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Table 9.20
Feed Frequency & Volume Guide
(Guide for determining how often, and
how much, plants should be fed )
 Feed 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 that there are no dry spots.
- The duration of pumping time will
depend upon the flow rate from
feed
outlets.
- To automate feed times and duration,
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.
- 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 media are also less complicated in regard to
diseases and pests, and the build-up of contaminants.
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NFT
Feed outlet 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. Be wary of
sudden increases in air temperature because increased transpiration
and evaporation can cause roots to dry out if under watering or pump
failure occurs. |
Perlite
Water sufficiently to ensure the entire
root mat is fed and flushed. Feed once every 2-3 hours during
daylight. Generally use a 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 a lower feed frequency at night.
** Maximized using a flood & drain
system.
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Soil / Coconut fibre
/ Rockwool
These media 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 experienced 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
nutrient run-off is being collected: |
Feed times:
To avoid water-logging, generally do
not feed at night. However, this 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 that feeding less frequently will allow for
longer drainage intervals and therefore potentially better aeration.
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.
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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 x 100 ÷ Volume of feed
- ‘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.
- ‘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 x 100 ÷ 100ml = 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 media
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 media are also less complicated in regard to diseases and
pests and the build-up of contaminants.
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