Lawn Nutrition

Why does my grass look greener after a thunderstorm?

A good summer thunderstorm provides a natural fertilizer boost! Lightning breaks atmospheric nitrogen bonds, and the heavy rainwater drives this highly bioavailable nitrogen directly into your soil, causing a sudden flush of green grass. This massive surge of nitrogen and deep watering is also why many landscape plants and shrubs will suddenly push out a new flush of flowers immediately following a storm.

Part of the Learning Center · Natural-First Fertilizing Guide

Lawn Nutrition Guide

Viewing NPK Basics

Understanding the numbers on a fertilizer bag

Every bag of fertilizer displays three bold numbers separated by dashes — for example, 28-5-12. This is the NPK ratio, and it's the most important thing to understand before buying anything.

What the bag numbers mean:

28 Nitrogen (N) %

5 Phosphorus (P) %

12 Potassium (K) %

Example: a 28-5-12 bag is 28% nitrogen, 5% phosphorus pentoxide, and 12% potassium oxide by weight.

Nitrogen (N) — The Green Number

Purpose Nitrogen is the primary driver of green, leafy shoot growth. It's why grass looks lush and dark green after a good feeding.

Too Much Excessive nitrogen — especially quick-release — causes rapid soft growth that is more disease-prone, burns the lawn, and depletes root energy. Streaky brown stripes after application are the classic sign.

Too Little Grass turns pale, yellowish-green. Growth stalls. Fescue especially can look washed out heading into fall without adequate nitrogen.

Phosphorus (P) — The Root Number

Purpose Phosphorus supports root development, energy transfer, and establishment. It's most critical when laying new sod or seeding bare ground.

Too Much Excess phosphorus ties up micronutrients (iron, zinc, manganese) and can damage Centipede grass, which is extremely sensitive to high phosphorus. Most established Triangle lawns have adequate phosphorus from clay — a soil test will tell you for sure.

Too Little Poor root development, especially at establishment. Seedlings and new sod struggle to anchor. A starter fertilizer with moderate phosphorus addresses this.

Potassium (K) — The Stress Number

Purpose Potassium regulates water movement inside the plant and strengthens cell walls. It improves drought tolerance, disease resistance, and cold hardiness — all critical for Triangle lawns.

Too Much Rarely a practical concern at normal rates. Very high K can block magnesium uptake, but this is uncommon with standard lawn fertilizers.

Too Little Grass is more susceptible to heat stress, Brown Patch fungus, and winter injury. K is often under-applied. Look for a fertilizer with meaningful K — like a 28-5-12 or a 15-0-15 for Centipede.

Before applying any nutrients to your lawn, it is highly recommended to perform a soil test to confirm exactly what is needed. Learn how to get a free soil test and read your results in the Soil Testing tab →

Before adding any nutrient, run a soil test — and it's usually free.

Free April 1 – November 30. The NCDA&CS Agronomic Division processes soil tests at no charge when samples arrive at their Raleigh lab between April 1 and November 30.

Small fee December 1 – March 31. During peak season, the charge is around $4 per sample. The fee is based on when your sample arrives at the lab — not when you drop it off at the Extension office — so plan ahead if submitting in late November.

How to submit: Pick up free sample boxes and mailers at your local NC State Extension office (Wake County Extension is at 4001 Carya Dr, Raleigh). Collect soil from 8–10 spots around your lawn, mix, fill the box, and mail or drop off. Results come back in 1–2 weeks with a full recommendation report.

NCDA&CS Agronomic Services Portal →

Soil Testing & Diagnostics

A soil test is a chemical blueprint of your land. In our compacted, acidic native clay, understanding what your soil actually needs is the difference between wasting hundreds on generic fertilizers and feeding your turf precisely what it lacks.

Why soil tests are critical for sod

Sod is grown in highly fertile sandy loam. When harvested, its roots are shaved extremely thin. Installing it onto compacted, highly acidic native North Carolina red clay is a major shock. A soil test confirms if the clay's pH will lock out nutrients, preventing roots from knitting. Adjusting pH beforehand guarantees successful rooting and prevents sod failure.

What about trees, shrubs, and ornamentals?

