Quick answer: what do mycorrhizae do for new sod?
Mycorrhizae are beneficial fungi that colonize grass roots and grow a vast thread-like network into the soil, effectively extending the root system and ferrying water and phosphorus back to the plant. For freshly laid sod — which arrives almost rootless — that head start matters. Under Sod™ puts live, named endomycorrhizae right in the soil-to-sod rooting zone — alongside a *Bacillus* and *Trichoderma* biology package — at one 25 lb bag per 500 sq ft (one pallet), so the partnership forms from day one.
Why the rooting zone is where mycorrhizae matter
When sod is harvested it leaves nearly its entire root system behind in the field. What arrives is grass on a thin pad of soil that now has to grow a brand-new root system down into the ground beneath it. That thin boundary — the soil-to-sod rooting zone — is the only place those new roots can find partners, and it is exactly where mycorrhizal fungi do their work.
Mycorrhizae form a living partnership with the root: the grass feeds the fungi sugars, and the fungal threads (hyphae) reach into soil pores far smaller and far farther than roots can, pulling back water and nutrients — phosphorus especially. The result is a root system that behaves as if it were many times larger. Place that biology in the rooting zone before the sod goes down and the colonization can begin in the first hours, instead of waiting for fungi to arrive on their own.
The old recommendation: a high-phosphorus starter
For decades the standard move was a high-phosphorus starter — products labeled something like 18-24-12 or 10-20-10 — on the logic that phosphorus drives root growth. Those products were built to seed a lawn from scratch, and the phosphorus spike is meant to push germinating seedlings. The problem under sod is twofold: most graded soil already holds enough phosphorus, and a heavy phosphorus load actively works against the very fungi that build roots.
Here is the mechanism. A plant invests in mycorrhizae mainly to scavenge phosphorus. When soil phosphorus is already high, the grass senses it no longer needs the partnership and reduces colonization — fewer roots get infected, and the fungal network that would have extended the root system never fully forms. So a hot starter can deliver early color while quietly suppressing the biology that would have built a deeper, more resilient root system.
| High-P starter (e.g. 18-24-12) | Under Sod™ (4-2-5 + biology) | |
|---|---|---|
| Designed for | Seeding bare soil | The soil-to-sod rooting zone |
| Phosphorus level | Spike (well above need) | Moderate, steady (2%) |
| Effect on mycorrhizae | Tends to suppress colonization | Leaves room for colonization |
| Living fungi included | No | Yes — named endomycorrhizae |
| Root-system outcome | Shallow, fungi-light | Extended by the fungal network |
The purpose-built answer: live fungi plus moderate phosphorus
Under Sod™ is formulated so the biology can actually establish. The K-forward, moderate-phosphorus 4-2-5 ratio supplies enough phosphorus to support root initiation without the spike that shuts colonization down — keeping the rooting zone in a range where the grass still values its fungal partners, while the raised potassium drives drought, cold, and stress tolerance through the transplant-shock window. Then it adds the partners directly: 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 (from leonardite) plus fulvic acid to condition the soil interface the fungi grow into. It is iron-free by design — we root the sod, we don't green the bag.
Because it is a granular product at SGN 90, applied straight onto graded soil, the inoculant stays exactly where the new roots will be — not stranded on top of the sod hoping to wash down. See the full breakdown on the product page, or read how it compares head-to-head in starter fertilizer vs. Under Sod™.
Where Under Sod™ fits
If you want the mycorrhizal partnership working in the rooting zone from the first day, Under Sod™ is built for exactly that — moderate phosphorus plus live fungi, in one bag per pallet. 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 — it tells you whether your soil even needs added phosphorus, which is the whole point for protecting mycorrhizae. Many states, including New York, New Jersey, Connecticut, and Florida, restrict phosphorus lawn fertilizer, though most rules exempt new-lawn establishment or a soil-test-shown deficiency, which is 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™ so the fungi take hold
- Finish grading and final soil prep so you have a smooth, firm surface 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 in the rooting zone; 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 it simple on the job site: count your pallets, count your bags.
Frequently Asked Questions
What are mycorrhizae and what do they do for grass?+
Mycorrhizae are beneficial soil fungi that colonize grass roots and grow a fine network into the soil. They extend the effective root system, pulling back water and nutrients — especially phosphorus — in exchange for sugars from the plant. For new sod, that means faster, deeper establishment.
Why does too much phosphorus hurt mycorrhizae?+
A plant invests in mycorrhizae largely to find phosphorus. When soil phosphorus is already high, the grass senses it no longer needs the partnership and reduces colonization — so a hot high-phosphorus starter can quietly suppress the very fungi that build a strong root system.
Does Under Sod™ contain live mycorrhizae?+
Yes. Under Sod™ includes live, named endomycorrhizae (*Rhizophagus irregularis, Funneliformis mosseae, Glomus aggregatum, Glomus etunicatum*) — alongside a *Bacillus* and *Trichoderma* biology package, seaweed extract, and humic acid — placed directly in the rooting zone so colonization can begin from the first hours after the sod is laid.
Can I add a mycorrhizal inoculant and still use a high-P starter?+
You can apply an inoculant, but pairing it with a hot phosphorus spike works against it — the excess phosphorus tends to reduce colonization. The point of Under Sod™'s K-forward, moderate-phosphorus 4-2-5 is to keep phosphorus moderate so the fungi can establish.
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.
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.