What is the best time of year to fertilise a lawn?

For cool-season turf the most valuable application of the year is the early-autumn one, which builds root reserves for winter. For warm-season turf it is the late-spring application after full green-up. In both cases soil temperature at 50 mm decides, not the calendar.

Can I apply all the nitrogen at once in spring?

You can, but you will waste most of it. A single 15 g per square metre application on a cool-season lawn loses much of its nitrogen to leaching and volatilisation and produces a soft growth flush that adds thatch. Split it into three applications and the same product produces a better lawn.

Why is my lawn yellow after I fertilised it?

Either salt burn from overlapping the spreader, which shows as stripes, or iron deficiency, which shows as yellow leaf with green veins. Burn recovers in two to three weeks with watering; iron deficiency needs ferrous sulphate, not more nitrogen.

Do I need a soil test?

Yes, once every three years, for pH and phosphorus. Nitrogen is not usefully measured in a routine soil test. Lime and phosphate decisions should never be made without the numbers.

operation2026-09-23By The Turf Deskfertilizing

Nitrogen: split ratios, single-application caps and the soil temperature trigger

Parameter sheet

Operation

fertilizing

Implement

Rotary broadcast spreader for granular products, calibrated on a hard surface. Backpack or hose-end sprayer for liquid urea or iron. A soil thermometer for the trigger.

Season window

Cool-season turf is fed in two main windows, late spring after the flush has slowed and early autumn, with a smaller early-summer application. Warm-season turf is fed from full green-up in late spring through midsummer, and stops eight weeks before the first expected frost. The trigger is soil temperature at 50 mm, not a date.

Frequency

Three to four applications a year on a home lawn, which is what a split ratio is for. A single annual application of 15 g per square metre loses most of its nitrogen to leaching and volatilisation and pushes a flush of growth that feeds thatch.

Annual nitrogen, cool-season lawn

Annual nitrogen, warm-season lawn

Annual nitrogen, low-input fescue

Maximum in a single application

Cool-season split ratio

Warm-season split ratio

Iron for colour without growth

Warm-season stop date

10 to 20g per square metre

3 to 6g per square metre

5g per square metre

40:30:30 or 45:20:35 across spring, summer, autumnratio

40:35:25 or 45:30:25ratio

2g per square metre

8 weeks before the first expected frostlead time

Prerequisites

Calibrate the spreader on a hard surface over a 1 square metre sheet, weighing the product collected, not reading the dial.
Water 6 to 10 mm after granular application, or apply immediately before forecast rain of the same amount. Urea left on dry leaf volatilises.
Soil test first if the lawn has never been tested: pH and phosphorus decide whether the nitrogen you buy will do anything.
Mow and remove clippings before applying, so the granules reach the soil surface instead of sitting in the canopy.

Do not run it when

Do not apply nitrogen when soil temperature at 50 mm is above 27 degrees Celsius on a cool-season lawn. The roots are shutting down and the leaf growth it produces is disease-prone.
Do not apply on frozen ground or to a lawn that is in drought dormancy; the nitrogen will not be taken up and will run off.
Do not apply within 2 metres of open water, or on a slope above one in ten without a buffer strip, whatever the product label claims about being slow release.
Do not apply high-nitrogen product to a low-input fine fescue lawn. Above 6 g per square metre per year that lawn thins and goes diseased, not greener.

Calibration

Weigh 100 g of product, spread it over a measured 10 square metre area, then weigh what is left to confirm the drop rate.
Make two passes at right angles at half the target rate each, rather than one pass at the full rate. Striping is the most common cause of burned patches.
Check the spread pattern with a tray test: 6 to 8 trays in a line, one pass, and measure what each tray caught. An uneven pattern means the ports need cleaning.
Rinse the spreader over a lawn area, never over a driveway drain.

Schematic for the operation Nitrogen programme with split applications — Operation section and season window, generated from this page

Nitrogen is the only nutrient you will apply every year, and the only one where the timing matters as much as the total. The method is an annual budget per species, a split ratio that puts the weight where the roots can use it, and a cap of 5 g per square metre in any single application (per Purdue Extension).

Setting the annual budget by species

The range across the species in this library is 3 to 20 g of nitrogen per square metre per year, a factor of nearly seven. Buffalograss needs 3, low-input fine fescues 5 to 6, centipedegrass 6, zoysia 10, Kentucky bluegrass and tall fescue 12 to 15, hybrid bermudagrass 20.

Applying a bluegrass programme to a buffalograss lawn is the most common fertiliser error in a low-water garden, and the damage is not visible immediately: the extra nitrogen pushes leaf growth, which increases water demand, which in a no-irrigation garden means the lawn goes into drought dormancy earlier and recovers later.

