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Feed frequency and volume is largely responsible for ensuring the optimum
balance of water, oxygen and plant nutrients at the root zone. It
also helps ensure
salt build-up, root exudates (etc) are flushed away. This objective
is most easily achieved in NFT,
and flood & drain systems containing
perlite and expanded clay. Note, coconut fibre and soil
systems are relatively difficult in this regard.
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Table 6.1
Feed Frequency & Volume Guide
This
chart is a guide only. Requirements may vary according to
plant variety, stage of growth, air temperature, humidity, etc.
Note the following:
+
Regularly inspect the root zone at
various points to ensure feeds are effective and consistent
(Fig
5.7).
+
Pumping time duration will depend upon the flow rate from
feed
outlets.
+
To automate the feed times and quantity utilize a
pump and timer to automatically
switch the pump on and off.
+
If growing under artificial lights, “daylight” refers to the “lights on”
period. |
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Feed flow rates must ensure the entire
root mat is covered. During daylight, the pump can be operated
either continuously or intermittently (”pulsed”) e.g. 10-20 feeds
per hour. Feed frequency can be less at night. Beware of sudden
increases in air temperature (or pump failure) because under
watering, especially if using pulsed NFT, can cause roots to quickly
dry out. |
Perlite
Water sufficiently to ensure the entire
root mat is fed and flushed. Feed once every 2-3 hours during
daylight. Generally use lower feed frequency at night. |
Expanded clay
Water sufficiently to ensure the entire root mat is fed and
flushed*. Feed once every 1-2 hours during daylight. Generally use
lower feed frequency at night.
* Maximized using a flood & drain
system. |
Soil / Coconut fibre
/ Rockwool
These mediums are typically used in ‘run-to-waste’
systems. Because of the number of variables it is difficult to
generalize. However, these are some guiding principles recommended
by run-to-waste growers:
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Be
careful not to over-water. This can cause oxygen starvation and fungal
diseases. With soil, use a sufficiently coarse grade to provide adequate
drainage and aeration.
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If excess nutrient
(run-off) is being collected (Fig 5.5b & 5.6): |
Feed times: To
avoid water-logging, generally do not feed at night – but depends on
humidity. Begin feeds at the beginning of daylight and cease at least 2
hours before night.
Feed
frequency: Space feeds evenly apart. Feed 3-6 times per day for larger
plants; 1-3 per day for smaller plants. Note, feeding less frequently will
allow longer drainage intervals for aeration to occur.
Feed volumes: Although
relatively complex, feed volumes are often determined by measuring the
‘percentage run-off’ (see below):
+ Warmer weather (or salty
make-up water): Generally aim for ~15-20% run-off. Although higher
run-off will help prevent salt build-up, it may cause water-logging.
+ Cooler weather: Generally aim for only ~10% run-off.
However, lower percentage run-off will require more frequent
flushing.
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How to calculate 'Percentage Run-off’
This is calculated as follows:
Volume of run-off ÷ Volume of feed x 100
- ‘Volume of run-off’ is the
total volume of nutrient collected at the drainage point over 24
hours.
- ‘Volume of feed’ is determined
by measuring the total volume (over 24 hours) delivered by all feed
outlets that drain to the drainage point. Assuming all plants are of
equivalent maturity, make sure they are fed equal volume – see “Equilibrating
outlet flow rates”.
- ‘Percentage run-off’ -
Achieving the desired percentage is determined by trial and error
e.g. If aiming for 10%, adjust feed volumes until 10ml of run-off is
collected for every 100ml fed to the plants (10ml ÷ 100ml x 100 =
10%). Realistically, because a plant’s demand for nutrient will
depend upon the stage of growth and environmental conditions, day to
day consistency is difficult to achieve.
EXAMPLE: A large plant grown in
cooler weather (requires ~10% run-off) may, for example, need to be
fed 1,000ml over the course of a day in order for 100ml of run-off
(10%) to be collected. If it is decided to feed 3 times per day and
the day length is 12 hours long, then the feeds should occur every
3.3 hours (10* hours ÷ 3 feeds), and individual feeds should be
~333ml (1,000ml ÷ 3 feeds).
*Generally cease
dosing 2 hours prior to night time.
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Growing mediums
Popular mediums include (from left to
right) Rockwool, coconut fibre, expanded clay and perlite.
Be careful when choosing a medium. Its
physical characteristics will determine how easy it is to achieve
the correct balance of water, nutrient and oxygen at the root zone.
Certain mediums are also less complicated in regard to diseases and
pests, and the build-up of contaminants.
See our
complete guide on mediums. |
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