Quick answer: what to put under sod
Under new sod you want loose, well-graded soil and a biological starter fertilizer worked into the rooting zone — not a hot high-phosphorus blend broadcast on top. Under Sod™ is the product built for that step: a 4-2-5 granular starter with named endomycorrhizae, a *Bacillus* and *Trichoderma* biology package, seaweed, and humic acid (plus fulvic acid), applied to the finished soil at one 25 lb bag per 500 sq ft (one pallet of sod) right before the rolls go down. It is iron-free by design.
Why what goes under the sod matters most
Freshly harvested sod arrives almost rootless — grass on roughly half an inch of soil. It survives on stored energy until it can drive new roots down into the ground beneath it. That thin boundary between the back of the roll and your graded soil is the soil-to-sod rooting zone, and it is the only place those first roots can go.
So the question is not really "what do I spread on the lawn?" — it is "what is waiting in the rooting zone when the roots arrive?" Get that layer right and the sod knits down in days. Get it wrong and the rolls sit on top like a rug, green for a couple of weeks and then struggling.
The old recommendation: bagged topsoil and a high-phosphorus starter
The traditional checklist was a layer of bagged topsoil and a high-phosphorus "starter" — products labeled something like 18-24-12 or 10-20-10 — broadcast on top of the new sod and watered in. The phosphorus spike was meant to push fast roots. But those starters were formulated for seeding bare soil, and a granule sitting on the leaf blades has to dissolve and migrate down through the sod before roots ever meet it.
| Old checklist | What the rooting zone needs | |
|---|---|---|
| Soil | Whatever fill is on site | Loose, graded, de-compacted topsoil |
| Fertilizer type | High-P seeding starter | K-forward biological starter (4-2-5) |
| Soil biology | Usually none | Endomycorrhizae + *Bacillus* + *Trichoderma* + seaweed + humic acid |
| Placement | Broadcast on top, watered in | On the soil, sod laid directly over it |
| Phosphorus | Hot spike | Moderate, steady (2%) |
The better answer: prep the soil, then feed the rooting zone
Two things go under sod, in order. First, prepared soil: relieve compaction, grade smooth and firm, and aim for a few inches of loose topsoil the roots can actually penetrate. Second, a purpose-built biological starter spread on that finished surface before the sod is laid.
Under Sod™ is formulated for exactly that second step. Its K-forward, moderate-phosphorus 4-2-5 ratio supplies enough phosphorus for root initiation without an excess that can suppress beneficial fungi or run off near water — while the raised potassium drives drought, cold, and stress tolerance through the transplant-shock window — and the biology does the heavy lifting: named endomycorrhizae (*Rhizophagus irregularis, Funneliformis mosseae, Glomus aggregatum, Glomus etunicatum*) to colonize and extend the new roots, a *Bacillus* consortium (*B. amyloliquefaciens, B. megaterium, B. subtilis*) and *Trichoderma* (*T. harzianum / T. virens*) to work the root zone, a microbial carbon source to feed them, 2% seaweed extract (Ascophyllum nodosum) for natural rooting compounds, and 6% humic acid (leonardite) plus fulvic acid to hold nutrients at the soil interface. It is iron-free by design — we root the sod, we don't green the bag. At SGN 90 it is granular enough to stay exactly where you spread it. Compare it to a conventional starter in starter fertilizer vs. Under Sod™, or see the full breakdown of the best fertilizer to put under sod.
Where Under Sod™ fits
If you are laying sod and want the feed and biology in the rooting zone from day one, Under Sod™ is built for that moment. It launches Fall 2026 across the Northeast through the CT Sod network. See the full specs or join the notification list to know when it ships.
Pair any fertilizer choice with a soil test. Many states — including New York, New Jersey, Connecticut, Florida, Maryland, Michigan, and Wisconsin — restrict phosphorus lawn fertilizer, but most rules exempt establishing a new lawn or a soil-test-shown deficiency, which is exactly the sod-installation case. Restrictions are often stricter near lakes, rivers, and shorelines, so confirm your local and municipal requirements before applying.
Application rate
How to apply Under Sod™ before laying sod
- Relieve compaction and finish grading so you have loose, smooth, firm soil ready for sod.
- Spread one 25 lb bag per 500 sq ft — 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 rolls 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
What should I put under new sod?+
Loose, well-graded topsoil plus a biological starter fertilizer worked into the rooting zone. Under Sod™ is a 4-2-5 starter with named endomycorrhizae, a *Bacillus* and *Trichoderma* biology package, seaweed, and humic acid made specifically for that layer beneath the roll.
Do I need to put fertilizer under sod?+
It is the single best place to put it. Applying a starter to the finished soil before laying sod puts nutrients and soil biology directly in the rooting zone, where new roots grow down — rather than on top of the leaves, where they have to wash through.
Should I put topsoil under new sod?+
Yes — a few inches of loose, de-compacted topsoil gives roots somewhere to grow. The goal is a smooth, firm surface; then spread the starter on top of that before laying the sod.
Can I just use regular starter fertilizer under sod?+
You can, but most starters are high-phosphorus blends built for seeding bare soil and meant to be broadcast on top. Under Sod™ is a moderate-phosphorus 4-2-5 formulated for the soil-to-sod interface instead. See best fertilizer under sod.
How much do I put under sod?+
One 25 lb bag of Under Sod™ covers 500 sq ft, which is one standard pallet of sod. Count your pallets to count your bags.
Is it safe near water?+
Its moderate 2% phosphorus — and iron-free formulation — makes it easier to justify in phosphorus-restricted and waterfront areas than a hot starter, but 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.