Quick answer
Soil drainage and infiltration means how fast water enters the soil surface, usually in inches per hour, and how it then moves down through the soil profile or fails to leave it.
NRCS says soil texture is the major inherent factor affecting infiltration, and that the rate falls as the soil gets wet. NRCS groups soils A to D by how much runoff to expect when they are thoroughly wet.
Quick facts
The key facts about Soil Drainage and Infiltration, with sources:
- Unit
- Inches per hour; NRCS notes field checks often record minutes needed per inch of water 1
- Biggest inherent factor
- Soil texture; water moves faster through large sandy pores 1
- Rate over time
- Starts fast and slows to a steady rate as the soil wets 1
- Hydrologic soil groups
- Four groups, A (low runoff potential) to D (high), set mainly by saturated conductivity and depth to a restriction or water table 2
- Dual groups
- A/D, B/D and C/D name wet soils that fall in D undrained and in A, B or C if drained 2
- Local test example
- A Maryland county program passes a hole that drains within 36 hours on each fill 4
Key takeaways
- Infiltration is the speed water enters the soil. Drainage is what happens next. A soil can take water in well and still drain slowly below.
- Texture sets the rough range, but compaction, wetness and cracks can move a soil far from it. Always test your own spot.
- Hydrologic soil groups A to D rate runoff potential when a soil is thoroughly wet. Look up yours in Web Soil Survey.
- A hole test gives a local, practical number. One Maryland program calls a hole that drains within 36 hours a pass for rain gardens.
- Never send runoff to a neighbor or a sanitary sewer. Call 811 before you dig the test hole.
Two ideas: how fast water goes in, and where it goes next
The NRCS defines soil infiltration as the ability of the soil to allow water to move into and through the soil profile.
The infiltration rate is a measure of how fast water enters the soil, typically expressed in inches per hour 1.
It matters because rain that falls faster than the soil can take it moves downslope or ponds on the surface 1.
Drainage is the next step. Water that has entered must pass down through the profile. A layer that is compacted or dense with clay can slow it.
NRCS says the steady rate depends on how fast water moves through the saturated soil, the most restrictive layer, such as a compacted layer, or a dense clay layer 1.
The two ideas can disagree. A lawn with a thin crust may refuse water at the surface even though the soil below is loose. A sandy bed over clay may take water fast and then fill up from below. Our yard ponding page covers the causes of standing water and a different hole test.
For design and mapping work, NRCS sorts soils into hydrologic soil groups. These rate runoff potential when a soil is thoroughly wet 2. This page owns rates and groups. The page on soil texture covers grain size, and soil compaction covers packed soil.
How a hole test works, step by step
The is below the method Montgomery County, Maryland, publishes for its RainScapes program. It is local guidance for rain gardens and permeable pavement, not a national standard 4.
- Locate utilities first. The county says to contact Miss Utility, Maryland’s 811 service, 48 hours in advance 4.
- Dig the hole. Make it 2 feet deep and 1 foot wide, or as deep as the planned excavation. If the planned excavation is 3 feet, the hole must be 3 feet deep 4.
- Fill it to the top with water and record the time 4.
- Cover the hole for safety and check it from time to time 4.
- Record when it is empty and work out how long it took to drain 4.
- Fill it a second time. If the first fill drains in under 36 hours, repeat in the same hole. If either fill takes longer than 36 hours, the test has failed and the county suggests another kind of project, such as a conservation landscape 4.
- Refill the hole with soil after testing 4.
- 811 firstWait for marks
- Dig 2 ft by 1 ftOr planned depth
- Fill and timeFirst fill
- Fill againSame hole
- Pass or failBoth under 36 hours
Steps and the 36-hour limit are from Montgomery County, MD, RainScapes. The limit is a local program rule. Source: [4]
The four hydrologic soil groups, with NRCS definitions
| Group | Runoff potential when thoroughly wet | Typical makeup | Saturated conductivity of the slowest layer |
|---|---|---|---|
| A | Low; water is transmitted freely | Typically less than 10 percent clay and more than 90 percent sand or gravel 2 | More than 5.67 in/h 2 |
| B | Moderately low; transmission unimpeded | Typically 10 to 20 percent clay and 50 to 90 percent sand; loamy sand or sandy loam 2 | 1.42 to 5.67 in/h 2 |
| C | Moderately high; transmission somewhat restricted | Typically 20 to 40 percent clay and less than 50 percent sand; loam to silty clay loam 2 | 0.14 to 1.42 in/h 2 |
| D | High; movement restricted or very restricted | Typically more than 40 percent clay and less than 50 percent sand; also soils with a shallow restriction or a water table within 24 in. 2 | 0.14 in/h or less 2 |
Dual groups mean drainage changes the answer
NRCS gives some wet soils a dual group such as A/D, B/D or C/D. The first letter applies if the soil is adequately drained, meaning the seasonal high water table is kept at least 24 inches below the surface, and the second applies to the undrained soil 2. See surface vs. subsurface drainage.
