Revealing channel controlled nutrient release mechanism of bio-oil polymer coated controlled-release fertilizer

• Nutrient release mechanism of bio-oil polymer coated controlled-release fertilizer was revealed. • Nutrient release rate was closely related to the channel of the coated fertilizer. • Large channels played a decisive role for nutrient release. • Moisture, temperature, and coating technology affected the nutrient release rate. • The dosage of bio-oil polymer affected the release channel and longevity of the controlled-release fertilizer. Nutrient channel release mechanism of bio-oil polymer coated controlled-release fertilizer (BCRF) was revealed. The bio-oil polymer was developed by co-polymerization of castor oil and epoxy soybean oil for the preparation the coating layer of the BCRF. Scanning electron microscopy (SEM) technology was introduced to investigate the channels in the coating layer of the BCRF and their effect on the nutrient release. Three types of channels affected the nutrient release rate of BCRF. The longevity of the BCRF increased due to decreasing nutrient release channels as the dosage of the bio-oil polymer increased. The number, length and tortuosity of the channels were decisive for the nutrient release rate of BCRF, while moisture, temperature and coating technology were sufficient factors. Higher the ratio of bio-oil polymer resulted in an increasing number and the cross-sectional area of channels in the coating layer. The dosage and ratio of the bio-oil polymer had no effect on the surface structure of the BCRF before culture. The small channel was 48.9 times that of the large channels, but cross-section area of the large channels was 79.9 times that of small channels. The large channels played a decisive role in controlling nutrient release rate of BCRF. The quantitative information of BCRF nutrient release rate depended on adjusting the amount of coating material, the ratio of bio-oil polymer in the material, coating technology and channel-forming agents. When the ratio of bio-oil polymer to polyisocyanate was 5:4–5:5 and the amount of coating material was at least 3 % of the weight of the core fertilizer, the BCRF exhibited excellent controlled release performance. The experiment will help the production of environmentally friendly controlled-release fertilizers through the application of bio-oils to adjust nutrient release performance and reduce fertilizer pollution in agricultural production.