Quick answer
Milorganite is a slow-release organic nitrogen fertilizer with iron that greens up an established lawn; Under Sod™ is an iron-free, potassium-forward biological starter made for the soil beneath new sod. For a new install, you want root establishment over blade color — which is exactly why Under Sod™ leaves the iron out and leads with potassium and soil biology. Milorganite is better suited to feeding and greening a lawn that's already rooted.
Milorganite, fairly
Milorganite is a genuinely good product for what it's for: a slow-release organic nitrogen source that also supplies iron, which is why it's a favorite for deepening the color of an established lawn without a nitrogen surge. Gardeners reach for it because it's forgiving, organic, and hard to burn a lawn with. Nothing here is a knock on Milorganite.
Why it isn't purpose-built for under new sod
The mismatch is about the job, not quality:
- Iron: Milorganite's iron is a feature for greening established turf — but under new sod, forcing blade color before the roots are ready is the opposite of the goal. Under Sod™ is iron-free on purpose.
- Potassium: Milorganite carries little to no potassium; Under Sod™ leads with it (the 5 in 4-2-5) precisely because potassium supports root establishment and stress tolerance in a new lawn.
- Placement and biology: Milorganite is a broadcast feed for turf that's already rooted. Under Sod™ is formulated to sit in the soil-to-root contact zone, applied before the sod is laid, and it includes the mycorrhizae and beneficial bacteria that help cut roots bridge into native soil.
| Factor | Milorganite | Under Sod™ |
|---|---|---|
| Primary job | Feed & green an established lawn | Root new sod at the soil interface |
| Nitrogen | Slow-release organic N | Modest, slow-release (the 4 in 4-2-5) |
| Potassium | Little to none | Potassium-forward (the 5) |
| Iron | Contains iron (for color) | Iron-free by design |
| Soil biology | Not a biological inoculant | Mycorrhizae, *Bacillus*, *Trichoderma*, humic/fulvic, seaweed |
| Placement | Broadcast on rooted turf | On soil, sod laid on top |
Factor
Primary job
Milorganite
Feed & green an established lawn
Under Sod™
Root new sod at the soil interface
Factor
Nitrogen
Milorganite
Slow-release organic N
Under Sod™
Modest, slow-release (the 4 in 4-2-5)
Factor
Potassium
Milorganite
Little to none
Under Sod™
Potassium-forward (the 5)
Factor
Iron
Milorganite
Contains iron (for color)
Under Sod™
Iron-free by design
Factor
Soil biology
Milorganite
Not a biological inoculant
Under Sod™
Mycorrhizae, *Bacillus*, *Trichoderma*, humic/fulvic, seaweed
Factor
Placement
Milorganite
Broadcast on rooted turf
Under Sod™
On soil, sod laid on top
The honest take
If you're feeding an established lawn and want slow-release nitrogen with a color boost, Milorganite is a reasonable pick. If you're installing new sod and want to prioritize fast, deep rooting in the first month, a low-P, iron-free, potassium-forward biological starter fits the job better. Under Sod™ is that: one 25 lb bag per 500 sq ft pallet, mixed into the top 4 inches of prepared topsoil before the sod goes down, up to 6 where the topsoil lift is deeper, then watered after installation.
Where Under Sod™ fits
Under Sod™ is a 4-2-5, iron-free, potassium-forward biological starter in an SGN 90 granule, with mycorrhizae, *Bacillus* and *Trichoderma*, humic and fulvic acid, and seaweed extract — made for the root-contact zone under new sod. See the full product specs or join the notification list for product updates.
Frequently Asked Questions
Can I use Milorganite under new sod?+
You can, but it's built to feed and green an established lawn — slow-release nitrogen and iron, with little to no potassium. Under new sod, a low-phosphorus, iron-free, potassium-forward biological starter fits the job better.
Does Milorganite have potassium?+
Little to none. Under Sod™ leads with potassium (the 5 in 4-2-5) precisely because potassium supports root establishment and stress tolerance in a new lawn.
Why is Under Sod™ iron-free when Milorganite contains iron?+
Iron greens the blade, which is a feature for established turf. Under new sod the goal is roots first, not top color, so Under Sod™ leaves the iron out on purpose.
Is Milorganite a starter fertilizer for sod?+
It's a slow-release nitrogen feed for established lawns, not a starter placed in the root-contact zone under new sod. For that job, Under Sod™ is built to sit at the soil-to-root interface before the sod is laid.
Under Sod™ product details
Under Sod™ is made for the 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.