Learning About Soil Permeability

What is Soil Permeability? (All You Need to Know)

In Gardening, Tips by Jamie

Whether you’re creating a rain garden or building a fish pond, you will come across the term soil permeability. And while a quick google search brings up very technical explanations, the concept itself is really pretty simple.

Soil permeability is how quickly water moves through the soil. Soil is made up of different particles with open pockets (pores) in between. Permeability is determined by how well-connected these open pockets are to each other – the more well-connected, the faster the water drains away, the higher the permeability. Along with porosity (the amount of open space in between soil), permeability determines how long a soil can hang onto water and how fast it can drain away.

What Does High Permeability In Soil Mean?

High permeability in soil means that water can move quickly and easily, so it drains away faster. Soils with high permeability have a lot of sand. Sand particles don’t absorb any water but their granular size and inability to stick together leaves plenty of little gaps (high porosity).

High permeability soils have:

  • High infiltration rates – water can soak into the soil quickly and fill all the gaps, reaching deep roots quickly.
  • Quick drainage – since water can move easily, it drains away faster. If your plants like to dry out in between watering (like rosemary), or live in an area with a lot of rain, then high permeability soils are fantastic. It’s also a great quality when creating a drainage ditch. But if you’re building a pond, the soil won’t be able to hold onto the pond water.
  • Low water-holding capacity – because sand doesn’t absorb any water but allows water to pass through quickly, sandy soils aren’t very good at holding onto water. For gardens, this means that you need to water more frequently as the soil dries out quickly.

What Does Low Permeability Mean?

 

Low permeability in soil means that water moves slowly. Soils with low permeability have a lot of clay and silt. Clay feels sticky when wet, and it’s this factor that makes clay useful for pottery. Silt feels silky smooth when wet as it absorbs water. Because clay has a fine texture (unlike sand, which has a coarse texture) that becomes sticky and plate-like when wet, robbing it of air pockets, it takes a lot longer for water to infiltrate and drain away.

Low permeable soils have:

  • Low infiltration rates – water slowly soaks into clay and silt-clay soil. The heavier the soil, the more likely it is you’ll see puddling or even flooding.
  • Slow drainage – since water cannot move easily through silt and clay-based soils, it takes much longer for the water to drain away. Slower drainage means you need to water less frequently, but if your soil gets too much water, plants will suffocate. Plant roots need the air that exists between soil particles to survive. If those air pockets are flooded for long enough, the roots will rot and the plant will die. These soils are better for building ponds and retaining walls.
  • High water-holding capacity – Because silt and clay-based soils drain slowly and absorb water, it holds onto water longer. This means you need to water less frequently.

Does Soil Have Consistent Permeability?

soil may have different layers with different soil textures

Your soil may have different layers with different soil textures. For example, while your upper layer of soil may have more sand in it, a layer of heavy clay may be inches beneath it. The water filters quickly through the upper layers, but then stops at the clay.

If you’re building something that needs high permeability (more water being moved away) – like a drain – then you’ll want it to rest on a highly permeable layer so the water can soak into the soil quickly and move away.

If you’re building something that needs low permeability (water staying longer) – like a pond – then you’ll want it to rest on an impermeable layer. (You can patch higher permeable layers with clay.)

How Is Soil Permeability Measured?

While there are several tests that you can use to figure out soil permeability, they all rely on observing how quickly water seeps out of a hole in the soil. Which test you use depends on how accurate a number (the coefficient of permeability) you need for other calculations.

The coefficient of permeability (K in equations) is just how many meters or centimeters per second it takes water to move through the soil. The higher the number, the more permeable the soil. Pure clay will have a coefficient of 0.05 cm/hour while pure sand will have a coefficient of 5.0 cm/hour.

Tests fall into two categories based on water pressure:

  • Constant head test. The water pressure being fed into the soil remains constant throughout the test. So, if you’re pouring water from a reservoir into the soil, you continue to fill up that reservoir so it remains at the same level from start to finish. (It’s like using a hose to water.) This test is best for sand or gravel soils, and even some clay, but not for silt.
  • Falling head test. The water pressure being fed into the soil decreases throughout the test. When pouring water from a reservoir into the soil, you allow the reservoir to empty without refilling it. (It’s like using a watering can to water.) This test is best for fine-grained soils.

If you need just a general idea, then all you need to do is look at the soil’s texture and structure. Use the jar method to figure out the different amounts of sand, silt, and clay, or send in soil samples to a laboratory. Dig down into the soil to find out if there are different layers. For example, your top layer of soil might have more sand in it, but if you dig four inches down, you find a layer of clay.

The United Nations’ Food and Agriculture Organization has an excellent visual guide to different methods of measuring soil permeability (to different degrees of accuracy) without fancy equipment. If you need a more accurate coefficient of permeability, then you may need to invest in equipment.

Factors That Affect Soil Permeability

  • Soil Composition. Soil is made up of different elements, like sand, clay, silt, and even organic matter. Sand has higher permeability, while clay and silt have lower permeability. The bigger percentage you have of sand, the higher the permeability, while the bigger percentage you have of clay and silt, the lower.
  • Particle Size. Not all particles are the same, with smaller particles providing more roadways for water to filter through. Very granular particles will have higher permeability, while blocky particles will have lower permeability.
  • Again, the more pores a soil has, the higher permeability it has. So if soil is very compacted, those pores disappear, making it much harder for water to filter through. Very light, aerated soil will have a lot more pores, allowing water to filter through quickly.
  • Yes, the liquid itself can affect the permeability. A thicker liquid will take longer to soak in than a thinner liquid (like contaminated water or heavy water vs distilled water).

Why Is Soil Permeability So Important?

Agriculture

Water management is a big deal in agriculture, with too much or too little water being able to decimate a crop. Rice fields need soils with low permeability or all the irrigated water would seep away. Other crops would rot away in such conditions. But if water drains away too quickly, plants will need to be constantly watered or die of dehydration. Not a problem in places with high annual rainfall (in which case, you need high permeability), but a big problem in areas where rainfall is scarce. 

Building New Ponds, Canals, or Other Waterways

When you’re trying to build a new pond, whether for fish conservation or as an additional feature in your garden, you don’t want the water to seep away. In these cases, low permeability (or impermeable) soils are important. Likewise, low permeability is important for crops like rice that need the field to be flooded.

Civil Engineering

Permeability affects the stability of slopes, retaining structures, and earthen dams, so when designing, civil engineers need to keep these factors in mind. These features need to stand up to water seepage in and around them, as high permeability could fracture or even shift the structure as water moves and destabilizes the soil.

Flood Management

Permeability plays a big role in flooding. Places with high permeability are less likely to flood than those with low permeability, because in places with low permeability, there’s no place for the water to go – especially when faced with a deluge. On a small scale, it’s why puddles linger on heavy clay soils. Knowing the permeability of soil can help craft better flood management plans by knowing which regions are most at risk.

About the Author

Jamie

Jamie is the founder of The Backyard Pros. When he was 15 years old he started working at a garden centre helping people buy plants, gardening products, and lawn care products. He has real estate experience and he is a home owner. Jamie loves backyard projects, refinishing furniture, and enjoys sharing his knowledge online.