Woodfibre; what growers need to know?

Woodfibre is seen as a sustainable inert option to peat based growing media, however there are issues around its use:

Slumping Due to its structure, particularly at high levels

Water retention At levels over 25 to 30% there are difficulties with water absorption and retention

Nitrogen draw down Produced predominantly from white woods, the material continues to break down within the growing media potentially using available N in the process

Buffering capacity With a low surface area woodfibre is not as efficient at holding on to available nutrients

Air Filled Porosity (AFP)

What is Air Filled Porosity (AFP)?

Air filled porosity is the percentage of air spaces present within the growing media.

What effect does it have?

Adequate oxygen is required around plant roots to help maintain growth whilst discharging waste

gasses, such as carbon dioxide and ethylene into the atmosphere. For plant roots to remain aerated, the growing medium must have enough pore space without exposing the plant to drought stress.

Low AFP: There are not enough air spaces present to sufficiently aerate the roots, resulting in root damage and potentially whole plant loss.

High AFP: The air spaces present are too large and the medium drains quickly. The growing medium will have to be irrigated frequently to prevent drought stress.

Water Holding Capacity

What is Water Holding Capacity (WHC)?

The water holding capacity of a substrate is the ability to contain water, so it can be available to the plants when needed. WHC is controlled by the materials texture, composition and the amount of organic matter they contain; thus peat, coir, woodfibre and bark all have different levels of WHC.

Low WHC: The media holds insufficient water and thus plants cannot extract enough water and nutrients, resulting in wilting and whole plant loss. A growing media with low WHC will require very frequent watering cycles.

High WHC: The media contains too much water so plants can become waterlogged. This can be overcome by careful irrigation management.

Cation Exchange Capacity (CEC)

What is Cation Exchange Capacity (CEC)?

Cation Exchange Capacity, or CEC, is a rating of how well growing media can hold plant nutrients. In other words, particles within growing media can be seen as negative magnets (anions) which attract cations such as Potassium K⁺ Calcium Ca²⁺ and Magnesium Mg²⁺. CEC is expressed as milligrams equivalents per 100cm³.

What is Meq?

Meq is a unit of measurement that is expressed as milligrams of the cations that the growing media can hold per 100cm3 of volume. Therefore the higher the meq the better the substrate is at holding on to nutrients.

What effect does it have?

The cation exchange capacity helps to regulate the supply of certain nutrients, such as Ca²⁺, Mg²⁺, K⁺, NH⁴⁺ and Na⁺. CEC also affects the regulation of pH. Growing media with a high CEC will recover more quickly from acidification compared with a media with a low CEC.

Low CEC: The media lack the ability to hold on to cations, resulting in nutrient leaching and deficiencies particularly in Potassium K⁺ and Magnesium Mg².

High CEC: The media retain sufficient concentrations of cations which are then easily available to plants roots when needed.