Lawn Fertiliser Guide: How Much, When and With What

What do the numbers on the bag actually mean?

Three numbers, always in the same order: nitrogen, phosphorus, potassium — N, P and K. A bag marked 20-3-3 contains 20 percent nitrogen, 3 percent phosphorus and 3 percent potassium by weight. The rest is filler and other minor ingredients.

The three nutrients do different jobs:

• Nitrogen (N). Drives leaf growth and green colour. It's the nutrient that makes a lawn look dense and vigorous, but too much at once scorches the grass yellow.
• Phosphorus (P). Builds roots, which matters most for newly seeded lawns. An established lawn needs far less phosphorus than a new one, which is why spring and autumn feeds for mature turf often carry a low P number.
• Potassium (K). Toughens cell walls and improves drought, cold and wear tolerance. That's why autumn feeds lean heavily on potassium.

As long as you know the nitrogen percentage, you can work out the correct rate for any product, any brand. It's the only number you actually need to track to avoid over- or under-feeding.

On slow-release granular products you'll often see "controlled release" or "coated" on the label. That means the nitrogen granules are wrapped in a thin, semi-permeable coating that breaks down gradually as soil moisture and microbial activity work on it, so nutrients leach out over weeks rather than dissolving all at once in the first rain. The upside is steadier growth and a lower risk of scorch; the downside is that results show up more slowly than with a straight water-soluble granular.

Good to know. A soil test, whether mailed to a lab or done with a simple home kit, tells you both the pH and whether the soil already has plenty of phosphorus and potassium. Many established garden lawns actually run a phosphorus surplus after a few years, in which case a lower-P product saves money without costing you anything in lawn quality.

When should you feed a lawn?

Timing matters more than the exact product. Fertiliser applied before the grass wakes up or after it has shut down for the season does nothing, no matter how good the bag is.

Spring

Wait until the lawn is actually growing, not just until the calendar says so. A simple test: you've mowed once and the grass has grown back within a week. Apply the spring feed after the first or second cut, using a nitrogen-forward product such as 20-3-3.

Summer

A light summer feed in midsummer helps hold colour through the hottest stretch, but only if the lawn is getting water. In a drought, or under watering restrictions, skip it entirely. No lawn has ever suffered from a missed summer feed.

Autumn

The most important and most skipped application of the year. Put down a potassium-rich, low-nitrogen product such as 5-1-15 while the grass is still actively growing — see the zone table below for the cut-off date where you garden. The roots then have time to store that nutrition before winter, rather than it simply leaching down through the soil.

First season on a new lawn

A lawn seeded or turfed this year doesn't follow the usual schedule. The first six to eight weeks are entirely about root development, not colour, so use a phosphorus-rich starter fertiliser rather than the nitrogen-forward spring product. Hold off on the first regular spring feed above until the following year.

If the lawn established over summer — after late-summer seeding, for instance — a light, low-nitrogen feed six to eight weeks after germination gives young plants a boost heading into autumn. Still wait a full year before applying a standard full-strength autumn feed, since young roots can't take up a full dose yet.

Common mistake. Feeding in March "to get a head start." The soil is too cold for the grass to take up the nutrients, the nitrogen leaches into the groundwater instead, and you've used up an entire application before the season has even begun.

Feeding calendar: USDA zones 3–6

The table shows approximate dates for a normal year. A cold spring can push everything back two to three weeks, so treat it as a starting point rather than a fixed rule.

The colder the zone, the more the summer and autumn feeds tend to merge into one. In zone 3, it's often more effective to move the whole autumn application earlier rather than splitting nutrition across three dates the grass doesn't have time to use.

How much fertiliser does your lawn need?

The baseline rule is no more than 0.5 kg actual nitrogen per 100 m² (1 lb per 1,000 sq ft) per application. Go higher and you risk scorch in warm weather and runoff in heavy rain. Here's how to work out the product amount, whatever brand you buy.

