Choosing the right fertilizer for outdoor plant boxes is essential for healthy, vibrant plants. The best options include organic fertilizers like compost or manure, which enrich the soil naturally. Slow-release granular fertilizers provide steady nutrients over time, ideal for low-maintenance gardening. Liquid fertilizers, such as fish emulsion or seaweed extract, offer quick absorption for fast-growing plants. Always consider the specific needs of your plants—leafy greens thrive with nitrogen-rich formulas, while flowering plants benefit from phosphorus-heavy blends. For optimal results, test your soil’s pH and nutrient levels before applying any fertilizer. Regularly amending the soil with organic matter ensures long-term fertility and robust plant growth in your outdoor plant boxes.