Tomatoes growing in pots benefit from an application of general multi-purpose fertilizer just after being planted and before they start flowering. All plants need nitrogen, phosphorus and potassium – seen as a ratio of NPK on a plant fertilizer label – and a balanced feed will have these in equal parts, eg. 10-10-10. Once the tomato plants start to flower, start giving them a regular feed weekly with a dedicated tomato food that is high in phosphorus and potassium.