Foundation advice written for Texas or Alabama often blames expansive clay that swells when wet and shrinks when dry. The soil survey data for Central Florida tells a different story. This post walks through what the USDA Natural Resources Conservation Service (NRCS) soil survey (SSURGO) shows for the four counties around Orlando, why fill and organic soils matter for a slab-on-grade house, and where the limestone under the sand comes in.
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The dominant soil series in each county
The NRCS soil surveys for Orange (FL095), Seminole (FL117), Osceola (FL097) and the Lake County Area (FL607) were queried for this site, with acreage weighted by each component's share of its map unit. The largest named soil series are:
| County | Largest soil components (acres) | Organic soils (approximate share) |
|---|---|---|
| Orange | Smyrna 145,956; Candler about 46,000; Basinger 34,127; Tavares 27,130; Samsula (muck) about 14,000–18,500 depending on tally | roughly 6 to 7% |
| Seminole | Basinger 23,156; Myakka 17,137; EauGallie 14,014; Astatula 13,121; Nittaw 9,203; Tavares 8,614 | roughly 6 to 7% |
| Osceola | Smyrna 160,339; Myakka 107,538; Basinger 79,125; Immokalee 63,206 | roughly 6 to 7% |
| Lake | Candler 102,504; Myakka 60,692; Tavares 23,817; Placid 22,182 | about 13% |
Source: USDA NRCS SSURGO via Soil Data Access, queried October 19, 2026. Organic soils cover roughly 6 to 7 percent of Orange, Seminole and Osceola and about 13 percent of Lake; the two NRCS tallies used in research differ slightly by method, so the shares are rounded. These are county-wide totals. They say nothing about the soil under a particular street or house.
Shrink-swell is negligible on most of these soils
NRCS rates shrink-swell with linear extensibility percent (LEP): under 3 is low, 3 to 6 moderate and 6 to 9 high. On the soils that dominate this area, the values are tiny. In Orange County's largest Candler map unit, clay is about 1.2 to 1.4 percent and LEP is 0. Smyrna runs about 2.5 to 5.2 percent clay with LEP 0.2 to 0.3; Myakka about 2 to 5 percent clay with LEP 0 to 0.3; Basinger about 2 percent clay with LEP 1.5. High-LEP soils make up under 1 percent of Orange County's major-component acreage.
The NRCS series descriptions explain why. Candler is "very deep, excessively drained" sand on uplands, where "thickness of the sand is 80 or more inches." Tavares formed "in sandy marine or eolian deposits ... on hills, ridges and knolls." Smyrna is "very deep, poorly to very poorly drained" sandy marine material "on nearly level flatwoods," where "the water table is at depths of less than 18 inches for 1 to 4 months in most years."
Nittaw: Seminole County's clayey exception
There is one notable exception. Nittaw, classed by NRCS as a fine, montmorillonitic Typic Argiaquoll, covers about 9,203 acres of Seminole County and has an LEP of about 7.5, in the high class. A few other wet-depression soils, such as Emeralda in Orange County, rate similarly. So "expansive clay" is not the general Orlando explanation, but it can apply in spots. Whether it applies at a given house is a question for a soil report, not a county map.
Organic soils and muck
The settlement risk that does show up in the county data is organic soil. Samsula, the main muck series in Orange County, formed "in moderately thick beds of hydrophytic plant remains ... underlain by sandy marine sediments ... in swamps, poorly defined drainageways and flood plains." In Orange County's largest Samsula unit, the top 61 cm (about two feet) is around 75 percent organic matter, with about 60 percent in the next layer, over sand. NRCS notes that "the water table is at or above surface of the soil except during extended dry periods."
Organic layers matter for a slab because they can lose volume. The Florida Geological Survey (FGS) lists "subsurface expansive clay or organic layers which compress as water is removed" among the causes of ground subsidence that can look like a sinkhole. Lake County has the largest organic share of the four counties, at about 13 percent, including the Everglades series and soils mapped simply as "Organic soil".
Why fill matters as much as native soil
A slab sits on whatever was placed and compacted under it, not only on the native series. FGS also lists "improperly compacted soil after excavation work, and even buried trash, logs and other debris" among subsidence look-alikes, along with "collapsed or broken sewer and drain pipes." On sandy lots, the settlement drivers that research points to are compression of organic layers, poorly compacted fill over former wetlands, erosion and washout at slab edges, and karst raveling.
Water ties these together. Orlando International Airport averages 51.45 inches of rain a year, and June through September brings 29.57 inches, about 57 percent of it (NOAA 1991–2020 normals). That makes drainage at the slab edge worth a look; see drainage and grading. 2026 has been one of the driest starts on record, a reminder that dry spells and heavy rain both affect the ground.
Karst under the sands
Below the sand is limestone. FGS staff, quoted in a December 2010 Florida Senate report, said a setting of shallow limestone "overlain by sands with various mixtures of clay" is found in Orange County and parts of Lake and Seminole counties. Florida law defines "sinkhole activity" as settlement from "movement or raveling of soils" into "subterranean voids created by the effect of water on a limestone or similar rock formation" (§627.706(2)(i)).
The 1985 state sinkhole-type map (FGS Map Series 110) describes cover "mainly of incohesive and permeable sand" where sinkholes "are few, shallow, of small diameter and develop gradually," and cover of "cohesive clayey sediments of low permeability" where they "develop abruptly." The map is regional, at roughly 1:2 million scale, so it cannot place a single house. FGS also notes that "Sinkhole activity is greatest when there have been drought conditions for some period of time."
The FGS database counts reported subsidence incidents, which are voluntary and mostly unverified reports: 232 in Orange, 172 in Seminole, 178 in Lake and 16 in Osceola as of October 19, 2026. Our post on settlement versus sinkholes explains how to read them.
What this means when a slab moves
County soil data explains why Central Florida settlement looks the way it does. It does not diagnose a house. A geotechnical soil report typically runs $500 to $3,000 and a structural engineer's report about $340 to $1,000, up to $3,000 for complex cases (national ranges, estimates, not quotes). Only a Florida-licensed professional engineer may diagnose the cause and write a structural report; a contractor's free evaluation is a sales estimate. Florida has no state agency that inspects sinkholes for homeowners, according to FGS.
If you see new cracks, sticking doors or a sloping floor, start with inspections and engineer reports, read about slab foundation repair, or request free quotes from independent contractors.
Frequently asked questions
Does Orlando have expansive clay soil?
Mostly not. NRCS soil survey data shows the dominant Orlando-area soils, such as Smyrna, Candler, Tavares, Myakka and Basinger, are sands with low shrink-swell; Candler's clay is about 1 to 1.4 percent with a linear extensibility of 0. A few wet-depression soils, such as Nittaw in Seminole County, are clayey and rate high, so a site soil report is the way to know.
What is muck soil and why does it matter?
Muck is organic soil formed from plant remains in swamps, drainageways and flood plains, such as the Samsula series. It covers roughly 6 to 7 percent of Orange, Seminole and Osceola and about 13 percent of Lake County (NRCS SSURGO). The Florida Geological Survey lists organic layers that compress as water is removed among causes of ground subsidence, which can affect a slab built above them.
Can a soil map tell me what is under my house?
No. County soil surveys summarize large areas and do not map individual lots, and fill placed during construction can differ from the native soil. A geotechnical soil report, typically $500 to $3,000 in national guides, and a Florida-licensed professional engineer's evaluation are how the ground under a particular house is assessed. A contractor's free evaluation is a sales estimate.