Soil evaluation and perc tests in Missouri

They are two ways of answering the same question, and Missouri accepts either. A percolation test measures how fast water actually drains out of a hole in the ground. A soil morphology evaluation reads the soil profile itself, describing the horizons, the texture, the structure and the depth to any limiting layer, and infers the loading rate from what the soil is.

The practical difference matters: where a soil morphology evaluation is used, no percolation test is required, although the administrative authority may still ask for one. Morphology is the more informative of the two, because it tells you why the ground behaves the way it does rather than only that it does.

Source: 19 CSR 20-3.060(2)(D)1. and 2., Missouri Code of State Regulations, read 2026-08-26.

Percolation tests are to be conducted by an engineer, a sanitarian, a registered geologist, a soil scientist, or a person trained and certified by the department. The soil morphology evaluation has a narrower list: a soil scientist, unless an engineer, registered geologist or sanitarian has had special training and field experience in determining the required soil characteristics.

The state publishes a county-filterable list of registered onsite soil evaluators with names, business names and phone numbers. Being listed is optional, so treat it as a starting point rather than a complete roster of everyone qualified near you.

Source: Missouri Department of Health and Senior Services, Onsite Wastewater Treatment, read 2026-08-26.

The method is set out in the rule rather than left to the tester. At least four test holes are required: three around the edge of the proposed absorption site and one in the middle. The holes are six to eight inches across with vertical side walls, dug to the depth of the proposed system bottom.

Then comes the part people are surprised by, which is the waiting. The hole is filled with clear water to at least twelve inches over the soil bottom and held there for no less than four hours, then left to swell for at least twenty-four hours before the measured readings begin. A test run without that presoak is not the test the rule describes, and a wet-season shortcut can flatter ground that will not perform in August.

Source: 19 CSR 20-3.060(2)(D) and (5)(A)1., Missouri Code of State Regulations, read 2026-08-26.

Slower than sixty minutes per inch. At that point the design has to be drafted and signed by a registered engineer, unless site suitability and system sizing have been established by a soil evaluation under section (7) of the rule. This is the single most common reason a routine-looking Missouri septic project acquires a second professional and a longer timeline.

There is a harder line beyond it. Absorption trenches should not be built in soils slower than sixty minutes per inch, and in no case may they be built in soils slower than one hundred twenty minutes per inch. Past that, a trench field is off the table and the design moves to something else entirely, which is where lagoons and engineered alternatives come in.

Source: 19 CSR 20-3.060(2)(D) and (5)(A)1., Missouri Code of State Regulations, read 2026-08-26.

At least one foot between the bottom of the absorption trench and the limiting layer or the seasonal high water table for a standard system. In highly permeable soils, and in cherty clay soils in areas of severe geological limitations, that requirement rises to four feet to groundwater or bedrock.

That four-foot case is not an edge case in Missouri. Cherty clay over shallow limestone is ordinary Ozark ground, and it is exactly the profile where a lot that looks perfectly buildable turns out to need a mound, a drip field or an engineered design. It is also why the same house plan costs differently in Christian County than it does on the northern till plains.

Source: 19 CSR 20-3.060(5)(A)2., Missouri Code of State Regulations, read 2026-08-26.

Because the evaluation can come back saying no. A lot with slow enough soil, shallow enough bedrock or a high enough water table can be unsuitable for a conventional system, and the alternatives cost real money. A soil evaluation ordered before closing is the cheapest version of that discovery. Ordered after, it is a renegotiation you no longer have leverage in.

Ask for the report to identify a replacement field area as well as the primary one. Fields do not last forever, and a lot with nowhere to put the second one is a different asset from a lot that has room.