Quick answer
For new sod on most Northeast soils, a low-phosphorus biological starter is the better choice than a traditional high-phosphorus starter — because most established soils already carry enough phosphorus, and the soil biology (mycorrhizae, beneficial bacteria) does more for root establishment than extra P ever will. A high-phosphorus starter is the right call in one situation only: when a soil test shows your soil is genuinely deficient in phosphorus.
What each one is built to do
A high-phosphorus starter (formulas like 18-24-12 or similar, from brands such as Scotts, Jonathan Green, or LESCO) is built on the older assumption that piling on phosphorus drives root growth. It feeds the new lawn broadly and quickly.
A biological starter takes a different approach: keep phosphorus low, lead with potassium for root establishment and stress tolerance, and add living soil biology — mycorrhizae, beneficial bacteria, humic acid, seaweed — so the young root system can reach water and nutrients on its own instead of depending on a heavy synthetic feed.
Why the high-P assumption breaks down under new sod
Phosphorus is nearly immobile in soil. Most of what you apply binds to soil particles within days and never reaches the roots. On top of that, excess phosphorus can suppress mycorrhizal colonization — the fungal partnership that actually extends a transplanted root system's reach. So on a soil that already tests adequate-to-high in phosphorus (which describes most established Northeast lawns — confirm with a soil test), a high-P starter is largely wasted, and it can work against the very biology that helps new sod knit into the ground.
When the high-phosphorus starter is the right choice
If your soil test comes back genuinely low in phosphorus, a conventional high-P starter is the correct, legal call for a new install — Scotts, Jonathan Green, and LESCO starters all do that job well. The soil test is what decides it. Don't assume; test.
Where Under Sod™ fits
Under Sod™ is the biological side of this comparison, purpose-built for the root-contact zone under new sod: 4-2-5 (low phosphorus, potassium-forward), iron-free, with mycorrhizae, *Bacillus* and *Trichoderma*, humic and fulvic acid, and seaweed extract, in an SGN 90 granule. You apply it to the prepared soil before the sod goes down — one 25 lb bag per 500 sq ft pallet. It's the low-P biological option for the adequate-to-high-phosphorus soils that describe most Northeast lawns. See the full product specs.
| Factor | High-phosphorus starter | Biological starter (Under Sod™) |
|---|---|---|
| Phosphorus | High (e.g. 18-24-12) | Low (the 2 in 4-2-5) |
| Potassium | Not the emphasis | Potassium-forward (the 5) |
| Soil biology | None | Mycorrhizae, *Bacillus*, *Trichoderma*, humic/fulvic, seaweed |
| Iron | Varies | Iron-free by design |
| Best fit | Soil that tests low in phosphorus | Adequate-to-high phosphorus soils (most Northeast lawns) |
This is general guidance, not a substitute for a soil test. Several Northeast states restrict phosphorus lawn fertilizer but typically exempt new-lawn establishment or a soil-test-shown deficiency. Confirm your local rules and test your soil before choosing.
Frequently Asked Questions
Is a biological or a high-phosphorus starter better for new sod?+
On most Northeast soils, a low-phosphorus biological starter is the better choice, because established soils usually already carry enough phosphorus and the soil biology does more for root establishment than extra P. A high-phosphorus starter is better only when a soil test shows a genuine phosphorus deficiency.
Does high phosphorus hurt soil biology?+
It can. Phosphorus is nearly immobile and mostly binds up before roots reach it, and excess phosphorus can suppress mycorrhizal colonization — the fungal partnership that extends a young root system's reach.
When should I use a high-phosphorus starter?+
When a soil test comes back genuinely low in phosphorus. In that case a conventional high-P starter from a brand like Scotts, Jonathan Green, or LESCO is the correct, legal call for a new install.
Under Sod™ launches Fall 2026
Under Sod™ launches in the Northeast in Fall 2026. 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.