Soilless Growing Media


Soilless Growing Media: How Water Holding Capacity & Air Porosity Impact Media Quality in a Float System ©1998 


Ray DeBruhl, Agronomist, Transplant Systems

When considering a growing media, there are some important questions that you should have answers to:

1-      What are its ingredients

2-      What is its potential water holding capacity

3-      What is its potential air porosity

The growing media supports the plant from germination to the time of transplanting. The growing media not only provides support for the plant but also must be a source of water, air and nutrients. The water and nutrients are absorbed from the media by the roots. In order for the media to best provide these essentials to the plant, the media quality must have certain characteristics.

These characteristics can be categorized into two areas:

1-      Physical Properties:

The physical properties include water holding capacity and air porosity, and also would include cation exchange capacity (CEC).

2-      Chemical Properties:

The chemical properties include the pH and soluble salts. This paper will focus on the Physical Properties of soilless media.

Your media quality will directly affect germination, root growth and plant growth. It is important to note that the media quality can be affected by your water quality.

(Please refer to the research paper on “Water Quality” on this site for a complete understanding of the importance of water quality and the impact that it can have on growing plants in the greenhouse.)

These characteristics are influenced heavily by the components that make up the media. The components of a quality media for the float system should contain:

1-        Sphagnum Peat Moss

2-        Vermiculite

3-        Perlite

Sphagnum Peat Moss has the highest water holding capacity of all the peats. It will hold up to 60% of its volume in water. It is also the most acidic, with a pH level of 3.0 to 4.5. While this peat has good water holding capacity due to its extensive surface area, it also provides for excellent gas exchange in the large pores between  the peat particles. This peat should not be finely ground down or this excellent gas exchange will be dramatically reduced.

Aggregates such as Vermiculite and Perlite should be added to provide aeration and drainage. Vermiculite adds water holding capacity as well, due to the extensive surface area of the particles. It also contributes to aeration and drainage due to the large pore space between the vermiculite particles. However the coarser grades of  vermiculite are tremendously better contributors to air porosity than the finer grades are.  The finer grades increase the capillary pore space, which hold water, while it decreases the non-capillary pore space which bring in air. In a float system the fine grades of vermiculite should be avoided.

Perlite is an aggregate that any good media should have in a float system environment. It is added to the media for aeration. Aeration is the main property that Perlite contributes to the media. Perlite is a volcanic rock that is crushed and heated to a very high temperature. It expands to form white particles of air filled cells and adds tremendously to Air Porosity. The absence of perlite in the soil mix will impede air porosity and reduce the quality of the mix.

The physical properties of your soil media that can affect your plants is contained within the pore spaces of your media, commonly referred to as soil porosity. Soil Porosity refers to the space between the peat particles. These pore spaces are the voids between the solid particles of your peat. Water, air and nutrients are stored in this soil matrix. It is within this matrix that the plant roots grow and absorb nutrients and get air.

The amount and the size of these pore spaces are very important constituents that make up a quality media. The amount of pores is referred to as porosity. It is this volume of pores that will directly affect the water holding capacity or in other words the ability to hold water in the growing media. This is very important especially since in a float environment the plant is sitting in water. Remember, there is no shortage of water in a float system environment. However, there can be a shortage of air porosity. And it is the water holding capacity of the media that will impact whether there is a shortage of air porosity in the cell media in a float system. A lack of air porosity can lead to the inability of the radical root to penetrate the soil surface. A phenomenon we often refer to as aerial root. A lack of air porosity can also lead to less vigorous growth or even stunted growth. Air Porosity is needed for good root growth, good plant growth and for helping control algae.

Therefore the water holding capacity of the media in a float system becomes extremely important as it will have a direct impact on air porosity.  The pore size distribution or Soil Porosity in the media will directly affect water retention and air space. More water retention means less air porosity. This is why fine texture soil mixes should be avoided in a float system.

This Soil Porosity is determined by the particle size distribution of the media. This is immediately evident by whether the media is coarse or fine in texture. A fine textured media has smaller but more numerous pores than a coarse texture media. It is these smaller pores between particles that hold water against gravity (Capillary Pore Space), which contribute to a higher water holding capacity. A coarse texture media has bigger pores (Non-capillary Pore Space) than a fine texture media, and it has less ability to hold water against gravity, thereby contributing to higher air porosity. Water is held tighter in smaller pores than larger ones. As a result, fine texture media will hold more water than a coarse texture media. It stands to reason, that if it is holding more water, then it is holding less air.

Water holding capacity is also influenced by the surface area of the peat particles themselves as they absorb water. Fine textured media has smaller particle size than coarse textured media, but the sum total of the surface area of the fine texture is much larger than the sum total surface area of coarse textured particles. Therefore, fine texture media will hold substantially more water than coarse texture media.

Remember in a float environment there is not a need for higher water holding capacity in a media because there is no shortage of water as the plant is sitting in water permanently. So, our emphasis should be on selecting a media that has higher Air Porosity  potential.

The media best suited for float system environments are ones that exhibit “broad particle size distribution”. Broad particle size distribution simply means that the media has small, medium, and large particle sizes. These media types are coarse in nature. Media that are fine in nature do not have this characteristic. Soilless media that has been ground too fine typically will hold too much water and will not have adequate air porosity.

This can best be illustrated by the following demonstration. Picture two glasses, one is filled with marbles all the same size, while the other one is filled with marbles of all different sizes. Now you fill both glasses with water, which glass will hold the most water? Most people would choose the glass with marbles of all different sizes. However the correct answer is the glass with marbles the same size.

A soil media that has been ground to fine, where the structured has been destroyed and the particles are all the same size is much like the glass with marbles of all the same size. It will hold more water than a mix that has a “broad particle size distribution” , like the glass with marbles of all different sizes. Media with a “broad particle size distribution” tend to be coarser in texture.

A soil media that is coarse in texture, is much better suited for the float system than a media that has had its structure reduced to particles all the same size and is a fine texture. A coarse texture soil mix offers greater air porosity than a fine texture mix.

A quality growing media will strike a balance between the retention of water and air porosity. Too much water (fine texture media) and your plants will suffer from poor soil aeration, a lack of air in the media. Normally an excess of aeration means that the plants will suffer from a lack of water, however, not in the float system.

Your soilless media is a valuable resource in your greenhouse production management systems. It supports plant life and is a source of water, nutrients and air. By understanding both the physical and chemical characteristics of soilless media, you can better evaluate the strengths and weakness of soilless media. And, make a better management decision on the best media to use in growing your greenhouse tobacco plants.


 Media Particle Size Distribution

Media Particle Size Distribution

Particle Size Distribution Demonstration

Particle Size Distribution Demonstration