninhydrin film. Similarly, the nano ZnO-containing film exhibited enhanced antibacterial activity, attributed to the intrinsic antimicrobial properties of ZnO nanoparticles.
The soil fertility sensing ability of the films was tested using four different soil samples: chicken manure-amended soil, cow dung-amended soil, black soil, and normal soil. The rate and intensity of colour development followed the order: chicken manure-amended soil > cow dung-amended soil > black soil > normal soil, reflecting their fertility order. The PVA–ninhydrin film exhibited a faster colour response, whereas ZnO incorporation slightly delayed colour formation, suggesting that ZnO influences reactant diffusion and the reaction kinetics within the polymer matrix.
Altogether, both films exhibited excellent potential as simple, portable, and low-cost colourimetric sensors for the rapid qualitative detection of soil fertility. Although the PVA–ninhydrin film provided a faster sensing response, the PVA–nano ZnO–ninhydrin composite film offered superior thermal stability, mechanical robustness, and antibacterial performance, making it a promising candidate for practical agricultural sensing applications.
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Introduction
Agricultural productivity and sustainable crop production are necessarily related to soil fertility. It determines the availability of essential nutrients required for plant growth and development (Khan et al., 2024). Among the primary macronutrients present in soil, nitrogen is predominantly important as it is a major constituent of amino acids, proteins, nucleic acids, enzymes, and chlorophyll, thereby playing a vital role in plant metabolism, growth, and crop yield. Consequently, rapid and reliable assessment of soil fertility in connection with nitrogen status is essential for efficient nutrient management and sustainable agricultural practices (Ali et al., 2025).
A significant source of soil nitrogen is organic matter such as cow dung, chicken manure, etc. Nitrogen produced through the decomposition of such organic matter makes soil fertile. The nitrogen present in the soil as amino acids can be qualitatively detected and provide information on soil fertility status. Conventional methods for soil fertility assessment, including Kjeldahl analysis, elemental analysis, and spectroscopic techniques, provide accurate results but often require sophisticated instrumentation, extensive sample preparation, skilled personnel, and laboratory-based analysis. These limitations highlight the need for simple, rapid, portable, and cost-effective sensing methods suitable for on-site applications.
The organic molecule ninhydrin (1,2,3-indanetrione hydrate) is one of the most widely used analytical reagents for detecting amino acids and other primary amine-containing compounds. It reacts with free amino groups through oxidative deamination and decarboxylation to produce the characteristic deep purple known as Ruhemann's purple. These characteristics make ninhydrin an attractive reagent for the development of colourimetric sensors for nitrogen-containing compounds. The intensity and rate of colour formation depend on the concentration of reactive amino groups. Due to this, ninhydrin reactions have been extensively employed in analytical chemistry, biochemistry, forensic science, chromatography, and protein analysis (Gelaye, 2024; Stauß et al., 2024).
Conclusion
In this study, PVA–ninhydrin and PVA–nano ZnO–ninhydrin composite films were successfully fabricated by the solution-casting method and evaluated as colourimetric sensing materials for qualitative soil fertility assessment. Structural, morphological, thermal, mechanical, optical, and antibacterial characterizations confirmed the successful incorporation of ZnO nanoparticles into the PVA–ninhydrin matrix. Compared with the PVA–ninhydrin film, the PVA–nano ZnO–ninhydrin composite film exhibited enhanced thermal stability, improved tensile strength and stiffness, and superior antibacterial activity, demonstrating the reinforcing effect of ZnO nanoparticles. Both films effectively detected nitrogen-containing compounds in soils through the ninhydrin colorimetric reaction, with the rate of colour development following the order: chicken manure-amended soil > cow dung-amended soil > black soil > normal soil, consistent with their relative fertility levels. Although the incorporation of ZnO nanoparticles slightly delayed the colorimetric response, the composite film retained excellent sensing capability while offering improved physicochemical properties and durability. Overall, the developed PVA–nano ZnO–ninhydrin composite film represents a simple, low-cost, environmentally friendly, and promising material for qualitative soil fertility assessment, with potential applications in sustainable agriculture and environmental monitoring. Future studies should focus on quantitative evaluation of soil nitrogen, long-term storage stability, and field validation under diverse agricultural conditions.
References
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