Fertilizer for growing veggies in containers


What kind of fertilizer should be used for veggies growing in container? How often should they be applied?
Some fertilizers contain 4-6-4, some has 20-20-20. Is 20-20-20 stronger than 4-6-4.
Should we use half the strength of fertilizers when applying them to the soils in the pot?
Would you have any resources to recommend for using fertilizers?
Thanks a lot.


Thank you for contacting the Toronto Master Gardeners with your fertilizer questions.

As you have not indicated what type of veggies you will be growing, here are some basic fertilizing information for growing veggies in containers which I hope you will find helpful.

There are basically two types of fertilizer – slow timed release granular or organic liquid.  Granules can be mixed into the potting soil or sprinkled on the soil surface, provides a 3 month supply of nutrients, gets released at each watering and only needs to be applied twice per growing season.  Organic liquid fertilizers are generally mixed with water following the package instructions, needs regular application but the nutrients are readily available for plant uptake and it is gentle on plant roots.

N-P-K are the 3 numbers you see on the package.  These are the macronutrients Nitrogen, Phosphorus & Potassium.  Plants need all three for healthy growth.  20-20-20 is considered a balanced fertilizer and is ideal for leafy veggie growth such as kale or swiss chard.  15-30-15 is more suited to growing fruiting plants or root veggies such as tomatoes, zucchini, carrots or beets.

All container plants benefit from a balanced fertilizer application initially to give them a head start into the growing season – this can be mixed into the growing medium before planting, so 20-20-20 slow release granules would be great.  After this switch your “fruits and roots” to a food with a higher phosphorus middle number and keep the greens on a balanced diet – these applications are best being in liquid form.

The challenge with container veggie growing is watering and feeding as the pots only hold enough soil mix to retain so much water and nutrients.  Daily watering is recommended but this may have to be increased to twice daily during hot summer months.  Feeding is also recommended for everything in containers every two weeks as the plants diminish the nutrient supply very quickly which is not helped by the frequent watering.

Whatever fertilizer you decide to use, please follow the manufacturers instructions for mixing carefully, container veggie plants are hungry so using half strength is not recommended.

Fertilizing Vegetables from the University of Maryland  gives information on the fertilizing of different vegetables

Good luck.