Quick answer: the best fertilizer under sod in New Hampshire
In New Hampshire, the smartest thing to put under new sod is a low-phosphorus biological starter worked into the rooting zone — not a hot high-phosphorus lawn starter that the state's fertilizer law (RSA 431:4-b) restricts on established turf and that new sod can't use fast enough anyway. Under Sod™ is built for exactly this: an iron-free 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) on the graded soil before the sod goes down.
Why this matters in New Hampshire
Freshly harvested sod arrives almost rootless on a thin pad of soil. It has days, not weeks, to push new roots down into the ground beneath it, and whatever is sitting in that soil-to-sod rooting zone is what those first roots find. Across New Hampshire that zone is usually well-drained glacial till or river-valley outwash sitting over acidic, granite-derived ground, and on all of it, feed placed at the interface does far more for a new install than a granule broadcast on top.
There is a second reason to keep phosphorus modest here. New Hampshire is a state of water — Lake Winnipesaukee and hundreds of smaller lakes and ponds inland, and Great Bay on the coast — where phosphorus (in fresh water) and nitrogen (in the estuary) drive the algae and water-quality problems the state works hard to prevent. Feeding the rooting zone directly, with a moderate 2% phosphorus, keeps nutrients where the roots are and lowers what can wash toward the nearest lake or the bay.
New Hampshire soils under new sod
New Hampshire soils are glacial and firmly granite country. The state's representative soil, the Marlow series (established in 1939 and named for the town of Marlow), is a well-drained loam formed in dense glacial till over schist, gneiss, and granite — acidic, with a natural pH commonly in the 4.5–6.0 range, well below what cool-season turf prefers. In the river valleys — the Merrimack and Connecticut corridors — the ground shifts to sandy, fast-draining outwash, while the uplands and White Mountain slopes run thin and shallow to bedrock.
That acidity matters for phosphorus specifically: in acidic soil, phosphorus binds tightly to aluminum and iron and becomes less available to roots, so simply adding more phosphorus is an inefficient fix. A feed also behaves very differently on leachy valley sand than on a shallow upland till, but on both the same principle holds — keep the nutrients and biology at the soil-to-sod interface, held there rather than washing through or perching on top. Living mycorrhizae that extend the root system and scavenge bound phosphorus do more for real uptake than a bigger P number on the bag — and you should still lime to your soil test.
What New Hampshire soil tests actually say about phosphorus
New Hampshire is the regional outlier on soil testing: the UNH soil testing program uses the Mehlich-3 extractant, not the Modified Morgan that most other New England land-grant labs use, so a New Hampshire report is read on its own scale and is not interchangeable with a Massachusetts or Maine result. Whatever the extractant, the logic is the same — once available phosphorus reaches the optimum range, the recommendation is to add no more. After decades of past fertilizing, many established New Hampshire lawns test at or above optimum, which is why the honest move is to soil-test the graded surface first rather than assume a new install needs a phosphorus spike.
The old recommendation: "grab a high-phosphorus starter"
For years the default sod-install advice was a high-phosphorus starter — products labeled something like 18-24-12 or 10-20-10 — on the theory that a big phosphorus spike drives roots. Those blends were designed to seed a bare lawn from scratch, not to feed the interface beneath a sod roll. In New Hampshire the fit is looser still: graded soils often already test at or above optimum, the spike can leach through valley sand or wash toward a lake, and — even though establishing turf is one of the exceptions the fertilizer law allows — feeding phosphorus the soil doesn't need is exactly what those rules are written to discourage.
