Fertilisers explained: NPK, labels and what to use

Walk into any garden centre and you'll find a wall of fertilisers. They all promise healthy plants, and the choice can feel overwhelming. The good news is that they all come back to a few simple ideas. Once you understand those, picking the right one gets much easier.

The big three: N, P and K

Every fertiliser label lists three main nutrients.

A simple way to remember it: N for leaves, P for roots, K for flowers and fruit.

Trace elements

Plants also need small amounts of other nutrients. Calcium, magnesium and sulfur are needed in moderate amounts. Iron, manganese, zinc, copper, boron and molybdenum are needed in tiny amounts. Many complete fertilisers include them.

In alkaline soils, which are common around Adelaide, plants can struggle to take up iron and manganese even when they're in the soil. Yellow new leaves with green veins are a common sign, often on citrus and azaleas.

Reading a fertiliser label

Australian labels usually show the N:P:K ratio as percentages by weight, for example 10:4:6. That means the bag contains 10 per cent nitrogen, 4 per cent phosphorus and 6 per cent potassium. The rest is other nutrients and filler.

When reading a label, look for:

• The NPK ratio. High N for leafy plants, higher K for flowering and fruiting plants, low P for sensitive natives.
• Rate and frequency. How much per square metre or per plant, and how often.
• Trace elements, if your plants need them.
• Whether it's slow-release or quick-acting.

Always follow the label. More is not better.

Organic versus synthetic

Organic fertilisers come from living things: manures, blood and bone, fish, seaweed and compost. Most release nutrients slowly as soil life breaks them down, and many add organic matter too.

Synthetic fertilisers are manufactured. Nutrients are in a form plants can use straight away. They're precise and quick but don't feed the soil life or improve structure.

Both can grow good plants. Many gardeners use organic fertilisers as a base and a quick liquid feed when plants need a lift.

Slow-release versus liquid

• Slow-release granules or pellets feed over weeks or months. Good for planting time and for trees, shrubs and long-lived crops.
• Liquid fertilisers act fast and wash through quickly. Good for a quick boost, for pots and for hungry crops, but they need repeating.

Common garden fertilisers

Chicken manure pellets. A popular all-rounder, fairly high in nitrogen. Water in well and keep pellets off stems. Don't use heavily around phosphorus-sensitive natives.

Blood and bone. An organic, slow-acting feed with nitrogen and phosphorus but little potassium. Many products add potash to balance it. Wear gloves and avoid breathing in the dust.

Seaweed. Best thought of as a tonic rather than a fertiliser. It's low in NPK but contains trace elements and other compounds that many gardeners find help plants cope with stress and settle in after transplanting.

Fish emulsion. A quick liquid feed with plenty of nitrogen. Good for leafy greens and seedlings. It smells for a day or two.

Sulfate of potash. Adds potassium. Useful when fruiting crops like tomatoes, capsicum and citrus are flowering and setting fruit.

Citrus food. A complete fertiliser with extra nitrogen and trace elements. Feed citrus in late winter and again in late summer or early autumn, spread around the drip line and watered in.

Don't over-fertilise

Too much fertiliser causes problems:

• Burn. Concentrated fertiliser draws water out of roots, leading to brown leaf edges or dead seedlings.
• Soft, sappy growth. Lots of nitrogen grows lush leaves that aphids and other pests love.
• Leaves but no fruit. Too much nitrogen on tomatoes and zucchini can mean big plants and few fruit.
• Run-off. Nutrients that wash out of the garden end up in drains, creeks and eventually Gulf St Vincent, where they feed algae.

Water the soil before and after applying granular fertiliser, keep it off leaves and stems, and feed little and often rather than all at once.

Feeding natives

Many Australian natives evolved in poor soils and need little feeding. Some go further: members of the protea family, such as banksias, grevilleas and hakeas, have special clustered roots (called proteoid roots) that are very good at finding phosphorus. Too much phosphorus can make them sick, with yellowing leaves and dieback, and can kill them.

• Use a fertiliser labelled for natives, which is low in phosphorus, or a slow-release product with very low P.
• Feed lightly, if at all, and only in spring.
• Avoid general-purpose fertilisers, chicken manure and blood and bone near banksias and grevilleas.
• Mulch with wood chips or leaf litter, which feeds gently.

Not every native is phosphorus-sensitive, but going light is the safe choice for all of them.

The simple approach

1. Build soil with compost and mulch first. Healthy soil needs less fertiliser.
2. Use a slow-release feed at planting time.
3. Add a liquid feed for hungry crops and pots during the growing season.
4. Match the NPK to the plant: leafy, fruiting or native.
5. Read the label, and when in doubt, use less.