How To Successfuly Choose Growing Media

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 crucialc 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.
   

However, there are also some disadvantages:

• Nutrient holding capacity is low
• Buffer capacity is low and therefore changes are rapid

This report we focus only on the physical properties. The chemical properties will be discussed in a separate report. 

Physical Properties of the 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.
 

Let's visualize...
 

Pores in a growing medium can be viewed as a series of capillaries. In our model, the width of the column represents pore size and the capillaries were "ironed out", so they are straight and can be easily compared to each other.
 

From this model it is easy to understand why the bottom of the medium is always saturated, while the top of the medium contains less water and more air.
 

The forces that make the water climb in the capillaries against gravity will not be discussed here. We will only mention that they are called "cohesion" and "adhesion". Cohesion is the affinity between water and the particles surface. Adhesion is the affinity between water and itself.  

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".

Let's take a look at an example of Water Retention Curves for two different growing media:

The two media have completely different behaviors. Medium B drains more easily and holds less water than medium A at any given height (tension). For example, at 20cm medium A holds 60% water, while medium B holds only 23%. This is because medium A contains higher percentage of smaller pores.

Therefore, under the same conditions (irrigation technique, container size and shape, cultivar) a grower who decides to grow in medium B, has to give more frequent irrigations. On the other hand, if the grower decides to grow in medium A, his main concern would probably be lack of aeration.

More information we can obtain from water retention curves is the amount of  water available to the plant roots. We know that very small pores may retain water very well, but they might also hold water so forcefully, that the plant cannot absorb it.

In growing media, water at tensions of 50-100cm is generally considered unavailable to the plant, because it is retained in very small pores. In addition, High content of unavailable water can set the stage for fungal and other diseases present in high humidity conditions.  

Containers Size and Shape 

It was 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.

Take a look at these containers:

            A                                 B                                 C                       D     

All containers are of the same volume, and are filled with the same medium. The blue area represents water. Since it is the same medium, the water reaches the same height in each of the containers.

Moreover, the same water content in % is 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 a 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 daily water consumption of the plant is higher than the amount of available water in the 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 fact, 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.