Fertililsers for edibles

Feed your edible plants well, and they’ll feed you well. TAMMY HUYNH explains how to choose and use fertilisers for potted fruits and vegies

Plants are just like us: they need water, sunlight and food to survive. The first two are easy, but choosing the right plant foods or fertiliser can be confusing, with aisles full of different products all claiming to be the best. Let’s look at what it all means for the gardener growing edibles in pots.

the science

Plants need a fertiliser that supplies the essential nutrients. Nitrogen (N), phosphorus (P), and potassium (K) are the ones plants use the most. Nitrogen supports leafy growth, phosphorus promotes flower, fruit and root growth, and potassium strengthens cell walls and improves the quality of flowers and fruit. A complete fertiliser (sometimes called an NPK fertiliser) contains all three, in varying amounts to suit the needs of specific plants. Plants also need other nutrients, including calcium, magnesium, and manganese, but these are required in much smaller amounts.

Fertilisers are available in organic and inorganic (or synthetic) forms, or they can be organic-based, which is a combination of the two.

Organic fertilisers are derived from natural or once-living sources, such as animal waste, blood and bone, and emulsified fish. Microbes in soil or potting mix slowly break down the materials and release nutrients over an extended period.

Inorganic fertilisers are made from both naturally occurring minerals and synthesised chemicals. These nutrients are in a format that is readily taken up by roots and, because the products are manufactured, the various nutrients are reliably well balanced.

Organic-based fertilisers contain the best of both worlds – slow-release organic nutrients to nourish the growing medium and gently feed plants, and fast-acting inorganics to quickly boost plant growth.

It’s important to note that organic soil conditioners, such as compost, aged manure, and worm castings, are not the same as fertilisers. Instead, think of them as good sources of organic matter that can be used to enrich the biology of potting mix and improve its water-holding capacity, which in turn provides plants with the strong foundation needed for growth. They contain small amounts of nutrients, but not enough to sustain the healthy growth of potted plants.

the right diet

Fertilisers suitable for the potted patch come in a variety of formats – pellets, prills, soluble and liquid – which perform in different ways. Pellets are mostly organic or organicbased, and break down slowly.

They’re great for nourishing microbes and other beneficial organisms in the mix and will gently feed plants over an extended period. Adding pellets every month or so should keep edibles happy and sustained.

Prills are usually controlled-release fertilisers that break down to gradually release nutrients over a certain timeframe. Depending on the product, one application can feed plants for anywhere between three and 12 months – perfect for the time-poor gardener. The short-term products (3–6 months) are best for fast-growing vegies, while the long term (9–12 months), are more suited to growing potted fruit trees. Not all prills are the same. Some are designed to release most of the nutrients soon after application, which could burn your plants, so always check the product label before use.

Soluble and liquid fertilisers are both dissolved in water and applied with a watering can. Plants naturally take up nutrients in liquid form, so these types of fertilisers are very fast-acting and ideal for encouraging potted edibles to grow quickly, or support them while they are fruiting. These can be applied to potted vegies weekly when used as the sole fertiliser source, or once every 2–4 weeks to supplement pellets or prills when you want to give plants an added boost.

which fertiliser is best for my plants?

Ultimately, choosing the right fertiliser comes down to personal preference. Compare the nutrition tables, price and ease of use, and see what you like best. Regardless of which format you choose, rest assured that all fertilisers will deliver valuable nutrients to your plants.