There are many different plant nutrients on the hydroponics
market today. Their function is to provide the optimum mix
of Nitrogen, Phosphorous, Potassium, Calcium and various
other trace elements, in order to sustain growth, improve
yields and allow the plant to achieve its potential. The plants
requirements will vary to some extent as it develops.
Concentrations and plant food components may also vary
with differing growing mediums. The food is absorbed
through the plants roots and transported to the leaves, where
it is converted into the sugars that the plant needs for
The most important thing to remember about plant nutrition
is that the NPK, (Nitrogen, Phosphorous, Potassium) Calcium
and trace element ratios are correct. There can be a wide
variation of ingredients in the various mixes for sale.
Because the plant will take whatever it requires from the
elements available and leave the rest, the balance will alter as
unused elements build up in the solution. If left unchecked
this will result in a toxic build up of salts and a subsequent
drop off in growth followed eventually by the death of your,
well loved and nurtured, plants. This same result will occur if
the water content is not replaced and the mixture strength
increases. If the plant transpires 50% of the water from the
supply tank, the concentration of elements within the solution
will become dangerously high.
The concentration of salts in the feeding solution is
measured using an Electrical Conductivity (EC) meter. The
EC meter measures the strength of the solution in parts per
million. This means that in a 1000 PPM solution there are
1,000 units of dissolved salts to every 1,000,000 units of
water. The meter measures the total salt concentration in
solution and does not discriminate between Potassium salts
say and Calcium salts. It cannot tell the difference between a
good and a bad mix, only their relative strengths.
The EC meter works by measuring the speed at which
electrons travel between probes immersed in a solution. In
distilled water, the electrons cannot find any impurities to use
as footholds to cross the water and so the meter returns a 0
reading in mMho or mS (these are units used to measure
electrical conductivity). As food is added to the water, the
concentration of impurities in the form of salts increases and
the electrons can find more footholds, and so cross the water
faster. Thus the meter reading rises. Of course this is a very
simplified explanation, but it should serve to give you an idea
of the basics. One other important thing to remember is that
as in all things chemical temperature plays an important part.
The higher the temperature, the faster the electrons move
and the higher the EC reading. This means that that in order
to accurately assess your mixture’s EC you must record the
PPM as mMho (mS) at a specific temperature.
As the PPM reading is a conversion from an electrical reading
and as each addition of a different salt will alter the electrical
properties, in order to obtain an accurate EC reading you will
have to use a reference solution of a known value. Because
the EC meter you are using will not necessarily have been
calibrated for the mix used by the people who prepared your
reference solution, these values can be quite inaccurate. In
view of this, any reference solution that does not show the
EC value in mS, or give you the conversion ratio that was
used, is of no use for nutrient evaluation purposes.
It is important to note that if the nutrient EC reaches 3,000
PPM (or the meter reads over 4.0mS) your plants will begin to
show signs of nutrient deficiency even though they will have
an excess. The reasons for this are quite complex, but
basically it is because the chemicals dissolved in the solution
are competing for the available water and the stronger ones
are blocking out some of the weaker ones. This leads to the
roots having to work harder to absorb the nutrients. By
working harder they have to expend more energy at the
expense of growth. If at this time the temperature rises and
the water level drops, due to evaporation, your plants will,
very probably, die.
Probably the most important factor that will affect your plant
growth in relation to nutrient uptake is pH. Different types of
plant prefer different pH values and it is important to
ascertain which the optimum for the species you are growing
is. The medium in which you are growing will affect the cation
exchange capacity of the plant. This is the ability of the
medium to hold nutrients on call for the plant roots to use.
Normal soil has a high cation exchange rate (CEC) of between
100 and 200 equivalent units. A number of growing
mediums and of course water cultures have a CEC of 0. This
means that once a nutrient has passed the roots it cannot be
taken up by the plant, and neither will it have any buffering
The nutrients, the gasses, the trace elements, the water and
the growing medium all have differing electrical charges and
are all exchanging positive and negative charges around the
roots of the plant. This ionic battle enables the roots to
absorb the nutrients it needs to sustain the plant. If the pH is
incorrect it stops the particle exchange. This is because the
shapes and sizes of the charged particles will be different
from the spaces available within the plant root tissue. The pH
can be looked at a bit like a Yale lock and key. If all is correct
the lock opens if the plant pH and the surrounding pH differ
then the lock cannot open.
Different plants need different nutrients at differing stages of
their growth. These nutrients have different charges and so
in order to get the greatest nutrient uptake the pH must be
closely monitored. If in doubt about the requirements of your
plant try asking the manufacturer of your nutrients for help.
After all he made the mix in the first place and so should
know all there is to know about it. If your plants are not
thriving look at the pH as the primary cause and try to work
out which of the nutrients is not being absorbed and why.
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