nutrient solution strength
L
ight is the major environmental variable that drives
photosynthesis in plants. However, if temperature, relative
humidity, CO 2 concentration, mineral nutrients, or media
moisture levels are outside of the optimal range for the plant
species you are growing, photosynthesis (and subsequent
plant growth) will be limited. There is a principal of limiting
factors that must be considered when cultivating plants, and
understanding how to properly measure and control each
variable is critical to the success of any controlled environment
plant growth facility. Therefore, taking a holistic approach to
managing environmental variables is recommended to optimize
growing conditions in a controlled environment. Measuring and
adjusting the strength of your nutrient solution will be the main
topic of this article, but we will also discuss how environmental
variables will impact water and nutrient uptake.
EC VS. TDS METERS
�
WHILE IT IS very
important to know
the strength of the
nutrient solution
applied to your crop,
it is equally (if not
more) important to
monitor the amount
of soluble salts in
your root zone.”
64
feature
It goes without saying that providing adequate mineral nutrients
to your root zone is critical to produce high yielding crops.
Nutrient deficiencies or toxicities are a problem that most growers
have dealt with at least once, and these nutrient imbalances can
significantly impact the yield and quality of a crop. Optimizing
the strength of your nutrient solution based on the stage of crop
growth and your environment is an important step to avoid
nutrient problems. There are two critical steps you can implement
to optimize the strength of the nutrient solution. The first step is
to accurately measure the concentration of the nutrient solution
being applied to your crops, and the second is to monitor the
amount of soluble salts present in your root zone. Most fertilizers
used for cultivation are composed of mineral salts that conduct
electricity when dissolved in a solution, and this conductive
property allows growers to accurately measure the strength of
a nutrient solution. The higher the conductivity, the higher the
concentration of dissolved salts present in a solution. Electrical
conductivity (EC) or total dissolved solid (TDS) meters are the
two most commonly used pieces of equipment to measure the
concentration of nutrients in a solution. However, TDS meters
(commonly referred to as parts per million meters) are really
EC meters in disguise. TDS meters measure the EC of a nutrient
solution then use a conversion factor of 500, 640, or 700 to estimate
the ppm of a solution (depending on the brand of the meter).
Whether you are using an EC or TDS meter, the steps used
to measure your nutrient solution are the same. You will first
measure the water that you will be mixing your nutrients with to
see if your source water contributes to the overall conductivity of
the solution. You will then mix your nutrients into the solution and
measure the overall conductivity of water and nutrients combined
(if you had any charge in your source water, you will need to
subtract that value from the reading if you are trying to achieve a
desired EC or ppm). Depending on your irrigation methods (hand
watering, drip, ebb and flood, etc.) and irrigation frequency, the
strength of your solution will need to be adjusted to maintain the
optimal level in the root zone for the crop you’re growing. While
it is very important to know the strength of the nutrient solu-
tion applied to your crop, it is equally (if not more) important to
monitor the amount of soluble salts in your root zone. A simple
method used to measure the conductivity of your media is called
the pour-thru method. The first step of this method is to irrigate
your crop with your nutrient solution until the media is completely
saturated. Wait 30 minutes, then place a saucer below the
container and pour distilled water over the surface of the media
until you collect a sample large enough (about 50 milliliters) to
be able to submerge the tip of an EC or TDS probe. This reading
will indicate whether you need to increase or reduce your nutrient
solution concentration, or if you need to flush the root zone with
pure water to remove soluble salts.