Most soilless growing media are composed of multiple components with distinct physical properties that fit together to form pore spaces of various sizes. Coarse particles create larger pores (macropores) that maintain air spaces and facilitate drainage, while fine particles create smaller pores (micropores) that hold available (and unavailable) water. Each component functions to meets a series of essential requirements. The media formulation is equally important because individual components will not retain their original porosity when combined. The combination of media components amounts added, and their distribution determines the character and porosity of a medium.

Why should growers be concerned about growing medium components, formulations, and essential requirements? Ensuring plant roots have sufficient air, water, and the right balance of available nutrients safeguards crop success. Stressed plants are more susceptible to pests and diseases as well as environmental stresses, which increases maintenance costs and decreases revenue.

Media Purpose and Properties

The purpose of growing medium is to support plants and their roots. A well-formulated medium creates a stable substrate that provides the right characteristics for a crop. Essential requirements must be met for the medium to offer the right amount of available oxygen and water for plant-root uptake while also providing a reservoir of available nutrients.

The essential requirements of a growing medium can be broken into two categories—physical and chemical requirements. The physical requirements are impacted more by component selection. The chemical requirements are impacted more by water and fertilizer selection and include pH and soluble salts. These requirements must be met to facilitate healthy root systems for strong growth.

The physical properties of a medium are generally dictated by the coarseness and fineness of its component particles. Key physical properties include bulk density, texture, porosity, available/unavailable water, and decomposition time. The cultural requirements of a crop determine the physical properties of a medium. Getting the balance of physical properties right for a given crop will aid in disease resistance, root respiration, and root development.

The chemical properties of a medium are equally important. Cation exchange capacity (CEC), electrical conductivity (EC), and pH are the three key properties that aid in retaining and releasing nutrients available for plant uptake. A good mix will maintain the right pH for a given crop while holding and providing needed nutrients for uptake.

Air and Water Pore Space

Particle sizes and the spaces they create are fundamental because particle size and distribution dictates the air-holding and water-holding capacity of a medium. Total porosity indicates a growing medium’s volume percentage of pore space. The number of macropores and micropores characterize media pore space.

There are general porosity and air space percentage ranges for soilless media in commercial ornamental production. On average, media has a “normal range” with a total porosity of 75-95%, air space of 10-30%, and container capacity of 65-80%. The recommended air-space range varies by crop: propagation mix should have 10-15% air space, general greenhouse production media should have 15-25% air space, and perennial mix should have no more than 25% air space.

Pores are the spaces between the solid components of the medium. Most soilless growing media contains 60% to 80% total pore space. Pore size determines the rate of drainage and gas exchange. The size and distribution of pores are the most critical factors in developing a growing medium with optimum physical characteristics. The amount of air and water pore space is determined by particle size. Larger or coarser particles have a greater percentage of air pore space, while smaller or finer particles have a greater percentage of water pore space. Adequate distribution of large and small pores is essential.

A mix containing coarse particles has fewer but larger pores. Large pores permit air to reenter the medium following irrigation. On average, most mixes contain 10% to 30% air following irrigation. Despite losing more water more quickly, larger pore spaces offer plants more available water at watering time.

A mix containing fine particles has more but smaller pores and a greater percentage of water pore space. The larger the quantity of water held by a medium, the lower the percentage of pore air space it maintains. The closer a water molecule is to a solid, the more tightly it is held through the forces of adhesion and cohesion. Therefore, a fine mix may hold more water than a coarse mix, but less water may be available to the plant. In general, the amount of unavailable water is relatively high in soilless growing medium.

Water Holding Capacity Stages and Media Components

Three stages are applied to water holding capacity: saturation, container or field capacity, and wilting or permanent wilting point. At saturation, most pores are full of water during irrigation. Gravitational water drains from the macropores due to gravity, and these pores refill with air. At container or field capacity, gravitational water has drained, and the medium contains available water for plant growth. Capillary action allows the micropores to retain water. At wilting point or permanent wilting point, no more water is available to plants and most plants wilt and fail to recover turgor when irrigated. All water between container or field capacity and wilting point is available water.

Different media components contribute to water holding capacity at various levels. Consider the following ingredients: coir, rock wool, peat, pine bark, vermiculite, perlite, and sand. Each lends unique characteristics with respect to saturation, container capacity, and the amount of water held. For example, components with smaller, finer particles and pores, such as Canadian Sphagnum peat moss, processed coconut coir, and rock wool, hold more water. In contrast, those with larger, coarser particles, such as pine bark, perlite, and sand hold little water. The percentage of each component making up the mix will determine the duration of container or field capacity.

Let’s consider an example of high- and low-bark media and how bark percentage impacts total porosity, container capacity, air space, and bulk density. A high-bark medium (50% peat, 10% perlite, 15% vermiculite, and 25% bark) maintains the following physical characteristics: total porosity of 87%, container capacity 71%, air space 16%, and bulk density 13-16 pounds/cubic foot. A low-bark medium (55% peat, 15% perlite, 15% vermiculite, 15% bark) maintains the following physical characteristics: total porosity of 90%, container capacity 78%, air space 12%, and bulk density 10-12 pounds/cubic foot. The higher bark medium is more heavyweight and maintains a lower container capacity, but it has more air space. As particle size increases, so does porosity.

 Container Height and Media Water

Container height also impacts the relative amount of water versus air in a medium. The taller the container, the less evenly water is distributed throughout the container, regardless of the media. Here is how it works.

Water is pulled down through the pot and drainage holes via gravity. Gravity in the container is constant but the gravitational potential at the container’s top is higher than it is at the bottom. Water is attracted to particles by adhesion, cohesion, and capillary and resist gravity. Matric potential, which is a medium’s ability to hold water through adhesion and cohesion, remains constant throughout the container.

As James Altland, Ph.D of North Willamette Research and Extension Center,
Oregon State University writes in the article, Physical Properties of Container Media: “Because of this gradual decrease in gravitational potential towards the container bottom, matric potential is higher at the container bottom and media particles are able to hold more water. This causes water to form a perched water table at the container bottom. The perched water table is a layer of saturation on the container bottom. In contrast, more air and less water exist towards the top of the container. With the same media, the perched water table occurs at the same height, regardless of the container size. Short containers will have the same perched water table as large containers. Thus, a greater percentage of container volume is filled with water. This explains why a taller 5-gallon container holds less water than a shorter 5-gallon container.”

Media Handling and Porosity

Handling impacts media porosity. Not only is it up to the grower to select the right growing media for their needs, they must also correctly manage it. Many handling and management mistakes can damage media structure and performance, resulting in poor plant growth.

Over mixing and fluffing or using inadequate equipment for fluffing compressed-bale growing media can damage the structure of various components, which can decrease porosity and structure. Pre-wetting media to 48 to 52 percent  moisture at the fluffing stage will help it maintain structure and perform well. On average, media are produced at 35 to 45 percent moisture. Pre-wetting will also help growers avoid irrigation problems, such as water channeling and dry areas in the media.

Media compaction will also decrease the presence of macropores, decrease air space and increase bulk density. Compaction can occur when a medium is manually pressed down at planting time, or when filled pots of flats are stacked. Proper media handling will keep it well aerated will allow it to perform as expected.

Conclusion

When selecting or formulating media for crops, be sure to choose media components carefully and consider the overall formulation with respect to air porosity and water-holding ability. Manage your media well to ensure it maintains the right characteristics and choose the right medium for the right pot size. Making good growing media decisions will ensure plant roots have sufficient porosity for air and water as well as the right balance of available nutrients for success.