Do You Need Starter Fertilizer for Sod? Only If Your Soil Test Says So

Short answer: only if your soil test says phosphorus is low. On the medium-to-high phosphorus soil that most established cool-season lawns sit on, a high-phosphorus "starter" adds a nutrient the sod does not need, is restricted without a test in a dozen-plus states, and works against the root fungi you want colonizing the new roots. Sod is not seed. What it needs on day one is potassium, water, contact with the soil, and biology in the contact zone.

This is part of a series: read your soil test → what goes under the sod → the 30-day feeding → why it is two steps.

What "starter fertilizer" actually means

A starter fertilizer is any lawn fertilizer with a high middle number. The middle number is phosphate, P2O5. Bags like 24-25-4, 12-18-8, and 18-24-12 are starters. They exist because a germinating seedling has a root a few millimeters long, and phosphorus does not move in soil. Putting phosphorus where that root will be is how you get a seed lawn out of the ground.

Extension does not dispute this. The University of Missouri's soil testing guide says new lawns "have a limited root system and require higher phosphorus levels for root growth." Wisconsin's A2303 says to use a starter "for new lawns or lawns with low phosphorus" and defines starters as fertilizers that "contain a high percentage of phosphate." Penn State's soil-test report for a home lawn to be planted spells out the starter it means: about 1 pound of nitrogen, 0.5 to 1.0 pound of P2O5, and 0.5 to 1.0 pound of K2O per 1,000 square feet in a 1:1:1 or 2:1:1 ratio, applied "just prior to seeding," on top of whatever phosphate and potash the test itself calls for, which Penn State says to mix "into a four to six inch soil depth."

So a starter is a legitimate tool. The question is never whether phosphorus matters to roots. It is whether your soil already has it.

Here is what the common starters deliver at their own label rates, next to what an incorporated bag delivers. Numbers are pounds of nutrient per 1,000 square feet, calculated from the analysis and the label rate.

Read the K column. Every starter on the market is phosphorus-heavy and potassium-light, because it was designed for a seedling. Sod's need, once phosphorus is adequate, runs the other way.

Sod is not seed

A seed has no roots. Sod arrives as a mature plant with a root mat cut off at the farm. Its job in the first 30 days is to grow new roots out of that mat and into your soil. Michigan State's sodding bulletin notes that thinly cut sod roots faster than sod carrying a lot of soil, because the plant has to push roots into the new bed either way and the thinner cut forces it sooner.

What helps that happen is contact with the soil, water every day, and potassium. Kansas State's turf nutrient guide describes potassium as "important in improving stress tolerance (such as heat, cold, wear, disease)," and notes that the stress benefit shows up at higher soil-potassium levels than basic growth requires. In Michigan State sod-farm work on a potassium-deficient soil, rhizome initiation was "nearly nonexistent" without applied potassium and increased "dramatically" with it, while phosphorus on the same soil "had less effect on rhizome growth." That study was on a low-potassium muck; the point is which nutrient the rooting plant reached for.

Phosphorus is immobile. On a medium or high test it is already sitting in the soil the roots are about to grow into. The starter's premise, phosphorus placed for a plant that has no way to reach it, describes a seedling. It describes sod only when the soil is low. Washington State University's extension sheet on phosphorus and home lawns makes the same point from the farm side: because sod is usually fertilized with phosphorus while it is grown, "newly sodded lawns may require less P fertilizer than newly seeded turf."

One honest caveat about the extension defaults you will see quoted. Rutgers' sodding sheet lists a 5-10-10, Michigan State's lists a 10-10-10 incorporated into the top 4 inches, and Penn State lists a starter just before seeding. Every one of those is a no-test default. The same institutions say, in the same publications, to test first and to add phosphorus only when the test calls for it. The default is what you do when you refuse to spend the money on a test. It is not a finding that sod needs phosphorus.

The soil test decides it

Cornell Turfgrass, on feeding a lawn: "The soil test results are the basis for choosing a fertilizer product with the necessary P or K. Only if P is low should a fertilizer with P be used." Cornell's Marty Petrovic, analyzing samples sent to the Cornell Nutrient Analysis Laboratory for lawn recommendations, found "at least 80 percent had enough phosphorus already." Cornell Cooperative Extension in Putnam County, after five years of research, "found virtually no need for phosphorous in soil." New York's Department of Environmental Conservation cites Cornell's finding of "no benefit to phosphorus levels above 4 parts per million" by the Morgan method.