| 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%) |
| Vs. typical NH soil P | Overshoots the optimum | Matches a real, tested need |
| Near lakes & Great Bay | Prone to run off | Held at the interface by humic acid |
| Soil biology | Usually none | Endomycorrhizae + Bacillus + Trichoderma + seaweed + humic |
| Fit with NH turf-fertilizer law | Often restricted | Lower-P, iron-free, easier to justify |
The better recommendation: a purpose-built biological under-sod starter
A starter built for the rooting zone fits New Hampshire far better than a generic high-P blend. Under Sod™ carries a K-forward, moderate-phosphorus 4-2-5 N-P-K — enough phosphorus to support root initiation without an excess that leaches or runs off, with the raised potassium (5%) driving the cold-, drought-, and transplant-stress tolerance a Granite State lawn leans on — plus the biology that does the real work: 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 (leonardite) with added fulvic acid to improve nutrient holding at the soil interface. It is iron-free by design — the goal is to root the sod, not to green the bag.
It is the right fit for the cool-season turf grown and laid across New Hampshire — Kentucky bluegrass, tall and fine fescues, and perennial ryegrass blends built to survive hard northern winters and freeze-thaw cycles. Because it is granular at SGN 90 and applied directly to graded soil, it stays where you put it instead of relying on a topical granule to wash down through the sod. Read the full breakdown on the product page, or see the broader best fertilizer to put under sod comparison.
Where Under Sod™ fits
If you are laying sod anywhere from the Seacoast and Great Bay to the Lakes Region and the Merrimack Valley and want the feed 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 to New Hampshire.
Always pair fertilizer choices with a soil test. New Hampshire restricts phosphorus in turf fertilizer under RSA 431:4-b, which was amended effective January 1, 2025 and is administered by the Department of Agriculture, Markets & Food. Fertilizer sold for established turf must be effectively no-phosphate (available phosphate no higher than 0.5%) unless it is labeled for a newly established or repaired lawn, or for a lawn a soil test shows is deficient in phosphorus — the sod-installation case. Near-water and frozen-ground limits also apply. Under Sod's moderate 2% phosphorus and iron-free formula fit the establishment exemption, but always confirm current state and municipal requirements before applying.
Application rate
How to apply Under Sod™ on a New Hampshire install
- Soil-test the graded surface first — note that UNH uses the Mehlich-3 extractant, so read your result on that scale, and confirm whether any added phosphorus is warranted.
- Finish grading and final soil prep so you have a smooth, firm surface — clear the rocks and root debris common in New England fill.
- 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
What is the best fertilizer to put under new sod in New Hampshire?+
A low-phosphorus biological starter placed in the rooting zone — not a hot high-phosphorus lawn starter that New Hampshire's fertilizer law (RSA 431:4-b) restricts on established turf. Under Sod™ is an iron-free 4-2-5 biological starter built for the soil-to-sod interface and suited to New Hampshire's acidic, granite-derived soils.
Does New Hampshire restrict phosphorus lawn fertilizer?+
Yes. RSA 431:4-b (amended effective January 1, 2025) requires fertilizer for established turf to be effectively no-phosphate unless it is labeled for a newly established or repaired lawn, or for a lawn a soil test shows is phosphorus-deficient — which covers a sod install. Near-water and frozen-ground limits also apply, so confirm current rules and use a soil test.
What soil test does New Hampshire use?+
The UNH soil testing program uses the Mehlich-3 extractant, which is different from the Modified Morgan test most other New England land-grant labs use — so a New Hampshire result is read on its own scale and is not interchangeable with a neighboring state's. Whatever the method, once phosphorus reaches optimum the guidance is to add no more.
What is New Hampshire's representative state soil?+
The Marlow series, a well-drained loam formed in glacial till over schist, gneiss, and granite, established in 1939. It is naturally acidic (commonly pH 4.5–6.0), which is one reason phosphorus binds up and mycorrhizae matter more than a high P number.
Does Under Sod™ work with New Hampshire grass types?+
Yes. It is formulated for the cool-season turfgrasses grown and laid across New Hampshire — Kentucky bluegrass, tall and fine fescues, and perennial ryegrass blends that handle hard northern winters.
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 in New Hampshire?+
Fall 2026, as part of the Northeast rollout 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.