Most of us feed the garden to get greener grass and more flowers. Few people think about where the fertiliser goes after it leaves the bag. Some is taken up by roots. The rest can end up in drains, groundwater and rivers.
That leftover fertiliser is a real problem for waterways. The good news is that small changes at home make a difference. You can still have a healthy lawn and garden while using less product.
Why garden fertiliser ends up in rivers
Plants only use what their roots can reach. Anything left over can travel through stormwater or soak down into groundwater. The Shire of Mundaring explains that unused nutrients can reach waterways this way. Its guide to fertilising your lawn and garden says this can cause algal blooms that release toxins and starve aquatic life of oxygen.
Sandy soils make the problem worse. Much of Perth sits on sand that holds very little nutrient. The Mundaring guide notes that most fertiliser applied to unimproved sandy soil is lost. It also says overwatering pushes nutrients down past the roots.
If you have read our piece on Perth water quality and your home plumbing, you will know that water at home is connected to the wider system. Fertiliser is part of the same picture. What goes on the lawn can turn up in the river.
Phosphorus limits in Western Australia
Excess phosphorus can cause algal blooms in waterways. Western Australia began limiting phosphorus in fertiliser sold for home use in 2011. A state government statement on phosphorus limits issued in December 2012 set out the next stage, which started on 1 January 2013. Under that stage, phosphorus in garden fertiliser dropped from 2.5 per cent to 2 per cent.
The same statement noted that lawn and all purpose fertiliser was limited to 1 per cent phosphorus. Controlled release products were also brought under the rules at that time. So were processed organic products such as blood and bone, composts and composted chicken manure. The statement linked excess phosphorus to algal blooms that can kill fish in the Swan and Canning rivers.
Those figures come from a 2012 statement and the rules may have changed since, so read the bag before you buy. Look for the phosphorus figure on the label. A lower number is usually the better choice for a home lawn.
Picking the right product
Fertilisers come in two broad types. Organic fertilisers are made from plant and animal remains, such as seaweed, blood and bone and manures. Inorganic fertilisers are mineral products sold as liquids or granules. Liquids are diluted and watered in, while granules are spread over the root zone.
Organic does not always mean gentle on waterways. The Mundaring guide warns that organic fertilisers can contain a lot of phosphorus. It suggests using them sparingly. Slow release products last for a set period, but heat and overwatering can make them release faster.
For lawns, the Fertilise Wise campaign run by the South East Regional Centre for Urban Landcare suggests a complete fertiliser. It names an N:P:K ratio of 10:1:6 as a guide. That means far more nitrogen and potassium than phosphorus. The campaign also suggests products that contain sulphur, magnesium, calcium and trace elements such as iron and zinc.
Timing and how much to use
Timing matters as much as the product. The Mundaring guide says it is better to apply a little fertiliser often during the warmer months. Feeding in winter or just before rain is expected is wasteful. Heavy rain can wash nutrients off the surface or carry them into groundwater.
Fertilise Wise gives Perth gardeners a simple calendar. It suggests feeding lawns only if needed, in September, October and November, or in March and April. It advises against fertilising in summer or winter. It also suggests waiting until you see signs of deficiency, such as yellowing leaves.
The rate on the packet is a ceiling, not a target. Mundaring describes the packet figure as the maximum amount to apply. Fertilise Wise gives a maximum of 25 grams per square metre for couch. For kikuyu and buffalo it gives 12 grams per square metre.
Measuring without scales
Most people do not weigh fertiliser in the garden. The Mundaring guide offers a rough kitchen conversion. A teaspoon holds about 4 grams and a tablespoon about 16 grams. A matchbox holds about 25 grams and a cup about 250 grams.
Simple habits that keep nutrients where they belong
A few small habits cut down how much fertiliser escapes the garden. None of them take long. Most will also save you money on product.
- Apply fertiliser near plant roots, using the canopy as a guide to where they are
- Sweep any granules off paths, driveways and roads before you water
- Water gently after feeding rather than flooding the area
- Hold off if heavy rain is forecast
- Improve sandy soil with compost, clay or a soil wetting agent
Hard surfaces deserve extra care. Granules that bounce onto concrete wash straight into the nearest drain. If you are planning paving for your landscaped garden, leave a clear edge between beds and paths. A short garden border helps keep granules on the soil where they belong.
The same thinking applies when you wash down outdoor areas. Our article on high pressure cleaning for home owners looks at the benefits of a clean driveway. Before you clean, sweep up any loose fertiliser first. That way it does not go down the drain with the wash water.
Native plants and smaller lawns
Native plants generally need less fertiliser than exotic plants. The Mundaring guide says a low phosphorus fertiliser applied in spring is usually enough for them. Fertilise Wise also encourages growing local plants, which need less fertiliser and water. It suggests keeping lawn areas to the size you really use.
Bringing back local plants is a theme in larger projects too. Our look at sustainable rehabilitation of former mine sites shows how native species help restore damaged land. A home garden works on a much smaller scale. The principle of choosing plants that suit local soil is the same.
Frequently asked questions
Is organic fertiliser always safer for rivers?
Not always. Organic fertilisers can contain a lot of phosphorus, which is linked to algal blooms. Use them sparingly and follow the label.
When should I fertilise my lawn in Perth?
Fertilise Wise suggests spring or early autumn, in September to November or March and April. It advises against feeding in summer or winter. Only feed when the lawn shows signs it needs it.
Should I water fertiliser in?
Fertiliser needs to reach the roots, so a light watering helps. Avoid heavy watering, which can push nutrients past the roots. Sweep granules off hard surfaces before you turn on the tap.
How do I know how much phosphorus is in a product?
Check the label on the bag. It lists the nutrient content, including phosphorus. For a home lawn, a low phosphorus product is the usual choice.