UConn's lawn fertilizer page says the same thing for Connecticut: if additional phosphorus or potassium is needed, "recommended grades and rates will be listed on your soil test results," and the maintenance fertilizers sold in the state carry 0 percent phosphorus because of the law. Massachusetts writes UMass's guidelines into its regulation. New Jersey requires the test before phosphorus goes on. The four labs, what their reports look like, and what "optimum" means on each one are in the soil test post.

The test costs less than one bag of starter. If the sod is coming tomorrow and there is no test, the honest default on an established residential lot in the states above is to assume what the data says, that the soil is probably fine on phosphorus, and use the incorporated low-phosphorus bag. The exception is scraped new-construction ground or imported fill, where phosphorus is often genuinely low. Kansas State: "Soil phosphorus levels are frequently low in new housing developments before turf has been established." Test that soil, and if you cannot wait, treat it as a low-P site.

Where a starter is restricted without a test

Minnesota passed the first lawn phosphorus law in 2005 and, by the state's own count, ten more states had followed by 2012. Every one of these laws has a new-lawn exemption, because the legislators understood the seedling problem. Every one of them also bans phosphorus on an established lawn without a test. This is the legal reason the bag says "under, once": phosphorus goes into the soil at establishment, under the new-lawn exemption, and then never on top again without a test.

Fifteen states, fifteen new-lawn exemptions. The details differ. Two are softer than the rest: Maine's law works through a sign at the register rather than a ban, and Illinois only binds applicators for hire. One is tighter: New Hampshire allows phosphorus on a new lawn but, since January 2025, caps retail turf fertilizer for that use at a quarter pound of phosphate per 1,000 square feet per application, the lowest number in the country. Everywhere else, the phosphorus that goes under the sod at establishment is legal, and the same bag on the same lawn a year later is not without a test.

Notice the New Jersey exemption for fertilizer placed under the soil surface, directly to roots. The law itself distinguishes between phosphorus broadcast on top, which runs off, and phosphorus tilled into the root zone, which does not. That distinction is the whole argument for incorporating whatever you use under sod. And notice Vermont's date: October 15, not December 1. Sod laid in the second half of October in Vermont has to wait until spring for its 30-day feeding.

Run your own situation through the rules above:

Why extra phosphorus works against the roots you want

Cool-season turf roots are colonized by arbuscular mycorrhizal fungi, which extend the root's reach for phosphorus and water. The plant pays for that partnership with carbon, and it stops paying when phosphorus is easy. The largest meta-analysis of field studies on the question, Treseder 2004 in New Phytologist, found mycorrhizal abundance fell 32 percent on average under phosphorus fertilization across the studies compiled, and 15 percent under nitrogen. That is a general finding across ecosystems, not a turf trial, and we label it that way. But it is the reason a bag built to put fungi in the contact zone keeps its phosphorus at 2 percent instead of 25: a high-phosphorus starter on soil that already has phosphorus sends the plant the signal to skip the fungi.

The turf-specific record is smaller but points the same way. The University of Minnesota turfgrass program's review of the literature lists Kentucky bluegrass, the fescues, perennial ryegrass, and creeping bentgrass among the grasses shown to form mycorrhizal associations, with benefits reported for establishment, biomass, and stress tolerance, and it notes the caveats: results vary with the grass, the fungal species, the soil, and whether the inoculant reaches the active root zone. One entry on that list matters for this bag: Pelletier and Dionne, 2004, reported faster establishment of Kentucky bluegrass, red fescue, and perennial ryegrass with Glomus intraradices, the species now named Rhizophagus irregularis and one of the four on the planned UNDER SOD™ label. Vermont's own survey before its law found 60 percent of lawns tested statewide already had too much phosphorus; Cornell's found at least 80 percent in New York. On soil like that, an inoculant wants low phosphorus in the contact zone and a mixing pass that puts it where the new roots are going, which is why the bag is tilled in rather than spread on top.

There is a second cost to loading phosphorus you do not need. Kansas State's turf guide notes that "excessive phosphorus can interfere with iron uptake by turfgrass roots and lead to an iron deficiency." The usual answer is to add iron to the bag, which greens the leaf without fixing anything below it. We took the other route: keep phosphorus at the test, and leave iron out of the bag that goes under the sod.

