Quick answer: what is under-sod fertilizer?
Under-sod fertilizer is a starter applied to the finished soil before the sod is laid, so nutrients and biology sit in the exact zone where new roots have to grow. Under Sod™ is the trademarked product built for that job — an iron-free biological 4-2-5 granular starter with named endomycorrhizae, a *Bacillus* + *Trichoderma* biology package, seaweed, and humic acid, spread at one 25 lb bag per 500 sq ft (one pallet of sod) right before installation.
Why the rooting zone is the only thing that matters
When sod is harvested it loses almost all of its roots. A fresh roll arrives as grass on about half an inch of soil, and it survives on stored energy until it can grow new roots down into the ground beneath it. That thin boundary between the back of the sod and your graded soil — the soil-to-sod rooting zone — is where establishment is won or lost.
Broadcasting fertilizer on top of new sod fights both timing and placement. Granules sitting on the leaf blades have to be watered in, dissolve, and migrate down through the sod layer before roots ever reach them. Putting the feed under the sod flips that: roots meet nutrients and soil biology on day one, exactly where they are trying to anchor.
The old recommendation: "just use a starter fertilizer"
For decades the standard advice was to reach for a high-phosphorus starter — products labeled something like 18-24-12 or 10-20-10 — designed to push fast establishment with a big phosphorus spike. Those products work in the sense that phosphorus supports early root growth, but they were formulated for seeding a lawn from scratch, not for the specific conditions under a sod roll.
| Conventional high-P starter | Under Sod™ approach | |
|---|---|---|
| Designed for | Seeding bare soil | The soil-to-sod rooting zone |
| Phosphorus | Very high (a spike) | Moderate, steady (2%) |
| Soil biology | Usually none | Endomycorrhizae + Bacillus + Trichoderma + seaweed + humic |
| Placement | Broadcast on top, watered in | Spread on soil, sod laid on top |
| Fit near water | Often too hot | Lower-P, iron-free, easier to justify |
The purpose-built answer: a biological under-sod starter
Under Sod™ is formulated around the rooting zone rather than the leaf. The N-P-K is a K-forward, moderate-phosphorus 4-2-5, so there is enough phosphorus to support root initiation without the excess that can suppress beneficial fungi or run off near water — and it is iron-free, because the job is to root the sod, not to green the bag. The biological package does the heavy lifting: named endomycorrhizae (*Rhizophagus irregularis, Funneliformis mosseae, Glomus aggregatum,* and *Glomus etunicatum*) to colonize and extend the new root system, a *Bacillus* consortium (*B. amyloliquefaciens, B. megaterium,* and *B. subtilis*) plus *Trichoderma* (*T. harzianum / T. virens*) to activate the root zone, 2% seaweed extract (*Ascophyllum nodosum*) for natural rooting compounds, and 6% humic acid with added fulvic acid to improve nutrient holding at the soil interface.
Because it is granular at SGN 90 and applied directly to graded soil, it stays exactly where you put it — no relying on a topical granule to wash down through the sod. Read the full breakdown on the product page, or compare it head-to-head in starter fertilizer vs. Under Sod™.
Where Under Sod™ fits
If you are laying sod and want the feed in the rooting zone from day one, Under Sod™ is built for exactly that. It launches Fall 2026 across the Northeast through the CT Sod network. See the full specs or join the notification list to be first to know when it ships.
Always pair fertilizer choices with a soil test. Many states — including New York, New Jersey, Connecticut, and Florida — restrict phosphorus lawn fertilizer, though most rules exempt establishing a new lawn or a soil-test-shown deficiency, which is the sod-installation case. Rules are often stricter near lakes, rivers, and shorelines, so check your local and municipal requirements before applying.
Application rate
How to apply Under Sod™
- Finish your grading and final soil prep so you have a smooth, firm surface ready for sod.
- Spread one 25 lb bag per 500 sq ft — that is one bag for every pallet of sod you are about to lay.
- Leave the granules on the soil surface; do not till them deep.
- Lay the sod directly on top, then water thoroughly to settle the sod and activate the biology.
One bag per pallet keeps the math simple on the job site: count your pallets, count your bags.
Frequently Asked Questions
Do you really put fertilizer under sod?+
Yes. Applying a starter to the finished soil before laying sod places nutrients and soil biology directly in the rooting zone, where new roots grow down. That is the entire idea behind Under Sod™.
What is the best fertilizer to put under new sod?+
A K-forward, moderate-phosphorus biological starter built for the rooting zone — named endomycorrhizae plus a *Bacillus* + *Trichoderma* package and humic acid — rather than a hot high-phosphorus lawn starter. See best fertilizer under sod for the full comparison.
Is Under Sod™ the same as a regular starter fertilizer?+
No. Regular starters are formulated to seed bare soil with a phosphorus spike. Under Sod™ is an iron-free 4-2-5 biological starter formulated specifically for the soil-to-sod interface. Compare them in starter fertilizer vs. Under Sod™.
How much Under Sod™ do I need?+
One 25 lb bag covers 500 sq ft, which is one standard pallet of sod. Count your pallets to count your bags.
Can I use it near the water?+
Its moderate 2% phosphorus and iron-free formula make it easier to justify in phosphorus-restricted and waterfront areas than a hot starter, but you should always confirm local rules and use a soil test. See best fertilizer under sod for waterfront.
When does Under Sod™ launch?+
Fall 2026, starting in the Northeast U.S. through the CT Sod network. Join the notification list for launch and shipping alerts.
Keep reading
Homeowners and installers are already asking what goes under new sod. UNDER SOD™ is the trademarked brand built specifically for that job.
Built for beneath the sod.
A 4-2-5, iron-free biological starter — named mycorrhizae, beneficial bacteria, Trichoderma, seaweed, and humic acid — at one bag per pallet. Launching Fall 2026 across the Northeast.