hOW TO SUCCESSFULLY CHOOSE GROWING MEDIA
surface) and adhesion (which is the
affinity between water
and itself).
“
in inches.
Let’s take a look at
an example of water
Water retention curves
retention curves for two
Labs can accurately measure the water
different growing media
percentage by volume (v/v in %) at given (see figure two). The two
heights of the medium, after saturation
media have completely
and drainage. The height is measured in
different behaviors.
inches and the data can be graphically
Medium B drains more
presented as a water retention curve.
easily and holds less water than medium
Some labs refer to the height as tension
A at any given height (tension). For
example, at 5 in., medium A
holds 65% water, while medium
B holds only 28%. This is because
medium A contains a higher
percentage of smaller pores.
Therefore, a grower who decides
to grow in medium B under
the same conditions (irrigation
technique, container size and
shape, and cultivar) has to give
more frequent irrigations. On the
Figure 2
other hand, if the grower decides
86
Maximum Yield USA | September 2012
Irrigation frequency and amount
of water applied with each watering
are determined by the available water
content of the medium and by the
container shape and size.
”
to grow in medium A, his main concern
would probably be lack of aeration.
Other information that we can obtain
from water retention curves is the
amount of water available to the plant
roots. We know that tiny pores can retain
water extremely well; however, they
might also hold water so forcefully that
the plant cannot absorb it.
In growing media, water at tensions
of 20 to 40 in. is generally considered
unavailable to the plant because it is
retained in very small pores. In addition, a
high content of unavailable water can set
the stage for fungal problems and other