When you do need a starter, and how to use it

You need phosphorus at establishment when the test says so: a "below optimum" or "low" phosphorus line on a UConn, UMass, Rutgers, Cornell, or Penn State report; scraped subsoil or imported fill on a new build; some sands. In that case a starter is the right tool and the new-lawn exemption is written for you. Use it the way the extension reports do:

1. Apply the phosphate the report calls for, and mix it into the top 4 to 6 inches. Penn State's report says exactly this: "Thoroughly mix recommended phosphate (P2O5) and/or potash (K2O) into a four to six inch soil depth." Michigan State's sodding bulletin: the top 4 inches. 2. Use the starter's rate, not the bag's marketing. Penn State's starter is 0.5 to 1.0 pound of P2O5 per 1,000 square feet. At label rate, Scotts 24-25-4 delivers 0.75, LESCO 18-24-12 delivers 1.0. 3. Once. Some starter labels tell you to repeat at 2 to 4 weeks. On sod, do not. Nothing goes on top of new sod until day 30. 4. Count everything. If you use UNDER SOD™ on a low-P site as well, its 1 pound of P2O5 per 1,000 counts toward the report's number. It was not built to fix a deficient site by itself; follow the report.

What to use instead when the test is optimum or high

This is the usual result, and it is where the starter habit costs people. The right bag for adequate-phosphorus soil is the one that does what a starter cannot: potassium ahead of nitrogen for the rooting plant, phosphorus held to what the fungi tolerate, living biology in the contact zone, and a label that says to mix it into the soil.

The 4-2-5 with no iron and four species of mycorrhizae, one 25-pound bag per pallet mixed into the top 4 to 6 inches, is UNDER SOD™. Per 1,000 square feet that is 2 pounds of nitrogen, 1 pound of P2O5, and 2.5 pounds of K2O blended through the rooting profile, which is why the rate reads high next to a surface bag: 12.5 pounds raked into the top inch and 50 pounds tilled through 4 inches land at about the same concentration in the soil, roughly 0.2 percent, and a third less at 6 inches. The nitrogen sits at the top of the incorporated range in the university sheets, and the share of it that is slow-release will be on the label before a bag ships. The 1 pound of P2O5 sits at the top of Penn State's own starter range, so on an optimum test you are not short, and on a high test you are not loading.

Until spring 2027, the closest bag on a shelf is Suståne BOLSTER 4-4-4, which scores 5 of 7 on our rubric because it carries 3 percent iron under the sod and its phosphate doubles at the sand rate. The plainer substitute is a phosphorus-free slow-release lawn fertilizer at a light rate worked into the surface before the sod goes down: it supplies nitrogen, brings no biology, and is potassium-light. Either way, do not add a high-phosphorus starter on top of them. The nine-bag scoring, the university precedent, and the bag calculator are in the Honest Comparison.

Then leave it alone for 30 days

Whatever went under the sod is the whole first-month program. The most common way a good install goes backwards is a second bag on top in week two: a starter "to help it root," or a maintenance fertilizer because the color faded. New sod in week two is supposed to look tired; it is growing roots, not leaves. Cornell's rule of never more than 1 pound of nitrogen per 1,000 square feet in one application is easy to break when a starter goes under and another bag goes on top.

Day 30 is when the lawn gets fed again, after it passes the tug test. The 8-0-6 with 2 percent non-staining iron, one 25-pound bag per four pallets at 30 days, is OVER SOD™. It is phosphorus-free by every definition in the table above, so it is legal on the now-established lawn in every state, and the timing rule is simple: if day 30 lands inside your state's blackout, wait for the first allowed date. Seven bags for that job are scored in the 30-day feeding post.

The procedure

1. Test the soil before the sod is ordered. Sample the imported topsoil if you brought some in, 10 to 20 cores about 4 inches deep, one lab, one method. 2. Read the phosphorus line. "Optimum" or "high" is the usual answer on an established lot. 3. Low phosphorus: a starter or straight phosphate at the report's rate, tilled into the top 4 to 6 inches, once. Optimum or high: one 25-pound bag of UNDER SOD™ per pallet, tilled into the top 4 to 6 inches, once. 4. Finish grade, lay the sod the same day, roll it, water it. 5. Nothing on top for 30 days. Water every day. Skip the "week two" bag. 6. Day 30, after the tug test: one 25-pound bag of OVER SOD™ per four pallets, outside your state's blackout dates and buffers. 7. Resume the normal seasonal program for your grass and state.

