hOW TO SUCCESSFULLY CHOOSE GROWING...
should also be heavy enough to provide physical support to
the plant.
Growing medium and production system
compatibility
It might be surprising, but in order to choose the best medium,
the first thing you should do is consider the production system’s
specifications. These factors include the type of irrigation
technique (dripper’s density and discharge), container size and
container shape. These specifications and the growing medium
must be compatible in order to obtain effective and uniform
distribution of the irrigation water.
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 contain pores of various sizes. Smaller pores can
retain water with more force than larger ones, and a large pore
cannot hold water against gravity. As such, the higher the pore is
positioned in the container, the smaller it has to be in order to
retain water against gravity. So, pores at the top of the container
that 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 (figure one), the width of the column
represents pore size and the capillaries have been “ironed out”
so they are straight and can be easily compared to each other.
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. There are forces that make the water climb
in the capillaries against gravity—however, these will not
be discussed here. We will only mention that they are called
cohesion (which is the affinity between water and particles
Figure 1
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Maximum Yield USA | September 2012