Example: a ring test on a tracked and an untracked patch, read against a storm
The numbers in the first two rows are NRCS’s own sample data sheet for a silty clay loam, dated May 1, 2012, shown to illustrate the method 1. The storm figure is NRCS’s example for Mead, Nebraska, which NRCS says should not be used for other places. The comparison is ours.
| Patch (NRCS example) | Time for the second inch | Steady rate | Against a 1-year, 1-hour storm of 1.1 in. (Mead, NE, example) |
|---|---|---|---|
| Tracked by wheeled equipment | 200 minutes | 0.30 in/hr 1 | Rain arrives at 1.1 in/hr and the soil takes about 0.30 in/hr, so runoff or ponding is likely |
| Not tracked by wheeled equipment | 120 minutes | 0.5 in/hr 1 | Better, but still below 1.1 in/hr, so some runoff is expected in an intense hour |
The point is the comparison between spots, not the absolute number. NRCS says to compare soils in different fields, of different types and under different management 1. Use the drainage diagnostic to trace where the extra water goes.
Where these terms show up
On drainage bids. A bid for a French drain, swale or dry well often assumes a soil type. See drainage and grading and French drain.
In rain garden and permeable paver guides. Many local programs set a hole test pass mark. Montgomery County’s is 36 hours 4. See permeable pavers.
In soil survey reports. Web Soil Survey maps soils and lets you build reports for your area of interest 3. NRCS says assigned hydrologic soil groups can be found through Web Soil Survey, the Field Office Technical Guide and published soil survey databases 2.
In stormwater plans. Local stormwater rules and engineers use the groups when they size runoff controls. See soil drainage and yard flooding.
On septic and well sites. Septic systems follow state and county rules for soil testing, which we do not cover. Ask the local health department.
What knowing your rate lets you do, and what it cannot tell you
What it can tell you
- Whether rain is likely to run off or pond. NRCS says rain that falls faster than the infiltration rate runs off or ponds 1.
- Whether a rain garden, swale or drain is realistic. The county test passes or fails a spot 4.
- Which patches of the yard are worst. NRCS suggests comparing spots under different management 1.
- A map-based second view of your lot through the hydrologic group 23.
What it cannot tell you
- A single rate for the whole yard. NRCS says rates are average values that should not be generalized for all soil types 1.
- The cause. A slow rate can come from texture, compaction, a crust, a high water table or a restrictive layer.
- What a heavy storm will do. NRCS shows rainfall depth and duration vary by place 1.
- Whether a design is legal. Local rules decide that.
Limits and where published rates disagree
Rates depend on wetness. NRCS says infiltration is fast at first and slows as the soil gets wet, and that steady-state values apply to soils wet deep into the profile and should be compared with the second inch of water applied 1. A test on dry soil can look quick and mislead.
Published tables differ. NRCS’s table lists loam at 0.2 to 0.4 in/hr 1.
University of Maryland Extension’s Table C-3 lists silt loam at 0.25 to 0.40, clay loam at 0.10 to 0.30, fine sandy loam at 0.35 to 0.50, fine sand at 0.50 to 0.75 and coarse sand at 0.75 to 1.00 in/hr 5. The textures differ, and neither is a guarantee for your yard.
Clay is the hardest to predict. NRCS says clayey soils can have a high infiltration rate when dry, because cracks are direct conduits, and a slow rate when moist, because cracks close 1.
The Maryland pass mark is one county’s rule for its rebate program. It is not a national standard, and other programs set other limits. We did not find a source that gives a single hole-test pass mark for every use, so we give none.
Web Soil Survey is a map. NRCS says surveys suit general local planning and that onsite investigation is needed in some cases 3.
What drives the cost of testing and of acting on the result
We publish no prices. The tests themselves cost little: a post-hole digger, water and a day. A professional percolation or infiltration test, and a soil investigation by an engineer, cost more. Ask what method will be used and how many holes.
The result drives the bigger costs. A soil that fails may need an underdrain, a different practice or a regrade. Montgomery County suggests an underdrained rain garden or a conservation landscape if a rain garden hole fails 4. Each choice changes a bid.
The result does not last forever. NRCS says infiltration can change drastically with management, especially near the surface, so a rate measured before construction may not hold after it 1. We give no lifespan.
Compare any quote with the cost hub and the estimates guide.
