Variable-Rate Fertilization in a Custard Apple (Annona squamosa L.) Orchard Based On the Spatial Variability of Soil Nutrients

Due to the soil nutrient distribution caused by complex mountain topography, it is difficult to achieve the precise fertilization. This study investigated variable-rate fertilization based on the spatial variability of soil nutrients in a custard apple orchard. Soil grid sampling was used to determine the variability of soil nutrients in a 1656‑m² orchard. The spatial variability of soil nutrients showed a moderate to high spatial dependency and variable-rate fertilization decision-making was made based on the nutrient distribution. The variable-rate fertilizer applicator controlled by code was assembled with a small self-propelled fertilizer applicator as the carrier to test its fertilizing effect in the field. Based on the results, variable-rate fertilization reduced nitrogen, phosphorus, and potassium fertilization by 42%, 46%, and 19%, respectively, compared with traditional fertilization, and custard apple yield increased by 22 and 34% in 2022 and 2023, respectively. In the 2‑year tracking experiment, the levels of soluble protein and vitamin C also increased significantly. The results showed that it was feasible to calculate fertilizer input at any location of the plot based on soil experiment data and to provide spatial data for the fertilizer operator control system for variable-rate fertilization, thereby improving fertilizer use efficiency and quality of custard apples.