Absolutely, it is not only possible but quite common to over-fertilize plants in the confined space of an urban planter box. Unlike plants in open ground, where excess nutrients can leach away, container plants are trapped with all the fertilizer you add. This buildup of salts can lead to "fertilizer burn," damaging the roots and causing symptoms like yellowing or browning leaf edges, stunted growth, and wilting despite moist soil.
The key to success is understanding that less is often more. Use a diluted, balanced liquid fertilizer at half the recommended strength, and only during the active growing season. Ensure your planter has excellent drainage and water thoroughly before feeding to protect roots. If you suspect over-fertilization, flush the soil with plenty of clean water to leach out excess salts. For a sustainable approach, incorporate slow-release fertilizers or nutrient-rich compost at planting time, which provides a gentler, steady supply of food for your urban garden oasis.