We all know plants need water, but they also need food, especially if you want that garden growing food for you.
But what should you use to fertilise your vegetable beds or pots and when is best to do it?
Why fertilise your garden?
Plants need food as well as water to survive and thrive, explains Melbourne/Naarm-based horticulturist and gardening coach Chloe Thomson.
And when it’s veggies or food plants, “you’re essentially taking those nutrients out of the soil … and [eating] them. So … you will be depleting the soil over time,” she says.
“[Plants’] food is macro and micronutrients that you can get from fertilisers, so think of fertiliser as food for your plants.”
Soil scientist Emily Leyden recommends a mixture of organic and inorganic fertilisers for beginner veggie gardeners. (Supplied: Emily Leyden)
Soil scientist and University of Adelaide research fellow Emily Leyden adds that soil structure is “critically important” for a successful veggie garden.
“You want to have your soil, you want to add your compost for your structure … and then you’re going to add your food, and having a balance of those two things.”
Is spring the best time to fertilise your veggie garden?
According to Ms Thomson, the best time to fertilise your vegetables is when they are actively growing, and for most veggies, that means the season you have planted them.
“Spring and summer are definitely peak growing season for a lot of those popular veggies like tomatoes, chillies, capsicums, eggplants, cucumbers, particularly if you’re in the southern states,” she says.
Most vegetables are annuals, which means they grow for one season before setting seed and dying. “So you need to be feeding them when they’re actively growing,” Ms Thomson says.
“They actually do need a fair bit of food in their growing window to get them healthy and happy.”
What types of fertilisers to use
Organic fertilisers are any that have been derived from living organisms, and can include granular or solid substances such as compost, manures, fish emulsion and blood, bone and fish meal.
Inorganic fertilisers are synthetic, concentrated forms of often naturally occurring minerals. Both organic and inorganic fertilisers can come in solid and liquid forms.
“Your granular, or your pelletised or your slow-release fertilisers, they’re considered your solid fertilisers, and they’re wonderful at feeding your plant, but they’ll [also] give it a longer feed for a longer period of time,” Ms Thomson says.
“Quite often, the inorganic ones can be heavily boosted in one of the nutrients to provide a really big kick of one of the nutrients.”
Dr Leyden says although some gardeners may rely solely on organic fertilisers, “that’s because they’ve built up their soil over time”.
“If you’re just starting out, I would recommend a mixture of both [inorganic and organic],” Dr Leyden says.
“There’s a smorgasbord of different options and it does get a bit complicated because it depends a bit on what your soil’s like and what you’re planting. So, for a novice, I always say just go for your basic, complete fertiliser.
“For your general veggie garden, they’re going to need the big stuff, the nitrogen, the phosphorus and potassium.”
Typically, fertiliser labels in a hardware or gardening store will have an “NPK” ratio. Dr Leyden explains this tells us the proportion of nitrogen (N), phosphorous (P) and potassium (K) it contains.
“Just say you’ve got something that’s 12, 24, 12, that means it’s got 12 per cent nitrogen, 24 per cent phosphorus and 12 per cent potassium, and that’s what we call a balanced fertiliser,” she says.
What else should you keep in mind?
Dr Leyden says the amount of fertiliser you should put in your veggie garden will differ depending on the texture of your soil.
“Generally, we say if you have a sandier soil, you want to be applying less fertiliser more often, and if you have a more clay soil, you want to be doing one application of fertiliser and then letting it sit,” she says.
Although soil pH levels are not necessarily the primary concern for all horticulturists and hobby gardeners, Dr Leyden believes having a basic understanding of where you soil sits on the scale of acidity and alkalinity “is quite important”.
“No matter what nutrients you put on that soil, if it’s a low pH, then they become unavailable. Or if it’s a really high pH, they also become unavailable,” she says.
“If you have an acidic soil, you might want to add some lime, and that will start to bring up the pH, because plants love a neutral pH, so between, say, six and 7.5 and that’s where all our nutrients are available in the root zone.”
Bringing down highly alkaline pH levels is more difficult, and Dr Leyden says you may need to replace your soil or consider planting in pots or raised beds.
Ms Thomson says some gardeners make the mistake of thinking certain seaweed-derived plant tonics offer the same nutrients as a complete fertiliser, but she says these types of products are “not classed as a fertiliser in their own right”.
“You could think of that more as a pick-me-up or a multivitamin”, she says, but “you still need some protein or some carbs”.
She also says worm juice — or worm wee — contains some macro and micronutrients that can be beneficial for your veggie patch.
“You get worm wee from flushing the worm farm or collecting the juice at the bottom,” she says.
“Quite often neighbours or community gardens will share it around and you have to dilute it down so it looks the colour of weak tea, and then you can water it over the plants and the nutrients will get taken up through the roots when it hits the soil.”
Ms Thomson says you should always monitor your plants’ health and if they are yellowing, wilting or stunted, “they’re all classic signs that they’re hungry” for nutrients.
“Possibly thirsty as well, but definitely hungry,” she says.
