Although most vegetables will grow perfectly well in fertile garden soils without feeding, additional fertilizers can maximise crop yields and correct deficiencies in poor soils. However, care must be taken not to over-fertilize, as this may actually decrease yields.

Nitrogen can be leached out of soils relatively easily, and so may need to be replenished regularly using artificial fertilizers or organic matter. Nitrogenous fertilizers should be applied to the soil in spring, a month before sowing or planting; additional applications may be made later in the growing season. Vegetables vary in the amounts of nitrogen that they require, so always check how much they need before applying. For example:

• None: Peas
• Low: Beans, courgettes, onions, parsnips, tomatoes and turnips
• Medium: Calabrese (broccoli), cauliflowers, kale, lettuces, early potatoes and spinach
• High: Beetroot, Brussels sprouts, celery, leeks and potatoes
• Very High: Cabbages, mizuna greens and pak choi

Phosphorus (P) and potassium (K) levels can be maintained with a yearly application of garden compost or manure. Alternatively, an artificial fertilizer such as potassium sulphate (for K) or superphosphate (for P) may be applied. Phosphate and potassium fertilizers may be applied during any time of the year, although they are usually added in the autumn, or, as part of a compound fertilizer with nitrogen, in the spring.