What we are not claiming

UNDER SOD™ and OVER SOD™ are planned labels for spring 2027; the water-insoluble nitrogen share, salt index, and organism counts will be printed on the bag when they are locked, not before. There is no independent multi-site sod trial of either bag yet; the phosphorus and mycorrhizae finding above is a meta-analysis across ecosystems, not a lawn study. The fifteen rows above reflect the state statutes and agency pages as we read them in September 2026, not legal advice. Two of them bear directly on the planned label and we are not going to pretend otherwise: New Hampshire caps phosphate in retail turf fertilizer for new lawns at 0.25 pounds per 1,000 square feet per application, and the planned UNDER SOD™ rate delivers 1 pound, so a New Hampshire label or rate of its own is an open question. New Jersey and Pennsylvania cap nitrogen at 0.9 pounds per application for lawn fertilizer, with adjustments for soil-test recommendations and enhanced-efficiency products; how an incorporated 2-pound charge is labeled in those states is a registration question that will be settled before a bag ships, not here. And on a genuinely low-phosphorus site, a starter at the report's rate is the right call and we say so.

Sources

• Cornell Turfgrass Program, Lawn Care: Feeding. "Only if P is low should a fertilizer with P be used"; never more than 1 lb N per 1,000 sq ft per application. https://turf.cals.cornell.edu/lawn/lawn-care-the-easiest-steps-to-an-attractive-environmental-asset/advanced-care/feeding
• Cornell Chronicle, May 2007. Petrovic: at least 80 percent of lawn soil samples had enough phosphorus. https://www.news.cornell.edu/stories/2007/05/new-eco-lawn-guidelines-focus-mowing-not-early-fertilizing
• Soldat, Petrovic and Rao, 2005. Summary of soil test phosphorus results from home lawns and athletic fields in New York State, 2001–2005 (3,803 samples). https://crops.confex.com/crops/2005am/techprogram/P5105.HTM
• Cornell Cooperative Extension, Chautauqua County. 10 Tips: five years of research in Putnam County found virtually no need for phosphorus. https://chautauqua.cce.cornell.edu/resources/10-tips-how-to-fertilize-your-lawn
• New York State DEC, FAQ for Lawn Fertilizer. New-lawn exemption; 0.67 percent phosphate threshold; Cornell's 4 ppm finding. https://dec.ny.gov/environmental-protection/water/water-quality-management/lawn-fertilizer/faq
• UConn Soil Nutrient Analysis Laboratory, Suggested Fertilizer Practices for Lawns. Connecticut law, 2-year test, phosphorus allowed when seeding or sodding. https://soiltesting.cahnr.uconn.edu/suggested-fertilizer-practices-for-lawns
• Massachusetts Department of Agricultural Resources, Plant Nutrient Regulations for Turf and Lawns fact sheet (330 CMR 31.00). https://www.mass.gov/doc/plant-nutrient-regulations-turf-and-lawns-factsheet/download
• Rutgers NJAES, Quick Facts: 2011 New Jersey Fertilizer Law. Exemptions including fertilizer applied under the soil surface directly to roots; nitrogen caps. https://casite-791589.cloudaccess.net/images/Environmental/FertilizerLawquickfacts711.pdf
• Rutgers NJAES News, November 2013. New Jersey blackout dates for homeowners. https://sebsnjaesnews.rutgers.edu/2013/11/nj-fertilizer-law-restrictions-on-home-lawn-applications-now-in-effect/
• Minnesota Department of Agriculture, Phosphorus Lawn Fertilizer Law; Minnesota Statutes 18C.60. https://www.mda.state.mn.us/phosphorus-lawn-fertilizer-law and https://www.revisor.mn.gov/statutes/cite/18C.60
• University of Wisconsin Extension, A2303 Lawn Fertilization. Wisconsin law; starter fertilizer definition. https://barron.extension.wisc.edu/files/2023/02/Lawn-Fertilization.pdf
• Michigan State University Extension. Michigan Fertilizer Law: phosphorus only with a test, a new lawn, or repair. https://www.canr.msu.edu/news/protect_michigan_water_resources_and_grow_healthy_lawns_with_go_green_tip_s
• Michigan State University Extension, E2911 Nine Steps for Establishing a New Lawn Using Sod (2004). https://sanweb.lib.msu.edu/DMC/extension_publications/e2911/E2911-2004.PDF
• McBurney and Rieke, Michigan State University. Using Soil Testing to Solve Nutritional Problems in Sod Production. Potassium and rhizome initiation. https://archive.lib.msu.edu/tic/mitgc/article/198298.pdf
