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Characterization of Controlled Release Fertilizer by Infrared Microspectroscopy

ABSTRACT The feasibility and potential of infrared microspectroscopy for quantitatively characterizing the dynamic changes in the coating of controlled release fertilizer granules are discussed to better characterize the release of nutrients. Two polymer-coated compound fertilizers were selected as typical samples. A frozen section technique was used for sample pretreatment, where freeze-dried controlled release fertilizer granules were cut into 20-µm-thick sections. Optimal parameters for the spectral range, spectral resolution, and pixel dimensions in the transmission spectra were determined to be 4000–750 cm−1, 16 cm−1, and 6.25 × 6.25 µm, respectively. A mean spectrum of four scans was used in data processing for second-derivative analysis with nine smoothing points. This work is aimed to quantitatively distinguish between the coating and fertilizer core according to radial variations in second-derivative spectra using characteristic wavelengths that represent the coating and fertilizer core. During nutrient release, the fertilizer core gathered gradually and adhered in uniform distribution around the coating, while some nutrients were trapped in micropores. The coating thickness of the controlled release fertilizers fluctuated within 37.5––55 and 15–55 µm, respectively. Although measurement results based on infrared microspectroscopy were in accordance with the scanning electron microscopy observations, they were shown to be more accurate and were not influenced by the coating mixed with the fertilizer core. Furthermore, infrared microspectroscopy was effectively used to quantify the dynamic thickness of the coating of controlled release fertilizer granules and visually characterize fertilizer penetration through the coating.