Test how fast your soil absorbs water by digging a hole 30 cm deep, filling it with water and timing how long it takes to drain. Under 30 minutes is fast-draining soil, 30 minutes to 4 hours is moderate drainage, and over 4 hours is slow drainage in which water will pond and the site needs more surface drainage. Repeat the test in several places and after the ground is already wet, because a second fill is the slower, more honest figure.
Part of Flood Control and Drainage
Soil drainage capacity is how quickly water soaks into the ground. It decides how much rain runs off, whether a swale or rain garden will work, and whether an infiltration outlet is realistic.
Soil Decides How Much Of The Rain Becomes Runoff
Sand takes in water quickly. Clay takes it slowly and ponds it. Between them lies most ordinary soil. The type is set by geology (see Soil Texture), but the real test is whether water goes in the ground on your site, because compaction, a hard layer or a high water table can make a sandy soil behave like clay.
What You Need
- A spade and a measuring stick — to dig a straight-sided hole 30 cm deep.
- Water — enough to fill the hole several times.
- A timer — a clock, a burning-time candle or a counted pulse.
Method: Run The Drainage Test
Step 1: Dig the hole. Make it 30 cm deep with straight sides. Roughen the sides if the spade has smeared them smooth, since a smeared clay wall seals shut.
Step 2: Pre-wet the soil. Fill the hole with water and let it drain away. The first fill soaks the surrounding soil, so the second fill shows how the ground behaves when it is already wet, which is how it is during a flood.
Step 3: Fill and time. Fill the hole to the top again and note the time it takes to empty.
Step 4: Read the result. Less than 30 minutes: fast, sandy soil, good for absorption. 30 minutes to 4 hours: moderate drainage, typical. More than 4 hours: slow, clay, and water will pond, so you need more surface drainage.
Step 5: Repeat across the site. Test at least in the low spots, the high spots and wherever you plan a ditch, swale or rain garden. Soil varies over a few metres.
Worked Example: Turn Minutes Into A Rate
The test gives a time for a 300 mm fall. Dividing gives a rate in mm per hour.
- 30 minutes for 300 mm: 300 mm / 0.5 h = 600 mm/h.
- 4 hours for 300 mm: 300 mm / 4 h = 75 mm/h.
So fast soil drains faster than about 600 mm/h, slow soil slower than about 75 mm/h, and moderate soil falls between. These are rough rates because the water depth falls during the test. Their use is in comparison. A pond 20 cm deep draining at 75 mm/h empties in 200 / 75 = 2.7 h. A rain garden’s ponding depth of 20 cm draining in 24 hours needs only 200 / 24 = 8.3 mm/h, so soil rated “slow” by this test can still support a rain garden, though not a deep one.
Common Mistakes
- Skipping the pre-wetting — dry soil takes the first fill fast, and the result looks better than a flood will be.
- Testing one spot — a single hole may sit in the best patch.
- Smeared hole sides — a shovel can seal the wall of a clay hole and slow the test.
- Testing where groundwater rises — if water seeps into the hole, the water table is high and the test says nothing about absorption.
- Reading “slow” as hopeless — it means plan surface drainage, not give up.
Safety
Open holes and soft ground
Test holes left open trap children and animals, and soft or waterlogged ground near a bank can slide. Fill holes when you finish, keep clear of bank edges, and do not leave water standing in an open hole where mosquitoes can breed. See Mosquito Prevention.
What’s Next
- Learn next: Swale Design — use the result to size an infiltration channel
- Combine with: Safe Outlets — decide whether infiltration or a stream outlet is realistic
- Combine with: Soil Texture — why sand and clay behave differently