Dr Christopher Teh, soil biologist and senior lecturer at Universiti Putra Malaysia answers gardening questions in Star2’s monthly gardening column, Ready, Set, Grow!, which is brought to you by social enterprise Eats, Shoots & Roots. Send your questions to faqs@eatsshootsandroots.com.

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This month’s questions are about growing mediums and how to use rice water and kitchen waste in your garden.

What is the best growing medium for potted edible plants? Does it need to be changed after some time?

It is not easy to simultaneously get all the good factors in one potting media. Commercial soil-less media have been formulated to be rich in specific nutrients needed for certain plants, but they can be too light to support taller and heavier plants like tomato plants.

These media also dry out faster than soil, so more frequent watering is required, which in turn can also lead to more water wastage as these media may not hold onto the water as well as soil.

One good natural option is to mix your garden soil with other material. If you don’t have a garden or if your garden soil is infertile, try buying soil from nurseries. Avoid the red soils you often find at these places, as they contain too much clay and are generally not fertile. Instead, look for soil that looks dark and has a rich, fertile smell; the soil should also be heavy.

Mix one part garden soil with one part peat moss and one part perlite or coarse (not fine) sand. You can buy peat moss and perlite (or coarse sand) from most hardware stores or nurseries. The advantage of this soil mixture is that it is able to support heavier and taller plants, and it holds onto water and nutrients longer.

How often you change your potting media depends on how aggressively you use the media to grow your plants and how well you protect your soil – for instance, adding mulch or compost will prolong soil use. Once you find your plants are no longer growing as well as before, you should change the media; this happens anyway after a few years.

Is it true that the water from washing rice is good for watering plants? What other types of kitchen water waste can be recycled for plants?

Using rice water is a good way to reuse and, thus save, water. Rice water contains nutrients needed by plants, and the starches in it will promote beneficial microorganisms in the soil that will, in turn, promote plant growth in the long run. So by using rice water, you are actually simultaneously watering and fertilising your plants.

Whatever source of water you use, make sure it does not contain harmful chemicals. Substances to avoid are salts, soaps, strong acids (like vinegar), and oils. If you are unsure of your kitchen water content, it’s best to avoid watering your plants with it.

Can kitchen waste like fish parts or coffee grounds be used to fertilize plants?

Burying fish parts will aid in decomposition, and as they decompose, they will release nutrients into the soil for your plant. How deep you should go depends on your plant: the parts shouldn’t be buried so deep that the plant’s roots cannot reach the nutrients; but neither should they be buried so shallowly that they attract flies or dogs and cats that might dig the parts up again. This usually works out to between 15cm and 30cm deep.

The coffee beans can be buried as well or just applied over the soil’s surface. If you find yourself doing this often, you should try composting.

Ready, Set, Grow! is a gardening series brought to you by Eats, Shoots & Roots, a social enterprise that champions urban edible gardening.