Why Do Soils Contain Voids? The Science Explained

Soil voids are a concept often used in engineering and construction. While it may seem like a complicated scientific idea, voids are simply the spaces between solid soil particles, and a simple equation is used to represent them.

Soils contain voids because air and water fill the space between solid particles. All soils have voids, but many factors can impact the void ratio. The void space in soil affects water drainage and retention, land workability, and root resistance.

This article will discuss soil makeup, how to measure voids quantitatively, and how the grain of soil affects voids. We’ll also talk about the practical applications of soil voids in planting and gardening.

Solids, Air, and Water

To understand the science of voids, you need to appreciate the makeup of a given soil. Soil is made up of solids, air, and water. Air and water create void spaces between the solid particles. All soils contain voids, but the amount of void space can differ significantly between soils.

When there’s more water and air between the solids, there is a larger void space. Soil becomes more porous as the void space increases. When soil gets compacted, the solid particles become closer together, reducing the amount of air and water between them. So compaction decreases the void space in the soil.

Void space is generally determined by the soil texture and makeup. However, external factors such as rain and machine compaction can also affect the void space in the soil.

Soil Void Ratio

The amount of voids in a soil sample is represented quantitatively through a soil void ratio. The soil void ratio is the volume of air and water divided by the volume of solids in the soil. 

(air + water) / solids

As soil compacts, the void ratio gets smaller, reflecting the lower amount of air and water between particles. Since all soil contains voids, this number will never be 0. 

Soil Void Ratio

Factors That Affect Void Space in Soil

Many factors determine the void ratio in your soil:

  • Soil makeup. The soil makeup is the first determining factor, with grain size having the most significant impact on compaction. 
  • Natural factors. Natural factors also come into play, such as rain and erosion. 
  • Uncontrollable variables. These are variables you can control, such as machine and manual compaction.

Fine-Grained Soil vs. Coarse-Grained Soil

Soil grain refers to the size of solid particles in the soil. The larger the particles, the more coarse the soil becomes. For example, sand is a fine-grained material, while gravel is coarse-grained. 

Fine-grained soil has a higher void ratio than coarse-grained soil. This is because the smaller the grains of soil, the more space there is between them. 

Coarse-grained soil is also easier to compact, which is why it’s difficult to build structures on sand.

How Rain Affects Void Space

Rain can affect soil void space in different ways. While light rain increases void space, heavy rain decreases it. 

Remember that void space is made up of air and water, so it makes sense that more water would result in more void space. As water gets absorbed, the soil expands, becoming less compact. This is how light rain increases the void space in soil.

On the other hand, heavy rain can put pressure on the top layer of soil, compacting it. Rain that comes down too fast to be absorbed by the soil can also create the same kind of pressure.

Fortunately, the compaction that occurs during heavy rain does not penetrate deeply into the soil. Therefore it does not have any effect deeper in the soil. 

Machine Compaction

Some machines are commonly used for soil compaction, such as rollers. However, these are not the only machines that can reduce the void space in your soil. Any heavy machinery can contribute to soil compaction.

For example, if you have a truckload of dirt and proceed to drive the truck over to your garden and empty it, the soil the truck drives over will become compacted. If you plan to plant in that path later, it will be more difficult for seeds to sprout. 

On larger farms, you are typically dealing with even heavier equipment. Tractors, tillers, and harvesters are very heavy, therefore they decrease the void space in soil.

Manual Compaction

You can also manipulate the void space in your soil by hand. Soil without much void space can be tilled by hand using shovels, hoe, or rakes. You can also mimic the effects of light rainfall with a sprinkler or hose. Similarly, pressure compacts the soil, so using tools such as a tamper or roller will reduce void space. 

Compacting or tilling soil by hand isn’t realistic for large fields but it’s an excellent option for smaller gardens.

Why Soil Compaction Matters

The amount of void space in your soil has several effects on your ability to plant successfully. The void space changes the texture of the soil, giving it different properties.

Soil compaction affects water drainage, land workability, and root resistance.

In industrial settings, having a very low void ratio is essential. The soil must be compacted to support the heavy weight of structures. However, when it comes to growing plants, there needs to be enough void space to make the soil permeable.

Let’s explore how soil compaction impacts drainage, land workability, and root resistance.

Water Drainage and Retention

For soil to have good water drainage, there must be room for water to move between the solid particles. This means that soil with more voids will have better water drainage. 

Water drainage is essential because it prevents water from pooling around plant roots for extended periods. Although having access to water is important, if the water sits around roots for too long, it will cut off the plant’s access to air and minerals, causing the plant to develop root rot.

Without proper water drainage, soil can also quickly turn to mud, which makes growing plants much more difficult.

The opposite of water drainage is water retention, or the soil’s ability to hold water for longer periods. Water retention decreases the frequency you need to water your plants and is particularly useful in hot climates where soil tends to dry out quickly. 

Less void space results in higher water retention. The ideal soil for planting contains a balance of water drainage and water retention, which requires a moderate amount of void space. 

Land Workability

The workability of land refers to how easy it is to manipulate. Most small gardens don’t use large machinery, so the digging, tilling, and hoeing are done manually. 

Soil with less void space is more difficult to dig because it provides more resistance. If you consider the difference between digging a hole in sand (high void space) and digging one in clay (low void space), you can see how compacted soil is more difficult to work.

The opposite holds true; soil with more void space is easier to work with than more compact soil.

Root Resistance and Stability

If the ground is difficult for you to dig through, it will also be difficult for plants to grow their roots. 

Plant roots aren’t very strong, especially when they’re first sprouting. This is why some seeds are germinated in water. Roots need void space to establish a strong root system. When soil is compacted, and there’s very little void space, the roots become stunted hence cannot expand.

The other side of this is root stability. Although water can be a good way to germinate a seed initially, it doesn’t provide the plant with any root support. A plant needs to establish roots in the soil so that the plant doesn’t fall over. In this sense, some soil compaction is necessary to support the plants. 


All soils contain voids, which can be measured by the soil void ratio: 

(air + water) / solids

Soil with a higher void ratio is less compact, with the compaction increasing as the void ratio increases.

Ultimately, plants require void space to allow for water drainage, water retention, and root growth. However, too much void space does not provide enough stability for roots. Thus, the best soil for growing plants is soil with a moderate amount of void space.

Alexander Picot

Alexander Picot is the principal creator of TheGrowingLeaf.com, a website dedicated to gardening tips. Inspired by his mother’s love of gardening, Alex has a passion for taking care of plants and turning backyards into feel-good places and loves to share his experience with the rest of the world.

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