Unlike turfgrasses which generally prefer a slightly acidic pH (6.0 to 6.5), other plants have very distinct needs. For instance, acid-loving plants (like Azaleas, Camellias, Hydrangeas, and Japanese Maples) thrive in highly acidic soils (pH 5.0 to 5.5). General hardwoods and fruit trees prefer a nearly neutral pH (6.2 to 6.8). Soil tests prevent you from making the fatal mistake of sweetening soil for acid-lovers, or leaving your lawn too acidic.

How to read your soil report

Soil pH (The Master Switch): Measures acidity. If your soil pH is below 5.5, nutrients like nitrogen and phosphorus become chemically locked in the clay — grass roots literally cannot absorb them, no matter how much fertilizer you apply.
P-I & K-I (Phosphorus & Potassium Indices): The NCDA&CS uses an index scale of 0–100+. An index above 50 is optimum. Anything below 25 is severely deficient. Most Triangle clay is naturally high in phosphorus but very low in potassium.
Calcium (Ca) & Magnesium (Mg): Crucial secondary nutrients. They also guide the type of lime you need: low magnesium requires Dolomitic lime, while normal/high magnesium requires Calcific lime.
Humic Matter (HM%): Measures active organic matter. Triangle clay typically has less than 1% HM%. Increasing this through compost and humic acids dramatically improves fertilizer efficiency.
Cation Exchange Capacity (CEC): Your soil's nutrient-holding capacity. Higher CEC means the soil acts as a bigger battery, storing nutrients for longer rather than letting them leach away.

How to correct your soil deficiencies

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Soil pH Balance

Too Acidic (pH < 5.8)

Apply pelletized Lime (cal). Apply Dolomitic lime if Magnesium is low, or Calcific lime if Magnesium is high. Fescue lawns typically need 40–50 lbs of lime per 1,000 sq ft annually in the Triangle to counter native acidity.

Too Alkaline (pH > 7.0)

Apply Elemental Sulfur to lower it. This is rare in native clay but common in beds next to concrete foundations or sidewalks where lime leaches out.

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Phosphorus (P) Level

Deficient (Low P)

Use a starter fertilizer with a high middle number (e.g., 10-20-10 or 18-24-6) or organic bone meal to build root strength during establishment.

Excessive (High P)

Stop applying phosphorus completely! Excess phosphorus runs off into local lakes and streams, causing algae blooms, and blocks iron absorption. Transition to a phosphorus-free blend (e.g., 21-0-11 or 15-0-15).

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Potassium (K) Level

Deficient (Low K)

Apply Sulfate of Potash (0-0-50) or Potash (0-0-60). Potassium is essential for summer heat survival and winter hardiness.

Turf Varieties We Install in the Triangle

This is a conservative baseline for the sod varieties we install. Always confirm with a current soil test. Over-feeding — especially with nitrogen — causes more problems than under-feeding in the Triangle.

Grass	Primary Feeding Window	Approx. N Rate	Notes
Tall Fescue	Sep 15 – Nov 1 (main); light in early Mar	2–3 lbs N / 1,000 ft² / year	No nitrogen June–Aug. Slow-release preferred year-round.
Bermuda	Late May – Aug (active growth only)	3–5 lbs N / 1,000 ft² / year	Feed only after full green-up. Split applications beat one heavy dose.
Zoysia	Jun – Aug	2–3 lbs N / 1,000 ft² / year	Slower growing than Bermuda — less N needed. Avoid late-season feeding.
Centipede	May – Jun (one light application)	0.5–1 lb N / 1,000 ft² / year	Extremely sensitive to over-feeding and phosphorus. Less is more.

Product categories worth knowing

We mention brands below as educational examples — the same way a county extension guide would. We have no commercial relationship with any manufacturer.

Synthetic / professional blends

These offer precise, consistent nutrient ratios and are widely used by lawn care professionals. Look for slow-release coatings for safety and longevity.

Lesco / SiteOne 28-5-12 or 18-0-4: A professional staple across the Southeast, available at SiteOne Landscape Supply in Raleigh and Durham. Polymer-coated nitrogen (PCU) reduces burn risk. The 18-0-4 is useful when phosphorus is already adequate per a soil test.
The Andersons Products: The Andersons manufactures professional slow-release nitrogen (MESA/Mugration), humic acid products (Humic DG), and soil carbon/char amendments (like CharX) to improve turf rooting and moisture retention.
Lebanon Pro: Another professional slow-release brand with regional distribution. Lebanon's 22-0-14 and similar blends are popular for warm-season turf maintenance feeding.