Who does it, and a safe order of work
A homeowner can run the hole test. A drainage contractor or engineer decides what to build from it.
- Look up your lot in Web Soil Survey for the mapped soil and its group 32.
- Pick the test spots where you plan to build, not just a convenient place. Test several, as NRCS suggests three or more measurements at each location 1.
- Call 811 at least two working days ahead 6.
- Choose a day when the soil is moist, not frozen, and run the two-fill test 4.
- Write down depth, spot, date, days since rain and the drain times 4.
- Match the result to the decision guide below, and talk to a drainage professional if it fails.
Locate utilities, cover the hole and never aim water at a neighbor
USA North 811 says locating is free and not to dig until each utility has responded 6. Montgomery County says to cover the hole for safety 4.
If a test shows poor drainage, do not solve it by sending water onto a neighbor’s lot or into a sanitary sewer. Large grading changes may need a permit or an engineer.
Keeping infiltration working
The are habits, not dates. Climate and soil change the details.
- Avoid driving or working on wet soil. NRCS says equipment use on wet soils and tillage can result in compaction 1.
- Keep a plant or mulch cover. NRCS says unprotected soil is hit by raindrops that dislodge particles and form a crust that restricts water entry 1.
- Build organic matter. NRCS says organic matter binds particles into stable aggregates, increasing porosity and infiltration 1. See soil organic matter.
- Re-test after big changes. New patios, regrading or fill change the surface.
- Watch the water table. A seasonal high water table can turn a good soil into a poor one for a few months.
Warning signs of poor infiltration or drainage
- Puddles hours after a modest rain. NRCS says restricted infiltration and ponding cause poor soil aeration and poor root function 1.
- Runoff across bare or thin turf. NRCS says runoff on bare or sparsely vegetated soil can cause erosion 1.
- A hole that is still full the next day. The county test treats more than 36 hours as a fail 4.
- A mapped D group or a dual group. D soils have restricted water movement, and dual groups point to a seasonal high water table 2.
- Cracks that open in dry spells and then close. Clay soils can switch from fast to slow, per NRCS 1. See expansive soils and hardscape.
If water enters a basement or crawl space, or a slope shows seepage, stop and ask a drainage contractor or engineer.
Standards, programs and local rules
A classification, not a law. Hydrologic soil groups are defined in NRCS’s National Engineering Handbook, and NRCS says soil scientists assign map unit components to groups 2.
The 36-hour test is a program rule. It comes from Montgomery County, Maryland’s RainScapes program for rain gardens and permeable pavement 4. Other places set other rules.
Stormwater and grading. Many cities regulate runoff, grading and where roof water can go. Check yours before you build. See our drainage research for background.
Utilities. Call 811 before you dig a test hole 6.
Septic. Percolation tests for septic systems follow state and county rules. We do not cover them.
Infiltration compared with related terms
| Term | What it describes | Unit or output | Main difference |
|---|---|---|---|
| Infiltration rate | How fast water enters the surface | Inches per hour or minutes per inch 1 | About the entry point |
| Drainage | How water moves through or leaves the profile | Drain time or class | About what comes after entry |
| Hydrologic soil group | Runoff potential when thoroughly wet | A, B, C, D or dual 2 | A map-based class, not a test |
| Percolation test | A hole drain-time test | Hours to drain 4 | Local program method |
| Soil texture | Grain-size mix | Texture class | A cause of the rate, not the rate |
Common beliefs about soil drainage
- Myth Sandy soil always drains well.
- Reality Sandy soil usually takes water fast, but a restrictive layer, high water table or compaction can still cause trouble 21.
- Myth Clay never lets water in.
- Reality NRCS says cracked clay can have a high rate when dry and a slow rate when moist 1.
- Myth One test hole tells you about the whole yard.
- Reality NRCS says rates should not be generalized, and suggests three or more measurements at each location 1.
- Myth A map class replaces testing.
- Reality NRCS says surveys suit general planning and onsite investigation is needed in some cases 3.
The regional variable: rainfall and the soil map under your lot
Two things are local. First, rainfall intensity: NRCS’s example tables show different storm depths for Mead and North Platte in Nebraska, and say they should not be used elsewhere 1.
Second, the mapped soil and its hydrologic group in Web Soil Survey 32. Local stormwater programs also set their own test rules, as Montgomery County does 4.
When to test, and what to do with the result
Test when you plan a rain garden, a drain, permeable paving or a bed in a spot that stays wet, or when you want to know if a puddle is a soil issue. If the yard drains well after storms, you do not need to test.
Your next step is to look up the mapped soil, then run the hole test in the exact spot you plan to use. Use the guide below to read the result.