1. Read the nitrogen percentage on the bag. A bag marked 20-3-3 is 20 percent nitrogen, or 0.20 as a decimal.
2. Divide the target nitrogen rate by that percentage. 0.5 kg nitrogen divided by 0.20 comes to 2.5 kg of product per 100 m² (roughly 5 lb per 1,000 sq ft).
3. Scale up to your lawn's size. A 250 m² lawn (about 2,700 sq ft) needs 2.5 times that amount: around 6.25 kg of product for the whole area.
4. Weigh it out if you're spreading by hand. A scoop or mug rarely holds exactly the right amount. Weigh the first batch on a kitchen scale so you know how many scoops it takes for the rest of the season.

The table gives ready reference figures for a 20-percent-nitrogen product at the standard 0.5 kg nitrogen per 100 m² rate. If your product has a different nitrogen percentage, recalculate using the steps above.

Never rely on "a handful per square metre" or similar rules of thumb. A handful of granular fertiliser can weigh anywhere from 20 to 60 grams depending on the product and the hand doing the scooping — in practice, that's guessing with roughly a threefold margin of error.

Granular, liquid or organic — which suits you?

The three main fertiliser types work differently and suit different situations. None of them is objectively best; the right choice depends on how fast you want results and how much time you want to spend.

Slow-release granular is the most practical choice for most home lawns: two or three applications a year and nothing to manage in between. Liquid fertiliser acts faster but needs topping up more often, and works best as a supplement for patches that have visibly lagged behind the rest of the lawn.

Can you feed a lawn organically?

Yes, and for many lawns it's entirely sufficient. Organic fertiliser relies on soil microorganisms to break the material down and release nutrients gradually, which tends to give steadier growth without the sharp colour swings mineral fertiliser can produce.

• Grasscycling. Leaving clippings on the lawn after every mow returns a meaningful share of the season's nitrogen need for free.
• Pelleted poultry manure or composted animal manure. Nitrogen-rich and reasonably fast for an organic product, but it can smell strongly for the first few days.
• Bone meal. Phosphorus-rich and slow, a good match for overseeding since it favours new root growth. If you've just sown a new lawn, bone meal or a phosphorus-rich starter feed is a better choice than the nitrogen-forward product you'd otherwise use on established turf.
• Compost, applied thinly. Gives slow, broad nutrition and improves soil structure at the same time, but doesn't carry enough nitrogen on its own to keep a worn lawn green.

The downside of organic products is that the release is harder to control precisely and strongly temperature-dependent: in cold spring soil almost nothing happens, while the same product releases nutrients quickly once the soil warms up in summer. If you want a predictable spring green-up, a mineral feed works better for that one application, with organic products filling in the rest of the season.

Robot mowers change the feeding picture more than most people expect. A robotic mower typically runs several times a week and always leaves the finely mulched clippings behind, which amounts to a small, continuous return of nutrients all season rather than the occasional boost from a traditional mow-and-collect routine. Many households that switch to a robot mower can trim the fertiliser they buy, particularly the summer application, and simply watch how the lawn looks before adding more.

Good to know. Combine grasscycling with an organic autumn feed and many fine-fescue-heavy lawns get by on noticeably less bought fertiliser than the general 1–2.5 kg nitrogen per year budget suggests. Cutting back further often favours a more species-rich, bee-friendly lawn with clover and other low-growing flowering plants that cope well on leaner soil.

What happens when feeding goes wrong?

Most feeding mistakes show up within one to two weeks, and almost all of them can be identified by how the damage looks.

Fertiliser burn from too much nitrogen works on the same principle as salt on a slug: the fertiliser salts draw water out of the grass blade's cells faster than the roots can replace it. The fix for an acute overdose is always the same — water heavily and immediately to dilute and flush the salts down past the root zone.

How to spread fertiliser evenly

1. Split the dose in half. Walk the whole area in one direction with half the amount, then walk it again at right angles with the rest. That cancels out the overlap that causes striping.
2. Spread on dry grass, in still weather. Damp grass makes granules clump instead of falling through to the soil, and wind carries light granules off target.
3. Use a spreader for anything beyond a tiny lawn. A hand-held or push spreader gives a far more even result than broadcasting by hand, especially above roughly 1,000 sq ft.
4. Calibrate the spreader once a season. Lay a tarp or a couple of sheets of cardboard over a known area, run the spreader across it at your normal walking pace, and weigh what lands on it. If it doesn't match your calculation, adjust the spreader setting before covering the whole lawn.
5. Water it in within 24 hours. 5–10 mm (0.2–0.4 in) dissolves the granules and carries the nutrients down to the roots. If rain isn't coming, water it yourself.

