Quick answer
For new sod in most Northeast soils, the best NPK is a low-phosphorus, potassium-forward ratio — something like 4-2-5 — rather than a traditional high-phosphorus "starter" such as 18-24-12. Potassium supports root establishment and stress tolerance, while keeping phosphorus low respects both the soil biology and, in many Northeast states, fertilizer law.
What each number does for new sod
- N (nitrogen) drives top growth. For new sod you want it present but modest and slow-releasing — enough to carry the establishment window, not so much that the plant pushes blade growth before the roots can support it.
- P (phosphorus) is the number the old "starter" thinking inflates. Phosphorus is nearly immobile in soil and mostly binds up before roots reach it, so more isn't better — and excess phosphorus can suppress mycorrhizal colonization, the partnership that actually extends a young root system's reach for water and nutrients.
- K (potassium) is the underrated one for establishment: it supports root development, water regulation, and tolerance to the drought and cold stress a new lawn faces in its first season. This is why a potassium-forward ratio makes sense under sod.
Why not the classic high-P starter?
Two reasons. First, agronomy: most established Northeast soils already test adequate-to-high in phosphorus (confirm with a soil test), so a high-P number is often wasted. Second, regulation: several Northeast states restrict phosphorus application on lawns except when a soil test shows a deficiency or the lawn is being newly established. A low-P ratio sidesteps both problems.
Where 4-2-5 comes from
Under Sod™ is formulated at 4-2-5 for exactly this: modest slow-release nitrogen, low phosphorus, and a leading potassium number to prioritize roots over top color. It's iron-free by design — iron greens the blade, and the point under new sod is to build the root system first. Alongside the NPK 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, in an SGN 90 granule sized to sit in the root-contact zone. One 25 lb bag per 500 sq ft pallet. See the full product specs.
The one caveat: if a soil test shows your soil is genuinely low in phosphorus, a higher-P starter is appropriate for that specific site. The ratio that's "best" always defers to what your soil test says — 4-2-5 is the right default for the adequate-to-high phosphorus soils that describe most Northeast lawns.
Frequently Asked Questions
What is the best NPK ratio for new sod?+
For most Northeast soils, a low-phosphorus, potassium-forward ratio like 4-2-5 — rather than a traditional high-phosphorus starter such as 18-24-12. Potassium supports root establishment while low phosphorus respects soil biology and state fertilizer law.
Why is potassium (K) important for new sod?+
Potassium supports root development, water regulation, and tolerance to the drought and cold stress a new lawn faces in its first season — which is why a potassium-forward ratio makes sense under sod.
Is a high-phosphorus starter ever the right call?+
Yes, if a soil test shows your soil is genuinely low in phosphorus. The best ratio always defers to your soil test; 4-2-5 is the right default for the adequate-to-high phosphorus soils that describe most Northeast lawns.
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.