The best soil mix for vegetables in an urban planter box is not ordinary garden soil straight from the ground. It should be fertile, lightweight, well-drained, and able to hold moisture without becoming soggy. A reliable all-purpose blend is 50–60% quality topsoil or loam, 25–30% mature compost, and 10–20% perlite or pumice. This gives roots room to breathe, supplies nutrients, and keeps the planter from becoming too heavy.
For balconies, rooftops, or very large containers where weight matters, use a lighter soilless mix: about 50% coco coir or peat moss, 30% compost, and 20% perlite. Coco coir and peat retain moisture; compost feeds soil life and vegetables; perlite improves drainage and aeration. Avoid using only compost, because it can shrink, become waterlogged, and burn plants if immature. Avoid heavy clay soil, which compacts in containers and restricts root growth.
Vegetables grow best in a slightly acidic to neutral mix, roughly pH 6.0–7.0. Most compost-based mixes fall in this range. If you are reusing old planter soil, refresh it by removing roots and debris, adding one-third new compost, and mixing in perlite if the mix feels dense. Do not reuse soil from diseased plants. For urban sites, choose compost and topsoil from a trusted supplier, and avoid soil from unknown industrial sites because of possible lead or other contaminants. If you plan to grow leafy greens or root vegetables, testing the mix is a sensible precaution.
Before filling, confirm the planter has drainage holes. Fill the box loosely, water thoroughly, and let it settle before planting. Leave a 2–5 cm (1–2 inch) gap below the rim for mulch and watering. Mulch with straw, leaves, or compost to reduce evaporation, which is especially important in hot urban spaces.
Feed vegetables according to their needs. Mix a slow-release organic fertilizer or worm castings into the top layer at planting, then side-dress heavy feeders such as tomatoes, peppers, and brassicas every few weeks. Water when the top few centimeters are dry, not on a fixed schedule. In small planter boxes, roots depend entirely on the mix you provide, so a balanced, airy, compost-rich blend is the best foundation for a productive urban vegetable garden.