It is important to control the nutrient’s
concentration. Burning of roots or foliage can occur if it is too high. If
the concentration is too low then deficiency symptoms will occur.
The concentration is usually set by following the manufacturers
recommended ‘volumetric’ dose rate e.g. “4-5 tsp/ gal” (Fig 2.3). However,
for the ‘main’ inorganic nutrient, a ‘conductivity’
(“EC”) meter** is a helpful dosing aid.
**Also includes cF and TDS meters
Uses of EC meters
1. EC meters gauge the total concentration of salts in the nutrient
solution. Hence they are useful for setting the target EC when a nutrient
batch is first made - see “Step 4” below.
Note, EC meters will not detect “organics”
e.g. vitamins, hormones, etc.
2. They are also useful for maintaining
the target EC as nutrient and water is consumed in “re-circulating
systems”.
However, because both good and bad salts are
included in the reading, re-circulating nutrients must be frequently “dumped”
because of the likelihood of there being toxic levels of bad salts (e.g.
chloride), and an imbalance of good salts.
Units
EC measurements are typically stated as “mS”
(cF and
ppm are common, but
unequal alternatives). For example,
distilled water and organics have an EC of zero mS. Mature flowering
plants however typically require a nutrient solution EC of around 2mS.
Obtaining conductivity readings
Step 1.
Make sure meter is
calibrated.
Step 2.
Remove a ‘representative’ sample from the nutrient reservoir:
+ For re-circulating systems ensure the
reservoir is always filled to the same volume i.e. if the reservoir is
only half full, then the conductivity reading will be twice as high as
what it would be when full.
+ Stir the nutrient thoroughly prior to
sampling.
+ Ensure the sampling container is
clean.
Step 3.
Rinse electrode in distilled water before immersing in the sample. Wait a
few minutes** before switching the meter on and recording the EC.
** Wait longer if the sample’s temperature
is significantly different from 25oC (77oF).
Step 4.
+ If the EC is below target, add
nutrient to the water until correct (to calculate this see
Table 2.40).
When making a new batch of nutrient, use the label’s volumetric dose rate
as a guide (e.g. 4-5 tsp/ gal - Fig 2.3).
+ If the EC target is exceeded then add
more water (“top-up” water). To calculate this see
Table 2.50.
Refer to Table 2.30 for commonly recommended
EC levels for the main stages of growth. These levels are suitable for the
majority of plant types (for specific plants see
Table 2.30b).
Within certain limits, there is no need to be precise with EC levels
because plants will generally only consume what they need. Most
species will tolerate being above or below the recommended value by at
least 10-20%.
Step 5. When
complete, rinse electrode with distilled water. Store the electrode
in distilled water when not in use.
