All soil is a mixture of clay, sand, and silt. The unique makeup of your soil determines its characteristics — such as permeability. So, how do the different materials compare in terms of permeability?
Clay is not more permeable than silt. Of the three materials that make up soil, clay is the least permeable. Clay is not very porous and is easily compacted. It also has smaller particles that limit the amount of space between the particles for water to drain.
In this article, I will discuss the factors that determine soil permeability, the permeability of different types of soil, and what all this means for your garden.
Factors That Determine Soil Permeability
Permeability describes how easily liquids can move through your soil. One way to think about permeability is that gravity naturally pulls water downwards, and the soil provides different levels of resistance.
The makeup of your soil (i.e., the ratio of clay, sand, and silt) determines the permeability or resistance of the soil. Three factors primarily affect the permeability of these materials: porosity and compaction. I will also briefly discuss infiltration and how it correlates with permeability.
While porosity is not the same as permeability, it does impact it. Porosity is the area between the particles of soil. Soil with larger particles will have more open space in between the particles. In general, the larger the space between particles, the more easily liquid and oxygen can move through the soil.
Although porosity affects permeability, the two are not always correlated. The reason for that is in order for water to pass through, the pores have to be connected. If the pores are cut off from one another by the particles, the porosity has no effect on permeability.
Clay is an exception to the above, however. Because its particles are so small, the ratio of pore space to clay is high. But since these particles sit close together, it cuts off the connection between the pores.
Soil becomes condensed if pressure is applied to it, meaning that it becomes compacted. The most common compaction causes are heavy machinery, such as tractors or cars. This is called mechanical compaction. Depending on the vehicle’s weight, the compaction can affect the top few feet of soil. As a result, this depth of compaction makes it difficult to reverse by hand.
Manual compaction is also possible, though it doesn’t produce as dramatic results. Walking on soil or leveling it with a shovel can also cause compaction, but it will often only affect the top few inches of soil. This compaction is more easily reversed by a rake or by hand.
More compact soil will be less permeable. As the particles are brought closer together, there’s less room for water to move through.
Heavy rainfall can cause compaction in all soil types due to the combined weight of several water droplets pounding on the top layer. Clay is particularly susceptible to this: It compacts more when wet because it becomes muddy and weighed down. Clay also has very small particles, so it can compact more tightly than the larger particles of sand and silt.
Infiltration occurs when water initially enters the soil. Infiltration is a direct result of permeability, so soil with low permeability will also have low infiltration. In other words, if water has a hard time penetrating the soil, it will likely not move well through the rest of the soil.
During heavy rainfall or watering, you may notice that water is absorbed quickly in sand and gravel but quickly pools on top of the clay. More pooling above the surface of the soil indicates poorer infiltration.
Compaction has the biggest impact on infiltration because its effects are concentrated on the top inches of soil. As mentioned above, whether it’s possible to reverse the effects of compaction depends on what causes it and to what extent it occurs.
Which Soil Is Most Permeable?
Sand is the most permeable type of soil. Soil with high sand content will have higher water drainage than its counterparts. Of the three soil materials, sand has the largest particle size. This leaves more connected space between the particles, allowing for greater drainage.
There are 12 different types of soil, but they are all made up of three different materials: sand, silt, and clay. The ratio of these materials to one another is used to classify the soil by texture.
Pure sand is the most permeable, but soil mixtures primarily made of sand will also provide great permeability. Sand has the largest particles, followed by silt, with clay particles being the smallest. This allows sand to have the greatest permeability.
Meanwhile, the small particles of clay cause them to fit together tightly like a puzzle, allowing little room for water to move between them. The larger particle sizes of silt and sand do not fit together as tightly, leaving more connected room between them, leading to a higher permeability. Sand is also resistant to compaction and is very easy to infiltrate.
Tip: I’ve written an extensive guide about the ideal soil texture for planting. Don’t miss it: The Ideal Soil Texture for Planting Explained
Permeability in the Garden
Permeability is an important concept to understand, but the most important thing to remember is its impact on your garden. The permeability of your soil directly impacts its water retention and drainage, which is crucial for the health and life of your plants.
Water retention and drainage go hand in hand. To meet the needs of your plants, you need a balance between the two.
Water retention is your soil’s ability to hold water. This retention provides plants with hydration between watering and rainfall.
Water retention is important because it provides plants with continued access to water. Water retention is especially important for plant hydration in hot climates where water evaporates more quickly.
Materials that have low permeability provide more water retention because it takes longer for the water to exit the soil. Clay retains the most water out of all the soils. Sandy soil will need to be watered more often than clay soil to give plants the same access to water.
Plants often grow best in soil with the same texture as their natural environment. For example, plants that grow naturally in bogs or marshes prefer soil with greater water retention. These plants include cattails, marsh marigolds, rhubarb, and cypress trees.
Although clay is the most permeable and therefore has the highest water retention, adding clay is not the best choice for increasing water retention in your soil. If you want to increase the water retention in your soil, you should supplement it with a light, high-retention additive such as vermicompost, vermiculite, or peat moss.
Conversely, drainage is a soil’s ability to remove liquid. Although plants need access to water, too much water can cause damage.
Drainage is important because it prevents water from pooling around the plant’s roots for too long. Pooling can cause root rot by surrounding the roots and cutting off the plant’s access to oxygen and nutrients.
Permeability determines how much drainage a soil provides. The more easily water can move through the soil, the faster it will drain.
Plants native to the desert and other arid environments thrive in soil with lots of drainage. This includes succulents, cacti, agave, and desert roses. Often, these plants grow in pure sand. They do not require as much water as other plants. Therefore, you should provide hot climate plants with very permeable soil.
The simplest way to increase the drainage of your soil is by adding sand. Pure sand is not the best option if your soil is very high in clay. Compost, peat moss, perlite, and gypsum also help improve drainage.
Clay is the least permeable of the three soil materials (sand, silt, and clay). Its small particles, though porous, do not have the well-connected pores necessary for water movement. Clay also compacts easily, reducing pore space even further and preventing infiltration.
Sand is the most permeable soil material. Its large particles allow air and water to travel easily between its pores. The permeability of silt falls between sand and clay.
The permeability of soil directly impacts its water retention and drainage. When soil is more permeable, it provides more drainage and less water retention.