Fabrication and Comparative Evaluation of PVA-Ninhydrin and PVA-Nano ZnO-Ninhydrin Composite Films as Colourimetric Sensors for Soil Fertility Assessment

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.