A soil test every three years is enough to track pH and phosphorus (per Purdue Extension). Nitrogen is not usually included in a standard test because it moves too fast in the soil to be a useful reading; use the species budget and the soil temperature trigger instead.

Annual nitrogen budget and split ratio by species

Species	Annual N	Split ratio	Single application cap
Hybrid bermudagrass	20 g per square metre	40:35:25	5 g per square metre
Kentucky bluegrass	15 g per square metre	40:30:30	5 g per square metre
St. Augustinegrass	15 g per square metre	40:35:25	5 g per square metre
Tall fescue, perennial ryegrass	12 to 14 g per square metre	45:20:35	5 g per square metre
Zoysia, colonial bentgrass	10 g per square metre	35:30:35	5 g per square metre
Centipedegrass, fine fescues	5 to 6 g per square metre	50:15:35	3 g per square metre

Why the cap is 5 g in a single application

Above roughly 5 g of nitrogen per square metre in one pass, cool-season turf takes up less than half of what is applied. The rest is lost to leaching below the root zone, to volatilisation if it is urea left on a dry surface, and to the growth flush it triggers, which is soft leaf that feeds thatch rather than roots.

The cap also protects against burn. Soluble nitrogen at 40 g per square metre in a single application will kill leaf tissue outright through salt injury. The injury appears as a striped or patchy brown pattern within 48 hours, exactly where the spreader overlapped.

Split into three or four applications the same annual total produces a steadier growth rate, a deeper root system, and no burn risk. The nitrogen bill is identical; only the labour differs, and a spreader pass takes fifteen minutes.

The soil temperature trigger, which replaces the calendar

Cool-season grass starts taking up nitrogen efficiently when soil temperature at 50 mm is above 10 degrees Celsius and stops when it falls below 5. The late-spring application belongs in the window after the flush has slowed but before soil temperature reaches 24 degrees Celsius, because nitrogen applied at the peak of the flush feeds leaf, not roots.

For warm-season turf the trigger is the other side: apply from full green-up, which is soil temperature above 18 degrees Celsius at 50 mm, and stop eight weeks before the first expected frost. Late nitrogen on a warm-season lawn produces soft growth killed by the first cold night, and on zoysia and centipedegrass that is the main route to winter kill.

A 150 mm soil thermometer costs less than one bag of fertiliser and replaces every date rule in this card.

Iron, slow release and the products that are worth the money

Iron at 2 g per square metre of ferrous sulphate deepens colour for about three weeks without pushing leaf growth. Where a lawn looks pale at full nitrogen, iron is usually the missing element, particularly above pH 6.5 where iron locks up. It stains paving, so water it in immediately.

Slow-release nitrogen is worth a premium where the risk of leaching or burn is real: sandy soils, slopes, and lawns where you can only make two applications a year. On a clay loam with a three or four application programme, plain urea or ammonium sulphate does the same job for less.

Do not buy a product for its phosphorus content. On an established lawn, phosphorus is needed only where a soil test shows it below 10 mg per litre, and on most soils the 30 percent in a starter fertiliser is a waste and a runoff risk.

Questions that come up on this page

What is the best time of year to fertilise a lawn?: For cool-season turf the most valuable application of the year is the early-autumn one, which builds root reserves for winter. For warm-season turf it is the late-spring application after full green-up. In both cases soil temperature at 50 mm decides, not the calendar.
Can I apply all the nitrogen at once in spring?: You can, but you will waste most of it. A single 15 g per square metre application on a cool-season lawn loses much of its nitrogen to leaching and volatilisation and produces a soft growth flush that adds thatch. Split it into three applications and the same product produces a better lawn.
Why is my lawn yellow after I fertilised it?: Either salt burn from overlapping the spreader, which shows as stripes, or iron deficiency, which shows as yellow leaf with green veins. Burn recovers in two to three weeks with watering; iron deficiency needs ferrous sulphate, not more nitrogen.
Do I need a soil test?: Yes, once every three years, for pH and phosphorus. Nitrogen is not usefully measured in a routine soil test. Lime and phosphate decisions should never be made without the numbers.

Sources

The public reference material the figures on this page were checked against:

Annual nitrogen budget15 g·m⁻²

Irrigation per week20 mm

recomputed on these two numbers · species profiles inside ±15%: 8/12 · operation cards touched: 6/7

Kentucky bluegrass, Turf-type tall fescue, Hybrid bermudagrass (Tifway 419), Perennial ryegrass, Zoysiagrass (Empire), St. Augustinegrass (Floratam), Common bermudagrass (seeded), Centipedegrass

Mowing to a set height, Hollow-tine core aeration, Nitrogen programme with split applications, Overseeding and slit-seeding, Irrigation to a deficit or ET replacement schedule, Scarifying and thatch removal

bench heat, dim to bright: 0 / 4 / 4 / 4 / 0

Adjacent entries

Symptom reading table

Four field symptoms, each read back to the parameter that produced it rather than to a product. The last column is the one number that row turns on, with the threshold drawn on it.

