What is Growing Medium?

growing medium holds water and nutrients, it is a place where gas and nutrient exchanges occur, and it also provides an anchor point for a plant’s roots. These physical characteristics of a growing medium are determined by the components used and the proportions in which they are blended together.

ns of a growing medium? A growing medium holds water and nutrients, it is a place where gas and nutrient exchanges occur, and it also provides an anchor point for a plant’s roots. These physical characteristics of a growing medium are determined by the components used and the proportions in which they are blended together. What is important to remember is that the resulting physical characteristics do not equal the sum of the ingredients.

First, let’s look at the components used in formulating growing media. Then, we will focus on the characteristics of growing media, and finally on biological additives.

Components:

Growing media components are either organic or inorganic. Organic components include peat moss, bark, coconut coir, rice hulls, etc. Inorganic components include perlite, pumice, vermiculite, sand, hydrogel, etc. Some of these components hold water on their surface, others hold water within their structure, while others hold little compared to other components.

There are also components, such as perlite, that hold very little water, if any.  Keep in mind that a specific type of ingredient can vary in its water holding capacity and physical structure, depending on its origin and how it is processed. For example, bark can vary greatly in its source and structure depending on how it is processed, aged, composted and screened. This is also true for peat moss. Light brown, fibrous peat moss has a porous structure and can hold up to 16 times its weight in water.

However, if this same peat moss is processed into fine particles, the available water can be cut in half and the air porosity decreases dramatically. If you blend your own growing medium, your source materials should be consistent to produce a quality and predictable growing medium. It is important to know the structure as well as the chemical and physical properties of the ingredients you use to be sure that the growing medium blend you produce is the same, batch after batch. 

organic-components

“Common organic components used in growing media. Starting in the upper left, going clockwise: bark, sphagnum peat moss and coir. Source: Premier Tech Horticulture.”

Inorganic components

“Common inorganic components used in growing media. Starting in the upper left, going clockwise: vermiculite, perlite and rice hulls. Source: Premier Tech Horticulture.”

Physical Characteristics:

While there are a number of laboratory tests for the physical characterization of growing media, the three most familiar measurements are bulk density (weight per volume), water holding capacity and air porosity.

Water holding capacity is the volume percentage of water retained after a saturated growing medium is allowed to drain.

Air porosity is a measurement of the volume of pore space occupied by air after a saturated growing medium is allowed to drain. For the most part, packaged growing media products have a low bulk density, since the majority are made with a base of sphagnum peat moss and have a higher water holding capacity.  Bark-based media are heavyweight products that are suitable when high drainage and container stability are required. Both products typically have good air porosity which is in a range of 10% – 18% by volume for most growing media.

Chemical Characteristics:

Two important measurements for growing media are pH and E.C. (Electrical Conductivity). pH is a determination of how acidic or basic a substance or solution is. E.C. reading measures the ability of soil solution to carry an electrical current and is an indication of the amount of nutrients available for crops to take up.For general purpose growing media, the ideal pH range is between 5.2 – 6.2 with a target of 5.8 when saturated. Desirable E.C. for general purpose growing media is between 1.0-2.0 mmhos/cm.

For seed germination and rooting of cuttings, the desired pH range will be slightly lower, between 5.0 – 6.0, with a target wet-out at 5.6. This pH range is slightly lower since pH can tend to rise during use from minimal fertilizer applications and water alkalinity of irrigation water from constant misting.  Desirable E.C. for germination and propagation growing media is between 0 – 1.0 mmhos/cm.

E.C. measurement in mud growing media

“A combination E.C. and pH meter is used to measure these parameters of a growing
medium sample. Source: Premier Tech Horticulture.”

Most commercial growing media are pH-adjusted with either calcitic or dolomitic limestone and contain a balanced starter fertilizer to help plants acclimate after planting. It is generally recommended to begin fertilization once new plant leaves begin to emerge and new roots develop.

The amount of fertilizer and frequency of applications will vary based on the crop type, stage of development, container size and frequency of plain water applications. Keep in mind that some ingredients used in formulating growing media may contain mineral salts, such as coir. It is recommended that coir should be leached thoroughly before use to reduce salt levels and potentially high nutrients (e.g.: potassium, chloride, sodium).

The same is true for bark, since aging and composting can release undesirable elements. To be sure, laboratory tests are recommended to check pH, E.C. and individual nutrient levels.