If the result is slow, talk to a drainage professional. Our drainage and grading page lists what the work includes. Do not fix a wet yard by sending water to a neighbor.
- If Both fills drain in under 36 hours (Montgomery County rule)
- Then Pass for a rain garden or permeable pavement under that program 4
- If Passes but drains slowly
- Then Choose wet-soil plants, per the county 4
- If Either fill takes longer than 36 hours
- Then Fail; consider a conservation landscape or an underdrained design 4
- If Pavement project that fails
- Then Add an underdrain 4
- If Slow rate and the mapped group is D or dual
- Then Ask a drainage professional about water table and options 2
Questions about Soil Drainage and Infiltration
What is the difference between soil drainage and infiltration?
Infiltration is how fast water enters the soil surface, usually in inches per hour. Drainage is how water then moves through the profile or fails to leave it. NRCS says a compacted or dense clay layer can restrict movement. A soil can take water in quickly and still fill from below.
What is a good soil infiltration rate?
No single number is right for every use. NRCS lists steady-state rates from above 0.8 inches per hour for sand to under 0.04 for sodic clayey soils, and calls them averages. For a rain garden or paving, a local program sets the pass mark. Montgomery County, MD, uses a 36-hour hole drain time.
What are hydrologic soil groups?
They are four NRCS classes, A to D, for runoff potential when a soil is thoroughly wet. A has low runoff potential and D has high. Dual groups such as A/D name wet soils that are D when undrained and A when adequately drained. Look yours up in Web Soil Survey.
How do I do a percolation test at home?
Montgomery County, MD, publishes one: call 811 first, dig a hole 2 feet deep and 1 foot wide, fill it with water, time the drain, then fill it again. Both fills must drain within 36 hours to pass. It is local guidance for rain gardens and permeable pavement, not a national standard.
How long should a perc test hole take to drain?
It depends on the rule that applies. Montgomery County, MD, uses 36 hours for each fill for its rain garden program. Septic rules use different tests. We found no single pass mark for all uses, so check your local program or ask a drainage professional.
Why does infiltration slow down during a storm?
Dry soil has pores and cracks that let water in fast. NRCS says that as soil moisture rises the rate slows to a steady value set by the most restrictive layer. A rate measured on dry soil can therefore look better than it will be in a long rain.
Does compaction reduce infiltration?
Yes. NRCS says compacted layers have less pore space, which restricts movement of water through the soil, and that equipment use on wet soils and tillage can compact it. The NRCS example sheet shows a patch tracked by wheeled equipment draining slower than an untracked patch.
How do I find my soil’s hydrologic group?
Use USDA Web Soil Survey. NRCS says assigned groups can be found through Web Soil Survey, the Field Office Technical Guide and published soil survey databases. Web Soil Survey says it suits general planning and that onsite investigation is needed in some cases.
Can clay soil have a high infiltration rate?
Sometimes. NRCS says cracking clays can take water fast when dry because cracks are direct conduits, and slow down when moist as the cracks close. Clay that does not crack is slow unless it has a high iron oxide content or formed in volcanic ash.
What if my soil fails a drainage test?
Do not send water to a neighbor or a sanitary sewer. Montgomery County suggests a conservation landscape, wet-soil plants or an underdrained design instead. For pavement, add an underdrain. A drainage contractor or engineer can design options for your lot.
Sources
- USDA NRCS, Soil Health Guide for Educators: Infiltration (May 2014; PDF text extracted locally), retrieved .
- USDA NRCS, National Engineering Handbook Part 630, Chapter 7: Hydrologic Soil Groups (210-VI-NEH, Jan. 2009; mirror hosted by the ASDSO Dam Safety Toolbox, PDF text extracted locally), retrieved .
- USDA NRCS, Web Soil Survey, retrieved .
- Montgomery County (MD) Department of Environmental Protection, RainScapes Rewards: How to Do a Perc Test (rev. 1/22; PDF text extracted locally), retrieved .
- University of Maryland Extension, Table C-3: Soil Infiltration Rates Based on Soil Texture (PDF text extracted locally), retrieved .
- USA North 811, DIYers (call 811 before you dig), retrieved .
Expert review
Written by the HyreYard Research Desk. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Infiltration definitions, steady-state rates and the ring method come from the NRCS Soil Health Guide for Educators (2014).
Group definitions and conductivity limits come from NRCS National Engineering Handbook chapter 7, read from a mirror on the Dam Safety Toolbox site because the NRCS directives copy was not retrieved.
The hole test is one Maryland county’s local program method. The University of Maryland table has no stated author or date. The example data sheet is NRCS’s sample, not measured on a yard.
Suggest a correction. We fix errors and say what changed.