Organic and natural-source options

For a gentler approach, organic sources feed slowly, build soil biology, and carry much lower burn risk. See our Natural-First fertilizing guide for full detail.

Milorganite (6-4-0): Heat-dried biosolids from Milwaukee. A widely available organic slow-release option — sold at most hardware stores in the Triangle. Well-suited for Fescue's fall feeding window and new-sod establishment periods.
Espoma Lawn Tone (3-0-3) and Organic Lawn Booster: All-natural pelletized products designed for easy broadcast spreading. Low NPK numbers mean lower burn risk. Good for light maintenance feeding on established lawns or Centipede.
Neptune's Harvest Fish + Kelp / Down to Earth All-Purpose: Liquid and granular options for those prioritizing soil biology. Best used as part of a broader biostimulant program alongside compost topdressing.

Soil conditioners (not fertilizers, but important)

These improve how your soil holds and delivers nutrients — they work alongside fertilizer, not instead of it.

The Andersons Humic DG: Dispersible granular humic acid from leonardite. Apply at core aeration time for best penetration. Works gradually to improve cation exchange capacity and microbial activity — both of which make your fertilizer dollars go further.
Yucca extract / wetting agents: Products like Primer Plex and Hydretain help water — and the nutrients dissolved in it — penetrate hydrophobic or compacted clay rather than running off. See the yucca section in our fertilizing guide for usage details.
Biochar and Azomite: Biochar improves water and nutrient retention in sandy or heavily amended soils. Azomite supplies trace minerals missing from heavily farmed clay soils. Both are long-term soil investments, not quick fixes.

Common questions

What NPK ratio should I use on Bermuda grass in North Carolina?

For Bermuda in the Triangle, look for a high-nitrogen ratio like 28-5-12 or 32-0-10. Apply only during active summer growth (late May through August) after full green-up. Slow-release nitrogen — coated urea or methylene urea — reduces burn risk and feeds steadily over 8–12 weeks. Avoid feeding in early spring before Bermuda is truly green.

What fertilizer is safe for new sod right after installation?

For the first 30–60 days, keep nutrition minimal. A light starter fertilizer with moderate phosphorus (e.g., 10-20-10 or similar) can support root development on bare soil before laying sod. Once roots knit and you pass the tug test, transition to a balanced slow-release. Never apply quick-release synthetic nitrogen to new sod during heat stress.

Is Milorganite good for Fescue in the Triangle?

Yes — Milorganite (6-4-0) is a popular organic slow-release nitrogen source that works well for Tall Fescue's fall feeding window (late September through early November). Its low-burn profile makes it forgiving. It releases slowly, which matches Fescue's preference for steady, modest nutrition rather than large flushes.

What does NPK mean on a fertilizer bag?

NPK stands for Nitrogen (N), Phosphorus (P), and Potassium (K) — the three primary macronutrients. Nitrogen drives green leaf growth. Phosphorus supports root development and is most important at establishment. Potassium improves drought tolerance, disease resistance, and winter hardiness. A bag labeled 28-5-12 contains 28% nitrogen, 5% phosphorus pentoxide, and 12% potassium oxide by weight.

Can I use the same fertilizer on Zoysia and Centipede?

No — Centipede is extremely sensitive to over-fertilization, especially phosphorus and high nitrogen. It thrives on a 'less is more' approach: one light application of a low-phosphorus fertilizer (like 15-0-15) in early summer is often all it needs. Zoysia can tolerate more nitrogen than Centipede but still far less than Bermuda. Using a Bermuda feeding rate on Centipede is one of the most common ways to damage it.

What are humic acids and do they actually work?

Humic and fulvic acids are organic compounds derived from decomposed plant matter (leonardite). They improve soil structure, increase cation exchange capacity (making nutrients more available), and support beneficial microbial populations. They work best as a long-term soil conditioner rather than an immediate fix. Products like The Andersons Humic DG are commonly applied at aeration time for best soil penetration.

What is slow-release vs. quick-release nitrogen?

Quick-release nitrogen (like ammonium nitrate or urea) dissolves immediately and produces fast green-up, but burns are more likely and the growth flush is short-lived. Slow-release nitrogen — including coated urea (SCU, PCU), methylene urea (MESA), and organic sources like Milorganite — feeds over weeks to months, reducing burn risk and supporting steadier growth. Most premium products blend both for an immediate green-up with lasting feed.