Understanding Lawn Fertiliser
Lawn feeds all look similar on the shelf, but the numbers on the bag make a significant difference. Here is what they mean and how to use them correctly.
What NPK means
Every fertiliser bag shows three numbers — for example 20-5-10. These are the NPK percentages by weight of:
Nitrogen
Drives leaf and shoot growth — the green, lush look. The most important nutrient for spring and summer.
Phosphorus
Supports root development. Most established lawns have adequate phosphorus — high-P feeds are mainly useful for new seedings or very sandy soils.
Potassium
Hardens cell walls, improves disease resistance, and helps grass tolerate cold and drought. Key in autumn feeds.
A 20-5-10 fertiliser contains 20% nitrogen, 5% phosphorus, and 10% potassium. Apply 35 g/m² and you are giving the lawn 7 g/m² of nitrogen.
Spring vs autumn feed
Spring / summer feed
High nitrogen (N), moderate potassium. Typical ratio: 20-5-10 or similar. Promotes vigorous green growth and recovery from winter.
When: March–July
Autumn / winter feed
Low or zero nitrogen, high potassium. Typical ratio: 4-5-20 or similar. Hardens grass for winter and improves disease resistance.
When: September–October
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Slow-release vs quick-release
Quick-release (standard granules, liquid)
- +Results visible in 7–14 days
- +Less expensive
- –Risk of scorch if over-applied or applied to dry grass
- –Feeds for 4–6 weeks, then needs repeating
Slow-release (coated granules, mini-prills)
- +Feeds evenly over 8–12 weeks
- +Lower risk of scorch
- +One or two applications per season usually sufficient
- –More expensive per kg
Iron and lawn sand
Lawn sand is a mixture of sharp sand, sulphate of iron, and sulphate of ammonia (nitrogen). It is not strictly a fertiliser but is widely used in spring for two reasons:
- •Iron kills moss — it turns the moss black and weakens it within 2–3 weeks, after which it can be raked out
- •The nitrogen encourages strong grass growth to fill gaps left by the dying moss
- •The sand opens up the soil surface slightly, improving drainage
Iron and application tools
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How to apply
- 1Use a spreader for even coverage. Uneven application causes stripes — over-fed areas can scorch; under-fed areas stay pale.
- 2Calibrate the spreader to the manufacturer's recommended setting. The bag will state grams per square metre — aim for that, not more.
- 3Apply to dry grass but ideally when rain is forecast within 48 hours. If no rain comes, water the granules in to prevent scorch.
- 4Do not apply in direct hot sun or during drought — risk of scorch is significantly higher.
- 5Keep children and pets off the lawn until the product has been watered in and the grass is dry.
When not to feed
- ⚠During drought or when the lawn is visibly dry — fertilising stressed grass causes scorch and makes the problem worse.
- ⚠On frozen or snow-covered ground — nutrients cannot be taken up and will leach away.
- ⚠In late autumn (after October) using a high-nitrogen product — soft growth is vulnerable to frost.
- ⚠Immediately after scarification or heavy work — wait 2–3 weeks for the lawn to recover.
Pale, yellow, or patchy lawn? Photograph it for an AI diagnosis — identify nutrient deficiency, moss, or disease.
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