Prepared by Maher Jabado


Media is composed of solid, liquid, and gaseous components

Solid materials usually constitute 33-50% of the media volume. Spaces, or pores , between the solid particles are filled with air or water. As water moves through container media, it is retained by smaller pores, but drains through larger pores.

The second fraction of the media, the liquid portion, consists of nutrients, organic materials, dissolved gases, and water.

The third media phase consists of gaseous materials including oxygen and carbon dioxide. Although media oxygen levels vary from 0-21%, a concentration of at least 12% oxygen is necessary for root initiation to occur. Roots of most plants fail to grow in a media atmosphere containing less than 3% oxygen. The carbon dioxide content of the media may range from 0.03% to 21%; however, very high carbon dioxide contents may be detrimental to plant health (Bilderback, 1982).

Understanding the attributes of these media components, as well as the interactions between these components is essential.

Media pH

Media pH is a measure of the acidity or alkalinity of a substrate, with a pH = 7 indicating a neutral pH. Measured on a logarithmic scale ranging from 0 to 14, a pH greater than 7 denotes alkaline media and a pH less than 7 signifies acidic media. The acidity of media is determined by the concentration of hydrogen ions [H+] on media particles and in the media solution. The chemical composition of media particles, the ratio of media components in the mix, and irrigation and fertilizer practices affect the pH of growing media. Container media can increase 0.5 - 1.0 pH units during the growing season as a result of alkaline irrigation water.

Microorganism activity

The pH of organic media influences the activity of microorganisms; bacteria are more prevalent at pH > 5.5, while fungi are most active at pH < 5.5. Nitrification occurs most readily at a neutral pH, contributing to the transformation of the ammonium-nitrogen cation (NH4+) to the nitrate-nitrogen anion (NO3-); this increases the potential for nitrogen leaching from the soilless media solution.

Nutrient availability

Micronutrient availability is optimal at media 5.0 < pH < 6.5. However, because these nutrients are furnished through fertilization, pH regulation is not as crucial with container-grown nursery crops as it is with field-grown woody ornamentals. It is usually unnecessary to modify the container media to a pH greater than 6.5 for most woody plant species if sufficient levels of nutrients are available; for Ericaceous crops, the media pH should not exceed the value of 5.5.

Soluble salts

Because media is restricted to a limited container volume, ions from dissolved fertilizers and irrigation water can accumulate and contribute to high soluble salts levels in the media water extract. Media, fertilizer materials, and irrigation water sources should be selected to minimize soluble salts buildup; in addition, media solution soluble salts levels should be monitored regularly.

Buffering Capacity

Buffering capacity is the ability of media to withstand rapid pH fluctuations. Media with a high buffering capacity requires incorporation of a greater quantity of acid or base to alter the pH than media with a low buffering capacity. Media characterized by low buffering capacities include sandy mixes containing little organic matter, while media exhibiting high buffering capacities are usually composed of greater quantities of organic matter such as peat moss, bark, punga fibre. Select a container media with as high of a buffering capacity as possible to alleviate unexpected pH fluctuations.

Low initial fertility

Nutrient levels can be more accurately monitored in media characterized by minimal inherent nutrient value than in purchased and prepackaged media containing pre-incorporated fertilizer materials. Low initial media fertility affords the grower the opportunity to develop a fertilization program targeted towards fulfillment of the nutrient requirements associated with the developmental stage of the species in production.

Some types of media may render certain nutrients unavailable for plant uptake. Use of media such as sawdust or bark that is not adequately composted can lead to the unavailability of nitrogen for plant absorption as microorganisms break down these materials and assimilate the nitrogen for their own use. Vermiculite can inhibit absorption of phosphorus and iron; likewise, certain kinds of pine bark can eliminate iron from the media solution.


The amount of pore space in container media is a critical physical characteristic which influences water and nutrient absorption and gas exchange by the root system. Pore space is related to the shape, size, and arrangement of media particles. Aeration porosity and water-holding capacity are two critical physical attributes of container media.

Total porosity reflects the total pore space present in growing media; it represents the percentage of the container media volume which is not occupied by solid media particles. Porosity is determined by media particle size and the extent to which the particles can be compressed. Total porosity is the sum of the aeration and water-holding porosity of media and should comprise over 50% of the container media volume.

Irrigating media to the point of saturation fills the total pore space with water. As the media drains by the force of gravity, smaller pores remain filled with water while larger pores empty and fill with air. When all water has drained from the large pores, the amount of water remaining in the medium's small pores is referred to as container capacity. Aeration porosity is comprised mainly of the large pore spaces, macropores, which drain water freely as a result of gravitational forces and remain filled with air after media saturation and drainage.