The difference between a spreader with a wide throw pattern and a basic drop spreader matters in practice. A wide throw pattern lets you overlap passes by an arm's length instead of walking exact edge-to-edge lines, which is much harder to hold straight across a whole lawn and a common cause of the striping described in the table above.

Sweep any fertiliser that lands on paths, driveways or patios back onto the lawn rather than hosing it away. Fertiliser that washes into storm drains ends up in lakes and streams, where the same nutrients that green up a lawn feed algae blooms instead.

Common mistakes

• Feeding by eye instead of calculating. A rough scoop can mean double or half the intended dose depending on the product. Weigh it out once and you'll know the right amount for the rest of the season.
• Using a nitrogen-heavy product for the autumn feed. Produces soft growth exactly when the lawn should be hardening off for winter, and raises the risk of snow mould.
• Feeding during drought with no way to water it in. The grass can't take up the nutrients, and the salts draw moisture out of the roots instead. Wait for rain or water it in yourself before applying.
• Double-dosing on overlaps. Covering the same strip twice by accident doubles the nitrogen on that band, which shows up as a dark green or scorched stripe for weeks.
• Feeding a brand-new lawn with the wrong product. Newly seeded grass wants phosphorus for root growth, not a nitrogen-forward product meant for an established lawn.
• Skipping soil tests year after year. Keep guessing for a decade and you can end up with a phosphorus surplus and a potassium shortfall without ever knowing it.
• Feeding an area that will never carry a lawn anyway. Dry shade under a mature tree won't grow a dense lawn no matter how much fertiliser goes down, because the nutrients end up feeding the tree's roots instead. Ground cover plants or shade-tolerant perennials are a better use of the effort than another year of feeding a lawn that keeps thinning out.

Frequently asked questions

How often should you fertilise a lawn?
Two to three times a year covers most lawns: a spring feed once growth is underway, an optional light summer feed in midsummer if the lawn is getting water, and an autumn feed, which matters more than the other two combined. More applications than that rarely produce a better lawn, just a higher risk of overdosing.

What's the best fertiliser for a lawn?
There's no single best product, only the right type for the right moment. A slow-release, nitrogen-forward granular suits the spring feed, a low-nitrogen, potassium-rich product suits autumn, and liquid fertiliser works well as a fast spot treatment on patches that have fallen behind.

Can you over-fertilise a lawn?
Yes, and it's the most common cause of brown or scorched patches after feeding. Fertiliser salts pull water out of the grass blade's cells faster than the roots can replace it. Stick to a maximum of 0.5 kg nitrogen per 100 m² (1 lb per 1,000 sq ft) per application, and water heavily right away if you suspect an overdose.

Should you water a lawn after fertilising?
Yes, with 5–10 mm (0.2–0.4 in) within a day if rain isn't due. Watering dissolves the granules and carries the nutrients down to where the roots can actually use them. Fertiliser left dry on the surface does less good and raises the risk of scorch in strong sun.

Can you fertilise and lime a lawn at the same time?
No, space them a few weeks apart. Lime raises soil pH while fertiliser adds nutrients, and mixing the two can cause the nitrogen in the fertiliser to escape as gas instead of being taken up by the grass. Apply lime in spring or autumn and wait at least two to three weeks before feeding the same area.

How soon after fertilising can you mow?
Wait at least two to three days after a granular application, ideally until after you've watered it in. Mowing too soon can collect some of the granules in the catcher before they've had a chance to dissolve into the lawn.

Is grasscycling enough instead of buying fertiliser?
It covers part of the need but rarely all of it, especially the potassium in the autumn feed. Left-on clippings supply a steady trickle of nitrogen and cut down how much you need to buy, but a lawn fed on clippings alone tends to thin out over time and becomes more prone to moss and weeds.

Why did the lawn turn yellow after fertilising?
Usually too much nitrogen at once, especially combined with heat or drought, which scorches the grass blades. Uneven spreading with overlapping double doses produces the same result in stripes rather than across the whole lawn. Water heavily to dilute the salts — the damage usually heals, though it can take a few weeks.

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