The soil evaluation decides almost everything about your project: what system you can build, how big it must be, what it costs, and in rare cases whether you can build at all.
It costs a few hundred dollars. Every estimate you get before it exists is a guess.
The two kinds of evaluation
Percolation test
Measures how fast water drops in a saturated hole, expressed in minutes per inch. It is the traditional method and still the legal standard in many states.
Soil morphology evaluation
A licensed soil scientist digs a pit and classifies the profile: texture, structure, consistence, and redoximorphic features — the grey and orange mottling that records where water has sat seasonally. From that they assign a loading rate directly.
Morphology is increasingly preferred because it reads the soil’s long-term behaviour. A perc test measures one afternoon; the soil profile records decades. In particular, mottling reveals a seasonal high water table that a dry-season perc test cannot detect at all.
Many states now require both, or accept morphology in place of a perc test.
How a perc test is run
- Dig the holes. 6 to 12 inches across, down to the proposed trench depth — usually 18 to 36 inches. Most codes require at least three, spread across the proposed field, not clustered.
- Scarify the sides. Digging smears and seals the soil surface. Scratch it open and remove the loose material from the bottom.
- Add gravel. Two inches in the bottom stops falling water scouring a crater.
- Presoak. Fill and keep filled for at least 4 hours, 24 hours in clay. This is the step amateurs skip, and it is the step that matters most — unsaturated soil percolates several times faster than soil in service.
- Take timed readings. Refill to a set depth, measure the drop over a fixed interval — usually 30 minutes — and repeat until consecutive readings agree within about a sixteenth of an inch.
- Divide. Minutes ÷ inches of drop = minutes per inch.
- Use the slowest hole. Codes size the field on the worst result, not the average.
What the number means
| Perc rate | Soil | Loading rate | Verdict |
|---|---|---|---|
| Under 1 min/in | Gravel, coarse sand | — | Too fast. Sand liner or advanced treatment required. |
| 1–5 min/in | Coarse sand | 0.80 | Fast. Some states require a liner. |
| 6–20 min/in | Fine sand, loamy sand | 0.60 | Excellent — the cheapest soil to build in. |
| 21–30 min/in | Sandy loam | 0.50 | Good. Conventional trenches. |
| 31–45 min/in | Loam | 0.40 | Good. Field about 25% larger. |
| 46–60 min/in | Silt loam | 0.30 | Fair. Large field; check local rules. |
| 61–120 min/in | Clay loam | 0.15–0.20 | Poor. Very large field or an alternative system. |
| Over 120 min/in | Clay | — | Unsuitable for conventional trenches. |
Note both ends fail. Too fast is a problem, not a bonus — effluent that races through gravel reaches groundwater untreated, which is why fast sites often need a sand liner or an ATU.
The hand texture test
You can do this yourself with a handful of moist soil, and it is a genuinely useful cross-check on a report.
Wet the soil to putty consistency, then try to form it:
- Will not hold a ball, feels gritty → sand
- Weak ball, mostly gritty → loamy sand
- Holds a ball, ribbon under 1 inch, gritty → sandy loam
- Even mix of gritty, smooth, sticky → loam
- Floury and smooth, ribbon under 1 inch → silt loam
- Sticky, ribbons 1–2 inches → clay loam
- Very sticky, ribbons over 2 inches → clay
A “ribbon” is what you get pushing soil between thumb and forefinger. The longer it ribbons before breaking, the more clay it has, and the slower it will accept water.
If your hand test says clay and the report says sandy loam, ask questions.
What else the evaluator is looking for
Depth to limiting layer. Bedrock, hardpan, or the seasonal high water table. You need 2 to 4 feet of unsaturated soil below the trench, depending on the state. This is the most common single reason a site fails.
Redoximorphic features. Grey depletions and orange concentrations in the profile record where water has sat seasonally. They reveal a high water table even in a bone-dry August pit, which is exactly why morphology beats a summer perc test.
Restrictive horizons. Fragipan, claypan, or a plough pan that will perch water above the trench regardless of what the soil below it does.
Slope and landscape position. Steep slopes need special design; a footslope or depression collects water from uphill.
When a site fails
A failure almost never means the land is unbuildable. It means a conventional trench field will not work. Your options, roughly in order of cost:
| Option | Cost | Requires |
|---|---|---|
| Shallow or at-grade trenches | $8,000 – $18,000 | Some permeable soil near the surface |
| Chamber system with area reduction | $7,000 – $16,000 | Marginal but workable soil |
| Aerobic treatment unit | $12,000 – $28,000 | Room for a smaller field; power |
| Drip dispersal | $14,000 – $30,000 | Room for a wide shallow area |
| Mound system | $18,000 – $45,000 | Room for the mound and its basal area |
| Holding tank | $8,000 – $15,000 + pumping | Last resort; many states allow only for repairs |
The genuine dead ends are narrower than people fear: bedrock or water table within a foot or two of the surface, or a lot with no room for a field plus its reserve area.
Buying land
Make the purchase conditional on a passing evaluation. Specifically:
- Ask whether a valid, unexpired evaluation already exists — many states expire them after 2 to 5 years
- Ask whether a permit was ever issued and then lapsed, which often signals a problem
- Test in the wet season if you can control the timing
- Confirm there is room for both the primary field and the reserve area
- Check the well location first; the setback may already rule out half the lot
A few hundred dollars and a two-week contingency against a potential $30,000 surprise is not a close call.
Reading the report
You should be able to find:
- Loading rate in gpd/ft² — the number that sizes your field
- Depth to limiting layer and what that layer is
- Soil texture classification by horizon
- Recommended system type
- Required absorption area
- Location of the primary and reserve areas on a site plan
If the report gives you a perc rate and nothing else, ask for the loading rate and the limiting-layer depth. Those two numbers are what every contractor will actually design from, and you need them to compare bids.
Common questions
How much does a perc test cost?
$300 to $1,200 depending on the state, the number of holes required, and whether a soil scientist or engineer must be present. It is the cheapest money you will spend on the project and the number that determines everything downstream.
When is the best time to test?
Late winter or early spring, when the water table is at its seasonal high. Many states restrict testing to that window precisely because a summer test on dry soil gives a flattering result that the system then has to live with for thirty years.
Can a failed site be made to work?
Usually, yes — with an alternative system. Mounds, drip dispersal, and aerobic units exist for exactly this. The genuine dead ends are sites where bedrock or the water table is too close to the surface to achieve the required vertical separation at all, and sites with no room for a field plus a reserve area.
Should I make a land purchase conditional on a perc test?
Always, if the lot is not already served by sewer. A failed or marginal result can mean $30,000 of extra cost or an unbuildable parcel. The test costs a few hundred dollars and the contingency costs nothing.
What is vertical separation?
The depth of unsaturated soil between the bottom of the trench and the water table, bedrock, or any other limiting layer. Codes require 2 to 4 feet because effluent is only treated while moving through unsaturated soil. Inadequate separation is the most common single reason a site fails.