Reading	What you see	Chain of causes	Parameter correction	Field confirmation	Scale
Bare patchesreleasedflag the species on the bench →	Bare patches and thin, worn ground recovery window 3–10 wk	Cutting height has been held below the species floor, thinning the stand from the centre of the lawn outwards until the soil is exposed. Concentrated traffic along a path, at a gate or in front of a goal has worn through the canopy to soil, and the species has no rhizomes to repair it. Drought dormancy has gone past the survival limit, four to six weeks for cool-season turf and six to eight for warm-season, and the crowns have died. A fungal patch disease, large patch on warm-season turf or summer patch on cool-season, has killed the crowns in a ring pattern. Grub or chafer larvae have eaten the roots under the patch, and the turf lifts away from the soil like a carpet when pulled. A spill of fertiliser, petrol, herbicide or a concentrated dog urine patch has killed the leaf and the crown together.	Raise the cutting height to the species band: 25 to 40 mm for Kentucky bluegrass, 30 to 50 for perennial ryegrass, 50 to 70 for tall fescue, 40 to 60 for fine fescue. Scarify or core-aerate the bare area to 75 mm depth so the seed has soil contact rather than a compacted surface. Seed at the species rate: 15 g per square metre of Kentucky bluegrass, 30 of perennial ryegrass, 35 of tall fescue, 20 of fine fescue. Where the area is over 1 square metre, use a blend rather than a single species. Where the species has no rhizomes, which is tall fescue, fine fescue and St. Augustinegrass, the only repair is seed or plugs. There is no parameter change that will fill the patch. For a dog urine patch, water 25 mm immediately and leach the salts below the root zone, then reseed. Do not add nitrogen; the patch is already salt-burned. For grubs, confirm by lifting the turf and finding 10 or more larvae per square metre, then treat; below that threshold the lawn recovers on its own. Replace the traffic. A worn path worn again and again needs a hard surface or a paving strip, not another reseed.	Pull a handful of turf at the edge of the patch. If it lifts like a carpet with no roots attached, the cause is grubs or a root disease; if it resists, the cause is surface wear or drought. Look for a ring pattern. Large patch and summer patch kill in expanding rings with a green centre or a green edge, which wear and drought do not produce. Check the cutting height against the species band; scalping produces a uniform thinning across the whole lawn rather than isolated patches. Dig a 100 mm deep spadeful at the edge of the patch and check for larvae, white fungal mycelium, or a hard compacted layer at 50 mm.	4 wk weeks to close
Mossreleasedflag the species on the bench →	Moss colonising thin or shaded turf recovery window 8–24 wk	The surface stays wet: compacted soil, a thatch layer over 15 mm, or a hollow that collects water keeps the top 20 mm saturated for most of the year. Light has fallen below the compensation point of the grass, usually under four hours of direct sun, so the turf thins and stops competing. Soil pH has fallen below 5.5, which suppresses the grass and the bacteria that decompose thatch while moss tolerates the acidity. The cutting height has been held below the species floor for long enough to thin the stand, opening the surface to colonisation. Nitrogen has been below the species budget for several seasons, or above it, in both cases leaving a stand too weak to close the surface. Soil has been compacted by traffic to the point where infiltration drops below 5 mm an hour and every rain sits on the surface.	Core-aerate to 75 mm depth at 50 to 100 mm spacing, two passes at right angles, to break the compacted layer and restore infiltration. Scarify 5 mm in early autumn where thatch exceeds 15 mm, then overseed at the species rate; a thin stand is what moss is colonising. Raise the cutting height to the top of the species band. In shade that means 60 mm for fine fescue and 70 mm for tall fescue, not 30 mm. Switch the species to creeping red fescue, shade rating 5 of 5, where direct sun is under four hours. Moss tolerates shade; most lawn grasses do not. Apply lime only if a soil test reads below pH 5.5, at 200 to 250 g per square metre on a loam, and retest after 12 months. Do not lime on the assumption that moss means acid. Bring the annual nitrogen to the species budget in a shaded lawn, which is a third less than in full sun: fine fescue 4 to 6 g per square metre, tall fescue 8. Improve drainage where water stands: a French drain, a surface swale, or the removal of a compaction pan at 150 to 200 mm.	Push a screwdriver into the soil. If it takes two hands and stops at 50 mm, compaction is the driver and aeration is the first action. Time how long a 100 mm ring filled with water takes to drain. Under 25 mm an hour is compacted; under 5 mm an hour is severely compacted. Map the sun: note how many hours of direct sun the mossy area receives in midsummer, not in spring. Under four hours, no grass species will win without changing the light. Test the soil pH. Moss tolerates a wide range, so pH is only sometimes the driver and should never be assumed. Read the thatch depth with a knife and ruler; above 15 mm the mat is holding the moisture that moss needs.	pH 5.5 soil pH
