Quick answer
Scotts Turf Builder Starter is a high-phosphorus synthetic starter built to feed a new lawn broadly; Under Sod™ is a low-phosphorus, potassium-forward biological starter built specifically for the root-contact zone beneath new sod. On most Northeast soils — which already test adequate-to-high in phosphorus — the low-P biological approach fits new sod better. Scotts' high-P starter is the better pick when a soil test shows a phosphorus deficiency.
Scotts Turf Builder Starter, fairly
It's a well-known, widely available synthetic starter formulated with a high phosphorus number to feed new grass — seed or sod — across the board. It's easy to find, consistent, and does what it's designed to do: deliver a broad synthetic feed to a new lawn. For a homeowner whose soil genuinely tests low in phosphorus, it's a sound, inexpensive choice.
Under Sod™, and how it differs
Under Sod™ is built around a different job — not "feed the new lawn broadly," but "give the freshly cut roots what they need in the exact zone they land." That leads to four concrete differences:
- Phosphorus: Under Sod™ is low-P (the 2 in 4-2-5); Scotts Starter is high-P. On adequate-to-high phosphorus soils, low-P is the better match.
- Potassium: Under Sod™ is potassium-forward (the 5) for root establishment and stress tolerance; that's not the emphasis of a traditional starter.
- Biology: Under Sod™ carries living soil biology — mycorrhizae, *Bacillus*, *Trichoderma*, humic and fulvic acid, seaweed — to help roots reach water and nutrients on their own. A synthetic starter feeds; it doesn't inoculate.
- Iron: Under Sod™ is iron-free by design, because the job under new sod is roots first, not blade color.
| Factor | Scotts Turf Builder Starter | Under Sod™ |
|---|---|---|
| Type | Synthetic feed | Biological starter |
| Phosphorus | High | Low (the 2 in 4-2-5) |
| Potassium | Not the emphasis | Potassium-forward (the 5) |
| Soil biology | None — feeds, doesn't inoculate | Mycorrhizae, *Bacillus*, *Trichoderma*, humic/fulvic, seaweed |
| Best fit | Soil that tests low in phosphorus | Adequate-to-high phosphorus soils (most Northeast lawns) |
Factor
Type
Scotts Turf Builder Starter
Synthetic feed
Under Sod™
Biological starter
Factor
Phosphorus
Scotts Turf Builder Starter
High
Under Sod™
Low (the 2 in 4-2-5)
Factor
Potassium
Scotts Turf Builder Starter
Not the emphasis
Under Sod™
Potassium-forward (the 5)
Factor
Soil biology
Scotts Turf Builder Starter
None — feeds, doesn't inoculate
Under Sod™
Mycorrhizae, *Bacillus*, *Trichoderma*, humic/fulvic, seaweed
Factor
Best fit
Scotts Turf Builder Starter
Soil that tests low in phosphorus
Under Sod™
Adequate-to-high phosphorus soils (most Northeast lawns)
Which one for your lawn
Test your soil. If phosphorus is low, Scotts Starter (or any high-P starter) is the right, legal choice. If phosphorus is adequate-to-high — most Northeast lawns — a low-P biological starter like Under Sod™ fits new sod better and keeps you clear of phosphorus fertilizer restrictions. Under Sod™ is one 25 lb bag per 500 sq ft pallet, mixed into the top 4 inches of prepared topsoil before the sod is laid, up to 6 where the topsoil lift is deeper.
Where Under Sod™ fits
Under Sod™ is a 4-2-5, iron-free, potassium-forward biological starter in an SGN 90 granule, made to sit in the soil-to-root contact zone under new sod. See the full product specs or join the notification list for product updates.
This is general guidance, not a substitute for a soil test. Competitor formulas vary by product and can change; check the current label, and confirm your local phosphorus rules before applying either product.
Frequently Asked Questions
Is Scotts Turf Builder Starter good for new sod?+
It's a solid high-phosphorus synthetic starter, and it's the right pick when a soil test shows your soil is low in phosphorus. On the adequate-to-high phosphorus soils that describe most Northeast lawns, a low-P biological starter fits new sod better.
What's the difference between Under Sod™ and Scotts Starter?+
Scotts Starter is a high-phosphorus synthetic feed for a new lawn broadly. Under Sod™ is a low-phosphorus, potassium-forward biological starter for the root-contact zone under sod — it adds mycorrhizae and beneficial bacteria and is iron-free by design.
Which is better for new sod?+
It depends on your soil test. Low phosphorus favors a high-P starter like Scotts; adequate-to-high phosphorus (most Northeast lawns) favors a low-P biological starter like Under Sod™.
Can I use Scotts Starter under sod?+
You can. It's built to be broadcast and watered in, though, while Under Sod™ is formulated to be applied to the prepared soil in the root-contact zone before the sod is laid.
Under Sod™ product details
Under Sod™ is made for the soil-to-root contact zone under new sod. Join the notification list for product updates.
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.