Popular mediums include (from top to bottom) NFT (no media), Rockwool, coconut fibre, perlite and expanded clay.
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.
(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.
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.
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.
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.
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:
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.
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.
● Cooler weather: Generally aim for only ~10% run-off. However, lower percentage run-off will require more frequent flushing.
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.