• Penn State Agricultural Analytical Services Laboratory, sample soil test reports for a home lawn to plant and for commercial turf to plant. Starter definition; incorporate test P and K 4 to 6 inches. https://agsci.psu.edu/aasl/soil-testing/fertility/handbooks/turf/forms/sample-soil-test-report-for-home-lawn-to-plant and https://agsci.psu.edu/aasl/soil-testing/fertility/handbooks/turf/forms/sample-soil-test-report-for-commercial-turf-use-to-plant
• University of Missouri Extension, G6954 Soil Testing for Lawns. New lawns require higher phosphorus for root growth. https://extension.missouri.edu/publications/g6954
• Kansas State Research and Extension, MF2311 A Guide to Turfgrass Nutrient Recommendations. Excess phosphorus and iron uptake; potassium and stress tolerance; low phosphorus in new developments. https://bookstore.ksre.ksu.edu/pubs/MF2311.pdf
• UConn Soil Nutrient Analysis Laboratory, Your Lawn and the Phosphorus Law. Two-year test; December 1 – March 15; 0.67 percent; starters allowed when seeding or sodding. https://soiltesting.cahnr.uconn.edu/your-lawn-and-the-phosphorus-law/
• New York State DEC, Lawn Fertilizer Law text. First-growing-season exemption; December 1 – April 1. https://www.dec.ny.gov/chemical/74956.html
• Maryland Department of Agriculture, Lawn Fertilizer Law blackout notice (November 2023). Test required on established lawns; November 16 – March 1. https://news.maryland.gov/mda/press-release/2023/11/10/blackout-dates-for-lawn-fertilizer-applications-begin-november-16-4/
• Code of Virginia § 3.2-3607. Lawn maintenance fertilizer containing phosphorus barred from sale after December 31, 2013. https://law.lis.virginia.gov/vacode/title3.2/chapter36/section3.2-3607/
• Turf Magazine, New Fertilizer Laws for Lawn Care Pros (2011). Virginia's maintenance-versus-starter distinction. https://turfmagazine.com/new-fertilizer-laws-for-lawn-care-pros
• Vermont H.26 (2011), 10 V.S.A. § 1266b, as passed. https://legislature.vermont.gov/Documents/2012/Docs/BILLS/H-0026/H-0026%20As%20Passed%20by%20Both%20House%20and%20Senate.pdf
• Lake Champlain Committee, Lawn P No More (2011). Vermont dates and buffers; 60 percent of lawns tested had too much phosphorus. https://www.lakechamplaincommittee.org/learn/news/item/lawn-p-no-more
• Maine DEP, Lawn Fertilizer and Phosphorus (38 MRSA §419). https://www.maine.gov/dep/land/watershed/fertilizer/index.html
• New Hampshire RSA 431:4-b, Phosphorus Content of Turf Fertilizer (as amended effective January 1, 2025). https://gc.nh.gov/rsa/html/XL/431/431-4-b.htm
• UNH Extension, New Hampshire's Turf Fertilizer Law: What You Should Know. https://extension.unh.edu/resource/new-hampshires-turf-fertilizer-law-what-you-should-know-fact-sheet
• Penn State Extension, Pennsylvania Fertilizer Law for Home Gardeners (2022 law). https://extension.psu.edu/pennsylvania-fertilizer-law-for-home-gardeners
• Pennsylvania Department of Agriculture, Fertilizer: Homeowners and Residents. https://www.agriculture.pa.gov/Plants_Land_Water/Fertilizer/Pages/Homeowners-and-Residents.aspx
• University of Illinois Extension, Phosphorus Law Reminder for Illinois Turf Managers. https://hyg.ipm.illinois.edu/article.php?id=724
• Washington State Department of Agriculture, Restrictions on Turf Fertilizers Containing Phosphorus. https://agr.wa.gov/Services/Inspections-and-Investigations/Inspections/Fertilizer-Compliance/Fertilizers-Containing-Phosphorus
• University of Minnesota Turfgrass Science, Sessoms 2020. Arbuscular mycorrhizal fungi and their interactions with turfgrass species (literature table incl. Pelletier and Dionne 2004). https://turf.umn.edu/news/arbuscular-mycorrhizal-fungi-amf-and-their-interactions-turfgrass-species
• Washington State University Extension, FS058E, Phosphorus and Home Lawns: Quick Facts and Recommendations. Newly sodded lawns may require less phosphorus than seeded turf. https://wpcdn.web.wsu.edu/wp-ecommerce/uploads/sites/2/product-3984-sku-FS058E.pdf
• Treseder, K. K. 2004. A meta-analysis of mycorrhizal responses to nitrogen, phosphorus, and atmospheric CO2 in field studies. New Phytologist 164:347–355. Mycorrhizal abundance down 32 percent under phosphorus fertilization. https://doi.org/10.1111/j.1469-8137.2004.01159.x (summary: https://eco.confex.com/eco/2007/techprogram/P1283.HTM)