Tests fall into two categories based on water pressure: Pure clay will have a coefficient of 0.05 cm/hour while pure sand will have a coefficient of 5.0 cm/hour. The higher the number, the more permeable the soil. The coefficient of permeability (K in equations) is just how many meters or centimeters per second it takes water to move through the soil. Which test you use depends on how accurate a number (the coefficient of permeability) you need for other calculations. 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.
(You can patch higher permeable layers with clay.) How Is Soil Permeability Measured? 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. 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. The water filters quickly through the upper layers, but then stops at the clay. For example, while your upper layer of soil may have more sand in it, a layer of heavy clay may be inches beneath it. Your soil may have different layers with different soil textures. This means you need to water less frequently. High water-holding capacity – Because silt and clay-based soils drain slowly and absorb water, it holds onto water longer.These soils are better for building ponds and retaining walls. If those air pockets are flooded for long enough, the roots will rot and the plant will die. Plant roots need the air that exists between soil particles to survive. Slower drainage means you need to water less frequently, but if your soil gets too much water, plants will suffocate. Slow drainage – since water cannot move easily through silt and clay-based soils, it takes much longer for the water to drain away.The heavier the soil, the more likely it is you’ll see puddling or even flooding. Low infiltration rates – water slowly soaks into clay and silt-clay soil.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. Silt feels silky smooth when wet as it absorbs water. Clay feels sticky when wet, and it’s this factor that makes clay useful for pottery. Soils with low permeability have a lot of clay and silt. Low permeability in soil means that water moves slowly. For gardens, this means that you need to water more frequently as the soil dries out quickly.
It’s also a great quality when creating a drainage ditch. 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.