Growing media – more than a bag of dirt!

23 May 2016
  • Whastsapp

Dr Jim Smith, ICL’s technical manager for growing media

Part 1 - Structure

A few people in the industry still make the mistake of thinking of growing media as a bag of dirt yet capable of growing every species of plant on the planet!  As most growers know, nothing could be further from the truth. Quality growing media is a highly technical, sophisticated material with an enormous amount of scientific thought behind it. In the first in a series of articles covering all aspects of growing media, Dr Jim Smith looks at structure.

Why is growing media so important?

Nurseries need mobile plants, hence they are grown in pots. The growing media provides necessary support, whilst also holding nutrients, water, and air (oxygen) for the roots to absorb.

What is structure?

Structure describes how the particles aggregate together and strongly influences how water moves through the profile.

Very uniform particles generate a greater amount of air space between them compared to variable particles, where smaller particles fill the spaces between the larger ones. The air space between the particles is known as the air filled porosity (AFP) and depends on particle size and uniformity. The diagram below shows how particles form together leaving air spaces between them.  

It is worth bearing in mind that there are differing ways of measuring AFP, which will give slightly different results, and that it is only a snapshot of the growing media at a specific time.  Many factors can cause degradation of growing media − such as handling, storage or even weather and irrigation types as well as plant roots − all breaking down structure within the growing media.

AFP is also an indication of how well the growing media drains – plants do not like waterlogged roots.  

Bark (5-12mm) is made up of hard relatively uniform particles which let free water drain away, leaving plenty of air space.  In contrast, peat (0-3mm) has very fine particles, which tend to mesh together filling most of the air space and generally restricting water from draining freely.

Different bulk raw materials are capable of absorbing and holding varying amounts of water.  Peat has the highest water holding capacity, followed by coir and bark (see table below).

Roots suck water out of the growing media. This is not a passive action and is measured in kilopascals (kPa).  Very simply the suction pressure is caused by osmosis, due to higher sugar levels in the root cells attracting water through the cell wall.  This is why plants wilt if the conductivity (salt level) in the growing media is higher than in the root cell.

What is kPa?

kilopascals (kPa) a unit of pressure named in honour of the French mathematician-physicist Blaise Pascal. 


Plants are capable of sucking water up to a pressure of 10kPa. The table below shows the % moisture available to the plant from different raw materials at a suction pressure of 5kPa – identifying peat (0-3mm) as the most readily available.


  • Particles are important in order to create air space and improve drainage
  • Water retention in the growing media is important, so irrigation can be less frequent
  • Growing system and management determines level of water retention vs drainage
  • Help is available from the ICL technical team.