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Feeding & Flushing: How often and how much?

Feed frequency & volume

Feed requirements will depend on medium and system type, stage of growth, air temperature and humidity.

NFT (no media)
Feed outlet flow rates must ensure the entire root mat is covered. During daylight, the pump can be operated either continuously or intermittently 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, Vermiculite, Expanded clay
Water sufficiently to ensure the entire root mat is fed and flushed. Generally feed once every 1-3 hours during daylight, and use a lower feed frequency at night.

Soil / Coco Coir / Rockwool
These media are typically used in ‘run-to-waste’ systems. As they are prone to water-logging, over-watering must be avoided as it can cause oxygen starvation, pest invasion (fungus gnat), and fungal diseases. Soil and coco coir can be improved in this regard by adding up to 30% perlite or expanded clay. Feed requirements will vary as the plant matures. Some guiding principles are:

Nutrients or plain water: For coco and Rockwool, use nutrients at each watering. For soils with high capacity to retain nutrients (high CEC), it may be beneficial to use nutrients at just each alternate watering. This will help minimize salt build-up and the need for frequent flushing.

Feed times: To avoid water-logging, generally do not feed at night (lights-off period). Begin feeds at the beginning of daylight and cease at least 2 hours before night.

Feed volume: Nutrient feed volume should produce about 10-20% run-off out the bottom of the pot i.e. Collect 10-20ml at the drainage point for every 100ml of feed volume. Run-off helps ensure plants obtain a correct balance of nutrients and that the root-zone and medium is flushed of nuisance salts and thoroughly watered.

Feed frequency: As a rule-of-thumb, generally feed once per day, or once every other day. To determine if plants are ready for feeding, lift the pot (or slab) to check if it feels light. Alternatively:
• For soil, only feed once the soil feels dry up to about 1 inch (25mm) below its surface.
• For coco, feed if the color of the coco’s surface has begun to lighten or feels dry. Note, when plants are more mature (mid-to-late veg and throughout bloom), some growers prefer to feed several times per day. Assuming water-logging and its associated issues are avoided, more frequent waterings help minimize root-zone salt build-up (and lock-out) and the risk of deficiency symptoms.

Flushing

Flushing is done to remove deposits of salt precipitates, general dirt and algae from the root zone, medium and other system parts. Pay particular attention to flushing the root zone and feed circuit. Inspect filters, inlets and feed outlets prior to replenishing the system with fresh nutrient. These are prone to becoming blocked with solid material dislodged during the flushing process.

Recirculating systems
Flushing is done immediately after the nutrient is discarded. First, do any necessary manual cleaning, such as the removal of any obvious build-up. Then partly fill the reservoir with fresh water and operate the pump to flush the feed circuit, medium and root zone. Flushing can be enhanced by spraying with a garden hose. Discard the waste, then repeat the process until the waste water is clear and its EC is equivalent to that of the input water.

Run-to-waste systems
As a general rule, coco coir and soil systems should be flushed weekly, and Rockwool flushed every second week. The frequency ultimately depends on factors such as stage of growth, salinity and temperature.

If flushing can be scheduled to occur when the nutrient reservoir is empty, then the existing system hardware can be used. Place low alkalinity water (pH 5-6) in the reservoir and operate the nutrient pump until the EC of the run-off water is no higher than about 0.5mS above that of the water in the reservoir. This is generally achieved once about 3 times the normal feeding volume is used. It can be a benefit to do additional flushing with a garden hose if the surface of the medium is readily accessible.

If the nutrient reservoir cannot be emptied to conduct flushing, it is beneficial to have another dedicated reservoir and pump for flushing. This can be connected to the existing feed circuit at a junction controlled by a 3-way valve. The valve is simply diverted to the second reservoir whenever flushing occurs.
© Andrew M Taylor (FloraMax)

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