Quick answer
Under new sod, apply a low-phosphorus starter fertilizer to the prepared, graded soil before the sod is laid — so the freshly cut roots make immediate contact with nutrients and soil biology instead of waiting weeks for a surface feeding to reach them. For most Northeast lawns a low-phosphorus biological starter is the right choice; a high-phosphorus starter is only the better call when a soil test shows the soil is genuinely deficient in phosphorus.
Why "under" the sod matters
When sod is harvested, the roots are cut. Those cut roots need to bridge into your native soil fast, and they do it best when the nutrients and biology they need are already blended through the top 4 inches of prepared topsoil they will root into during the first season, up to 6 where the topsoil lift is deeper.
Why low phosphorus, not a traditional high-P "starter"
The old rule was to reach for a high-phosphorus "starter" — something like 18-24-12 — on the theory that phosphorus drives root growth. But phosphorus is nearly immobile in soil; most of it binds to soil particles within days and never reaches the roots. Meanwhile, most established Northeast soils already test adequate-to-high in phosphorus (confirm your own site with a soil test), so adding more does little — and excess phosphorus can suppress the mycorrhizal partnerships that actually help young roots reach water and nutrients on their own.
What to use instead
A biological starter built for the root-contact zone: low phosphorus, potassium-forward for root establishment and stress tolerance, plus the soil biology (mycorrhizae, beneficial bacteria, humic acid, seaweed) that helps a transplanted root system connect to your soil.
Where Under Sod™ fits
Under Sod™ is a 4-2-5 biological product made specifically for this — iron-free by design, in a fine SGN 90 granule. It carries named endomycorrhizae (*Rhizophagus irregularis, Funneliformis mosseae, Glomus aggregatum, Glomus etunicatum*), a *Bacillus* consortium (*B. amyloliquefaciens, B. megaterium, B. subtilis*) and *Trichoderma* (*T. harzianum / T. virens*), 2% seaweed extract (*Ascophyllum nodosum*), and 6% humic acid (leonardite) plus fulvic acid. One 25 lb bag covers one 500 sq ft pallet: spread it over the prepared topsoil and mix it into the top 4 inches with a tiller, harrow, or power rake, up to 6 inches where the topsoil lift is deeper, finish-grade, lay the sod the same day, roll, and water.
The honest exception: test your soil first — it's inexpensive and it answers the only question that changes the answer. If your soil genuinely tests low in phosphorus, a conventional starter with phosphorus is the right, legal call. If it tests adequate-to-high — which describes most Northeast lawns — skip the high-P starter and use a low-P biological starter instead.
Frequently Asked Questions
What kind of fertilizer goes under new sod?+
A low-phosphorus, potassium-forward biological starter applied to the prepared soil before the sod is laid. Under Sod™ is a 4-2-5 starter with named endomycorrhizae, a Bacillus and Trichoderma biology package, seaweed, and humic acid built for that root-contact zone.
Should you use a high-phosphorus starter under new sod?+
Only if a soil test shows your soil is genuinely low in phosphorus. Most established Northeast soils already test adequate-to-high, so a high-P starter is often wasted and can suppress the mycorrhizal colonization that helps new roots establish.
Do you put the fertilizer down before or after laying sod?+
Before. Mix Under Sod into the top 4 inches of prepared topsoil before the sod is laid, up to 6 where the topsoil lift is deeper. It is applied once and is not reapplied. See fertilizer before or after laying sod.
How much do you need?+
One 25 lb bag of Under Sod™ covers one 500 sq ft pallet — one bag per pallet. See how much starter fertilizer per pallet.
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.