Thatch over 15 mmreleasedflag the species on the bench →	Thatch layer 15 mm or thicker recovery window 4–12 wk	Nitrogen has been applied above the species budget for several seasons, producing more lignin-rich stem and stolon tissue than the soil can decompose. Clippings have been bagged for years on a lawn that is now below 10 mm of thatch, removing the nitrogen the microbes need to break the mat down. Soil pH has drifted below 5.5, collapsing the bacterial population and shifting decomposition to slower fungal pathways. The lawn is a species with a thatch tendency of 5 out of 5 and has never been scarified. Irrigation is frequent and shallow, keeping the surface constantly moist and the mat compressed rather than allowing it to dry and fracture.	Scarify 5 mm, in early autumn when soil temperature at 50 mm is between 15 and 21 degrees Celsius, with the tines set to just graze the mat. Overseed the same day at the full species rate, 10 to 15 g per square metre for Kentucky bluegrass, 25 to 35 for perennial ryegrass, 30 to 40 for tall fescue. Topdress 1 to 2 mm of sand-loam matched to the root zone, then water 6 to 10 mm a day for ten days. Cut the annual nitrogen to the species budget: 15 g per square metre for Kentucky bluegrass, 10 for zoysia, 6 for centipedegrass and fine fescues. Raise soil pH into the species band with 200 to 250 g per square metre of lime on a loam, applied in the following autumn and retested after 12 months. Switch to mowing at the top of the species band and mulch the clippings, which returns 1 to 2 g of nitrogen per square metre per year.	Cut a 100 mm square flap of turf with a knife, lift it like a hinge and read the brown fibrous layer between the green leaf bases and the dark soil with a ruler. Do it in three places. Push a screwdriver into the profile; if it stops in the mat before reaching soil, the layer is thick and hydrophobic. Watch a hose test: water beading and running off a dry lawn instead of soaking in is the signature of a hydrophobic thatch layer. Check for the crown being lifted above the soil line by parting the grass with both hands; crowns 15 to 20 mm above soil dry out in two days.	15 mm mat depth
Broadleaf weedsreleasedflag the species on the bench →	Broadleaf weed species as indicators of soil and management conditions recovery window 8–30 wk	Compacted soil where the taproots of dandelion and plantain can penetrate but grass roots cannot, so the weeds have an exclusive hold. Cutting height below the species floor, thinning the canopy and letting light reach the soil surface where weed seed germinates. Soil pH outside the species band, which weakens the grass while the weeds in question tolerate the reading. Nitrogen below the species budget for several seasons, producing an open, pale stand that clover and speedwell colonise readily. Surface moisture that stays high, favouring species such as creeping buttercup and moss-adjacent weeds that indicate poor drainage. A thatch layer over 15 mm that keeps seed in contact with moisture at the surface, which is what crabgrass and goosegrass need to germinate.	Core-aerate to 75 mm depth at 50 to 100 mm spacing wherever dandelion, plantain or knotgrass dominate; the taproot species are the compaction indicators. Raise the cutting height to the species band and hold it for a full season. Density, not herbicide, is what keeps weed seed from establishing. Bring the annual nitrogen to the species budget: 15 g per square metre for Kentucky bluegrass, 12 for tall fescue, 6 for fine fescue. Clover and speedwell indicate a lawn that has been underfed. Correct the soil pH to the species band: 200 to 250 g per square metre of lime on a loam to raise half a unit, applied in early autumn. Scarify 5 mm where thatch exceeds 15 mm and remove the mat, which is the germination bed for summer annual grasses. Where drainage is the driver, indicated by creeping buttercup or rushes, add a French drain or a surface swale before any other measure. Overseed at the species rate in early autumn to raise density. A stand at 80 percent cover suppresses new weed germination; a stand at 50 percent does not. Where chemical control is used, treat only the species present, at the label rate, when the weed is actively growing and air temperature is between 15 and 25 degrees Celsius.	Read the indicator species: dandelion, broadleaf plantain and knotgrass indicate compaction; clover and speedwell indicate low nitrogen; creeping buttercup and rushes indicate poor drainage; sorrel and sheep sorrel indicate acidity. Push a screwdriver into the soil where the weeds are densest. Two hands and a stop at 50 mm confirms compaction. Count the stand: lay a 300 mm square frame in three places and count grass tillers per square metre. Below 3,000 tillers per square metre allows weed establishment. Measure the canopy closure by looking straight down in midday sun: if more than 15 percent of the view is soil, the stand is too open. Soil test for pH and phosphorus; weed composition is a hint, not a measurement.	15 g annual N