Soil Porosity: What Is It | Why Its Important | How To Improve It

Soil porosity is the term used to describe the amount of open space or pore space that can be found between particles of soil.

cut away of soil showing roots and porosity

Pore spaces can be formed in a number of ways. For example:

  • Insects and worms in the soil can cause pore spaces.
  • Gases can be trapped in the spaces by groundwater, and this can cause further expansion.
  • Root growth and movement can affect pore space.
  • Materials in soil may dissolve creating pore space.

Additionally, the texture of the soil will affect soil porosity. There are many different soil textures, but the main three are:

  1. Sand is a gritty material, and the individual soil particles can be seen with the naked eye.
  2. Silt is a slippery, smooth material when it is wet. The extremely tiny individual particles are difficult to see with unaided vision.
  3. Clay is a sticky material when wet. Its microscopic individual soil particles cannot be seen with unaided vision.

The combination of these three materials makes up the soil texture. Their different sizes, shapes, and qualities determine soil porosity.

Why Is Soil Porosity Important?

There are many reasons why the porosity of soil is important. Top among these is the fact that soil contains the groundwater that most living beings depend upon to sustain life. 

Healthy soil retains and filters water and is an important player in the water cycle. 

Soil porosity also determines the amount of oxygen that can be found in pore spaces. In addition to water, plants must have oxygen to grow and thrive. 

They need oxygen for respiration; therefore, it is important to have well-aerated soil to grow healthy plants.

Soil that has been compacted by heavy foot traffic, construction equipment or other causes has lowered soil porosity. This type of soil cannot contain enough water and oxygen to maintain clean, healthy groundwater or thriving plants.

Know-How to Judge the Porosity of Soil

Soil has many physical properties including:

Structure

The structure of soil affects porosity. Basic soil structure types are:

  • Coarse, sandy soil has a low percentage of organic matter and is not especially fertile. This type of soil is rapidly permeable and does not hold moisture well. In coarse, sandy soil nutrients may be leached away from plants. 
  • Medium textured, loamy soil has a higher percentage of organic matter. This type of soil holds moisture better and allows water and nutrients to move through the soil more slowly.
  • Finely textured, clayey soils have the greatest water and nutrient holding capacity, but this type of soil is difficult to work with because it is quite sticky when it’s wet and very hard when it’s dry.

The ideal soil is about half pore space. It should have equal amounts of water and air filling its pores. Soil that has too much air will cause plants to go thirsty. Soil that holds too much water hampers growth of plant roots and may cause root rot.

Water Holding Capacity

The ability of soil to hold water and react with nutrients is dependent on how much surface area is available. When soil contains a great deal of material with very small particle size, there is more available surface area. 

This is why clay, which has a very small particle size and a plate-like structure, is better able to hold water than are sand, gravel or rocks.

Texture

The texture is determined by the proportions of clay, silt, and sand. Soil texture is the most important property in determining how to correctly manage and use any given soil. 

Soil that has poor texture is very difficult to alter, and it is not economically feasible to attempt to change the texture of soil for commercial use.

Color

Soil color can be an indicator of the quality, texture, and porosity of the soil. For example, soil that is quite light in color may have a low percentage of organic matter and a high percentage of sand. 

This could result in a sharply draining soil with a high level of porosity.

Darker soil may contain a high percentage of organic matter. Additionally, dark-colored soil is often caused by poor drainage or low porosity.

Soil that is gray or red in color may contain a high percentage of clay. Interestingly, red clay is well aerated and has a good level of porosity. Gray clay is poorly draining and has a low level of porosity.

All of these properties combined determine soil porosity and how suitable a given soil maybe for use as a growth medium. 

What Damage Occurs With Too Little or Too Much Not Enough Soil Porosity?

Attempting to use soil that has not been amended to improve porosity is a recipe for failure in gardening. Some of the problems you’re likely to encounter include:

Compaction

As we have mentioned, when a great deal of pressure has been applied to soil through heavy foot traffic or heavy equipment or other means, the water and air are pushed out of the pore spaces. 

When soil is compacted, air and water cannot move through it and neither can plant roots. This state of affairs makes it very difficult for plants to thrive.

Erosion

When soil is compacted you can expect water to run off rather than soaking in and this can lead to erosion. Even though the sticky clay soil particles will not be washed away by water or blown away by the wind, the lighter weight soil particles will. This will leave you with soil that is predominantly clay, have poor water movement, be difficult to work with, and resistant to plant roots.

How To Improve Soil Porosity?

One of the most affordable and easiest ways to improve soil porosity is by adding organic matter. Although organic matter does not change soil texture, it does change soil structure and increases pore space while improving drainage. 

Follow these 5 smart tips:

  • Be sure to add organic matter to the soil both by tilling it in at the end of each growing season and by mulching with good organic matter during the growing season. Learn more about soil conditioning.
  • A thick mulch, 1″ inch to 3″ inches deep, will help hold moisture in the soil while slowly decomposing to add nutrients. At the end of the growing season, you can till this mulch into the soil to further nourish the soil and improve porosity.
  • Use a deep cultivation technique that mixes the top 6″ inches of soil, mulch, and organic matter into the top 24″ inches of soil. Note that you should not till the soil when it is very wet as this will damage soil structure.
  • Growing a cover crop during the winter helps retain moisture and protects your soil against erosion. 
  • If you have low spots on your property, create a rain garden to help capture runoff and sediment.

Be Patient & Consistent

It is possible to garden successfully with any texture of soil as long as you clearly understand all of the attributes of your soil. This knowledge can empower you to improve the soil to its greatest potential. 

It also informs you regarding which crops are most likely to be successful in your correctly amended soil.

There is no quick fix to improving porosity of soil. Use the 5 tips above a little at a time, on a consistent basis, year in, and year out. Simply working organic matter into your soil in increments, regularly will gradually improve the porosity of your soil.

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