You can buy simple soil test kits that tell you about your soil's pH, nitrogen, phosphorus, and potash levels. Then you know if/what to replenish. A university

You can buy simple soil test kits that tell you about your soil's pH, nitrogen, phosphorus, and potash levels. Then you know if/what to replenish. A university or lab near you may also offer a service where you send them a soil sample and you get a detailed report. Probably overkill for a balcony though.