Greenhouse Canada

Growing Media Inputs
Inside View: September 2012

August 21, 2012
By Gary Jones


As I write, severe droughts are hitting most of the continental U.S. As
well, sadly, extreme temperatures are causing loss of human life.

As I write, severe droughts are hitting most of the continental U.S. As well, sadly, extreme temperatures are causing loss of human life.

By contrast, here on the West Coast the weather was the coolest and wettest in decades, and water was available in abundance. Perhaps we should not be surprised. These extreme weather events are what many climate scientists have been predicting will become the new “norm.”


So, as water continues to be a big issue, how can we manage growing media to increase the resilience of our production systems?

When I was at university, I spent what felt like way too much time studying soil physics and hydrology. OK, so I get “field capacity” and “permanent wilting point.” But as for “Bernoulli’s equation” and “Penman’s method,” well, they left me high and dry. And still do. So, let’s stick with some basics of how to manage growing media with regard to water retention and availability.

In potted crops, there are a number of factors to consider, including:
Particle size and distribution: In most basic terms, individual particles making up a soil (or medium) can be divided arbitrarily into two major classes: the clay fraction (smaller than 2 µm) and the non-clay fraction (sand, silt and gravel, larger than 2 µm). Soil survey reports show the percentage of soil in each fraction based on an oven-dry soil passing a 2 mm sieve. Gravel and stones larger than 2 mm are reported separately. At its most simple, the larger the particle diameters, the more free draining will be the medium. However … 
Air-filled porosity: … firstly, the way in which a soil or medium behaves depends also on how such particles are “arranged.”

■ Sandy soils typically occur as a collection of individual grains. But in other media, particles can be linked together into clusters or aggregates, and this has a marked effect on the behaviour of the medium. This space between the particles forms an intricate system of chambers (sometimes called “air filled porosity”) that strongly influences the amount of and movement of both water and air.

Bulk density:… and secondly, particle size distribution affects the bulk density of the medium. Think of mixing billiard balls, gravel and sugar – the smaller particles tend to fit in between the larger ones. So for a given volume, the density goes up as more, smaller particles are added.

Talk to your supplier about how best to achieve a suitable compromise between a free-draining medium and one that holds on to water tightly while at the same time making it available to plant roots and being of suitable weight for use.

Incorporate organic material: While it may not always be possible, or indeed desirable, adding organic matter will typically make a growing medium retain more moisture and hence make it more resilient to drought.

Mulch (pot covers if necessary): Reducing the amount of water leaving a pot will reduce the need for water input. Preventing evaporation from the medium surface by using a mulch will increase the resilience of the medium to drought. Mulches can be paper, artificial (e.g., rubber) or organic (e.g., woodchips), although the latter may not be very practical when it comes to shipping.

Remove weeds (including mosses and liverworts): If changes to the medium are difficult (and even if they’re not!), remove weeds so as to reduce competition for water (and nutrients).

What about less commonly used techniques?

  • Biochar? As well as particle size, the type of particle is also critically important. Biochar (pyrolysed [= burnt in the absence of oxygen] wood) consists of charcoal-like sticks that are simply a carbon “skeleton.”
  • The surface area of those “sticks” is absolutely immense, and the space between those “skeleton bones” provides tremendous water-holding capacity. While this water can be relatively difficult for plants to obtain, it does provide a “reserve tank” of moisture.
  • Go high-tech if need be: There are a number of water absorbing materials (polymers, “hydrophyllic granules”) available (e.g., Hydrosorb™, Watersorb®).
  • These can help growing media maintain a water supply to plant roots and so reduce the need to irrigate as frequently.

Gary Jones is chair of Production Horticulture at Kwantlen University, Langley, B.C. He serves on several industry committees and would welcome comments at

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