For adequate gas exchange, aeration porosity should constitute at least 15%, but ideally, 20-35% of the media volume. Water retaining micropores should comprise 20-30% of the media volume. Water held in even smaller pores is not easily extracted by the plant. Conditions under which these very small spaces are the only pores retaining water often result in some stomatal closure and wilting. As the media dries and water is available only from the smallest pores, significant wilting can occur.

For sufficient gas exchange, drainage, and water-holding capacities, the proper proportion of macropores to micropores is necessary. The type of container media mix used determines the amount of macropores and micropores in the media. In addition, the size arrangement of pores is important in the ultimate water-holding capacity of the mix. A peat-sand mix contains a greater number of large and medium sized pores than a bark-sand mix. Media containing the greatest amount of medium-sized pores has the potential to hold more readily available water.

Media drainage

Sufficient media drainage is critical for optimal plant growth. The rate at which water drains from container media depends on the pore size and cohesive and adhesive forces between the water and container media. Media depth and pore size affect the height of the perched water table which is created when water saturates media pore spaces. If coarse materials like gravel or sand are placed in the bottom of the container, the smaller pores in the media above this layer will retain water until pressure forces the liquid downward. Water accumulation above these coarse materials elevate the perched water table.

If small pores are prevalent in the bottom layer of container media, water will pass through the larger pores above this layer fairly quickly and saturate the base layer, potentially creating an atmosphere too wet for vigorous root growth (Swanson, 1989). Avoid media saturation in the upper or lower layers of the container by thoroughly mixing the media.

Drainage, or hydraulic conductivity, is the rate at which water flows through the media.

Drainage is affected by the height of the container. Containers that have identical heights but different diameters have similar drainage characteristics when the same media is used in both. In general, water retention of container media decreases as the height of the water column increases. Media in a tall container characterized by a greater depth drains more readily than the same media in a short container with a shallower media depth. Media in a short container remains wetter than the same media in a tall container because of a lack of drainage; use a deeper container to improve media drainage.

When coarse material is placed at the bottom of a container, the height of the column is shortened, altering the drainage pattern. However, addition of coarse materials to the container bottom aids in drainage by constructing larger pore spaces.

Media capacity exists when large pore spaces do not contain any free water after drainage. Water is retained only in small pore spaces by adhesive and cohesive forces. After drainage, such a situation exists in the upper portion of container media.

Water retention

As an indication of sufficient water retention, media should absorb two inches of water per hour without runoff. In addition, if one quart of water can flow through media in a one gallon container per minute, adequate drainage exists.

Container media should also have the capability to retain sufficient amounts of water for root uptake. The ability of one cubic foot of media to retain three gallons of water is indicative of sufficient media moisture retention (Swanson, 1989).


Cation exchange capacity

Cation exchange capacity (CEC) quantifies the ability of media to provide a nutrient reserve for plant uptake. It is the sum of exchangeable cations, or positively charged ions, media can adsorb per unit weight or volume. It is usually measured in milligram equivalents per 100 g or 100 cm3 (meq/100 g or meq/100 cm3, respectively).

A high CEC value characterizes media with a high nutrient-holding capacity that can retain nutrients for plant uptake between applications of fertilizer. Media characterized by a high CEC retains nutrients from leaching during irrigation. In addition, a high CEC provides a buffer from abrupt fluctuations in media salinity and pH.

Important cations in the cation exchange complex in order of adsorption strength include calcium (Ca2+) > magnesium (Mg2+) > potassium (K+) > ammonium (NH4+), and sodium (Na+). Micronutrients which also are adsorbed to media particles include iron (Fe2+ and Fe3+), manganese (Mn2+), zinc (Zn2+), and copper (Cu2+).

The cations bind loosely to negatively charged sites on media particles until they are released into the liquid phase of the media. Once they are released into the media solution, cations are absorbed by plant roots or exchanged for other cations held on the media particles.

Anion exchange capacity

Some media retains small quantities of anions, negatively charged ions, in addition to cations. However, anion exchange capacities are usually negligible, allowing anions such as nitrate (NO3-), chloride (Cl-), sulphate (SO4-), and phosphate (H2PO4-) to leach from the media.

Percent base saturation

The concentration of potassium, magnesium, and calcium expressed as a percentage of the cation exchange capacity is referred to as the percent base saturation. Values for percent base saturation should be within the range of 1-5%, 10-15%, and 60-80% for potassium, magnesium, and calcium, respectively. Media nutrient analysis recommendations for the application of these nutrients are established from the ratios of potassium, magnesium, and calcium to each other in addition to the quantity of these nutrients present in the media.