Author: Mr. Guy Sela, CEO of SMART! Fertilizer Management software and an international expert in plant nutrition and irrigation.
Knowing the physical properties of your growing media is important both in the selection of the growing media and in its management.
The physical properties of growing media differ from those of soil and container production requires more attentive management.
Each growing media 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".
The growing media contains pores of various sizes. Smaller pores can retain water at a greater force than larger ones, and a large pore cannot hold water against gravity . 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, even the larger pores will retain the water, making the bottom layer saturated.
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". Adhesion is the affinity between water and the particles surface. Cohesion is the affinity between water and itself.
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.
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.