How to fertilise your raised garden bed

A raised bed is a contained environment, so what nutrients go in is entirely up to you. The good news is that feeding a raised bed is straightforward once you understand the basics.

What’s the right base mix for a raised bed?

A quality premium potting mix already contains starter nutrients, usually enough for the first four to six weeks. Do not fertilise immediately after planting. Give the plants time to settle in and use what is already there.

What’s a simple fertilising routine?

1. Slow-release granules. After the first six weeks, apply a slow-release fertiliser according to the pack instructions. These break down steadily over several months, so you are not constantly topping up.
2. Liquid fertiliser during growing season. Every two to four weeks while plants are actively growing, give the bed a liquid feed. A seaweed solution or a balanced liquid fertiliser works well. This is especially important for fruiting crops like tomatoes, capsicum and zucchini, which are heavy feeders.
3. Compost top-dress. Every few months, add two to three centimetres of compost across the surface. This feeds the mix over time and improves its structure. Do not dig it in, just let it sit on top and let watering carry it down.

Why watering decides how far your fertiliser goes

Feeding and watering are more or less the same job, and getting the watering wrong wastes the fertiliser twice over.

Soil microbes do the actual work. Slow-release granules, compost, manure pellets and blood and bone are not directly available to a plant — microbes have to break them down first, and they need steady moisture to do it. Let the mix dry right out and that population crashes, taking weeks to recover. The same handful of fertiliser simply goes further in a bed that never dries out.

Heavy, infrequent watering flushes nutrients away. Nitrogen and potassium in particular are highly soluble, so a big drenching carries a share of your feed straight out the bottom of the bed before the plants get a look in. Little and often keeps it where the roots are.

Which is a good argument for taking the guesswork out of it — here are three ways to make a bed water itself. And if you would rather that flushed-out fertiliser did not end up in your lawn, it can be caught underneath the bed and put back.

Signs your plants need more feeding: slow growth, pale yellow-green leaves, poor fruit set. Signs of too much: lush leafy growth but no fruit, or brown burnt leaf tips. For more on that, see our guide to overfertilising.

What to avoid: heavy doses of high-nitrogen fertilisers push leafy growth at the expense of fruit and flowers. Go steady and consistent rather than large doses occasionally. And avoid fertilising during a heatwave or when plants are stressed, as they cannot absorb it well.

Refreshing older beds: after a few seasons, premium potting mix loses some of its structure and nutrients. Top it up with fresh mix and compost each year to keep the bed performing well.

For more on filling your bed from scratch, see our filling guide.

More garden guides: can you overfertilise? · three ways to make a bed water itself · compost vs potting mix · what to plant in SEQ

What should I plant right now? Our free garden calendar shows what suits your postcode this month. Ready to grow? Browse our raised garden beds or build your own with the garden bed builder.

Image: Daderot, CC0, via Wikimedia Commons.