Container production has been expanding in recent years. Choosing the optimal medium requires special consideration, because it is much more than just an anchor for the plant: it can be crucial to successful crop. Physical and chemical properties of growing media differ from those of soil and container production requires more attentive management.
There are many advantages to using growing media:
* High yields can be achieved on a limited area
* Better control over Irrigation and fertilization
* Easier disinfection
* Recycling of drainage water is possible
* Growing media can be used as an alternative to an inadequate soil (due to drainage problems, salinity etc.)
Of course, there are also disadvantages:
* Nutrient holding capacity is low
* Buffer capacity is low and therefore changes are rapid
In this report we will focus only on the physical properties. The chemical properties will be discussed in a separate report.
What Are The Physical Properties of Good Growing Media?
A balance between air content and available water is one of the most important requirements of good media. Plant roots require air for oxygen supply and gas exchange, and therefore, aeration is critical for optimum plant development. Lack of adequate aeration results in poor plant growth, susceptibility to diseases and nutrient deficiencies. Ideal growing media provide plants with adequate water supply and at the same time contain enough air to allow gas exchange in the root system.
Good growing media are also characterized by high hydraulic conductivity, i.e. ability to transmit water.
Another important property is the medium's weight: it should be light weight for easy and less expensive transport and handling. But it should also be heavy enough to provide physical support to the plant.
Growing Medium And Production System Compatibility
It may be surprising, but in order to choose the best medium, the first thing you should do is consider the production system's specifications. These include: the type of irrigation technique (drippers density and discharge), containers size and containers shape. These factors and the growing medium must be compatible in order to obtain uniform distribution of the irrigation water and effective irrigation.
Porosity and Water holding Capacity
Each growing medium has a characteristic particle size distribution. The spaces (pores) between the solid particles can be filled with either air or water and are referred to as "total porosity".
Each medium contains pores of various sizes. Smaller pores can retain water with more force than larger ones. A pore that is too large cannot hold water against gravity, and empties. The higher the pore is positioned in the container, the smaller it has to be in order to retain water against gravity. At the top of the container, pores which are too large to hold water against gravity are empty. Therefore, the top of the container will always be dryer than the bottom. At the bottom of the container, all pores, including the largest, are filled with water, making the bottom layer saturated.
Water Retention Curves
Labs can accurately measure the water percentage by volume (v/v in %) at given heights of the medium, after saturation and drainage. The height is measured in cm and the data can be graphically presented as a "Water Retention Curve". Some labs refer to the height as "tension in cm".
Containers Size and Shape
We mentioned above that the size and shape of the containers, in which the medium is placed, determine the amount of water that the media hold.
If we fill a few containers of the same volume, but different shapes, with the same medium and irrigate to saturation, the water will reach the same height in each of the containers.
The same water content in % will be measured at each height (according to the water retention curve of this medium). But because of the different shapes, the actual amount of water is different in each container.
This results in different water/air ratio in each container and in different irrigation management.
Irrigation frequency and amount of water applied in each irrigation are determined by the available water content of the medium and by the container shape and size. For example, one irrigation cycle a day is not enough, if the water consumption of the plant is higher than the amount of available water in the container medium.
Hydraulic Conductivity
As the name suggests, hydraulic conductivity is the rate in which a medium transmits water. Hydraulic conductivity of media is not routinely measured in lab tests. Nevertheless, it is extremely important to understand its significance. Hydraulic conductivity is in effect the limiting factor of water uptake by plants in container media, rather than the water quantity in the medium.
When transpiration rate exceeds the hydraulic conductivity of the medium the plant cannot efficiently use the water contained in the medium and might wilt. In materials used for container media, the hydraulic conductivity decreases exponentially as the medium dries. This is because continuity of water is disrupted after the larger pores empty.
Guy Sela is an agronomist and water specialist. In 2005 Guy developed a unique software for fertilizer management that help growers to reach higher and better yields, save time and frustration and increase their profits.
