W
hen it comes to plant nutrients,
there are two main categories:
essential nutrients and beneficial nutrients. There are upwards of 17
essential nutrients, including three macronutrients—nitrogen, potassium and
phosphorus—and micronutrients such
as calcium, magnesium, sulfur, boron,
chlorine, copper, iron, manganese, molybdenum and zinc. Essential nutrients
formulated in the proper balance and
amount are what make a good hydroponic fertilizer.
The list of nutrients beneficial but not
essential to plants is a smaller category
consisting of four elements: aluminum,
sodium, cobalt and silicon. Before we
go on, it has to be said that not all of
these are all that beneficial to all plants.
In fact, aluminum, sodium and cobalt
can cause severe toxicities in most
plants. Silicon, however, has an interesting chemistry that prevents it from
causing harm to plants and is extremely
beneficial to many types of plants.
Silicon makes up 28% of the Earth’s
crust and is a major component of soil.
It is generally available to soil-grown
plants at low levels, but in some areas, it
has been depleted from the soil, which
can lead to a number of issues with crop
plants. As hydroponic growing became
more popular, the importance of silicon
became even more apparent. Even
when using hydroponic media such as
stonewool or hydroton, which are essentially silicon-dioxide/silica/quartz/
sand (SiO2), silicon does not become
available to plants because these media
are stable and do not break down into
available silicon (SiO42-). This attribute
makes these mediums ideal, as it means
they do not degrade or interact with
water chemistry, but they are a poor
source of silicon for plants.
Plants can either be silicon accumulators or silicon rejecters, which refers to
the amount of silicon that accumulates
in the tissues of the plant. For example,
rice, which is a silicon hyper-accumulator, can have as much as 10% of its
tissue dry weight be silicon. Tomato, a
silicon rejecter, has around 0.1% silicon
in its tissue. This method of classifying
plants may seem over-simplified because
although silicon may not be present in
some plants at high quantities, this does
“The bulk of research on silicon in horticulture
has been done on rice and sugar cane plants.”
not mean it is unimportant. Tomatoes,
for example, can suffer from silicon deficiency, which can impact flowering, yield
and plant quality even though the tissues
typically contain a smaller amount of
silicon compared to plants like rice.
The bulk of research on silicon in
horticulture has been done on rice and
sugar cane plants, which are silicon
hyper-accumulators. In these crops,
silicon has been shown to help with
issues such as improved resistance to
drought stress, heavy metals and salt
stress; increased structural stability; and
improved disease resistance. In recent
years, the focus of silicon research has
moved from silicon hyper-accumulators
to other plants. Current research at
the University of California, Davis is
focused on horticultural crops such
as dwarf citrus, chrysanthemum and
roses. Preliminary results point to
a decrease in pest populations on
both citrus and chrysanthemums.
Researchers have found a decrease in
leaf miner populations on these crops
when potassium silicate was included in
irrigation water. Past research by other
scientists has also shown a
decrease in powdery
mildew, specifically
on roses and
cucumbers.
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