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In Nutrient film technique systems a shallow stream of nutrient enriched water is continuously re-circulated past the plants root balls which are suspended in in watertight channels. It is best that the channels depth be relatively shallow and only a thin film of nutrient enriched water [nutrient solution] is trickled over the roots. This is where the name 'Nutrient film' is derived from.
If the flow is too deep, the roots will be unable to breathe and an anaerobic condition develops, basically the plants drown. The upper surface of the root balls or root mats should be exposed to the air for oxygenation. The channels should also be angled in a sloping fashion with the water flow originating at the upper most point and being gravity fed downwards to drain back into the reservoir at the lowest point.
As stated earlier in this article, the flowing water should be very shallow. The recommended flow rate for seedlings is 8 to 15 gallons per hour. For mature plants it should be doubled to 16 to 30 gallons per hour. Larger plants with more higher requirements would naturally be at the high end of the 15 to 30 gallons while smaller plants at the other end of the scale. Excess flow rates can easily cause nutrient deficiencies.
Flow rates for the most part are controlled by the slope of the conduits [channels] in conjunction with the quantity of water. The water pump has very little to do with it. The channels slope uses gravity to control how fast the water flows over the roots from end to end.
The optimal angle is one inch of drop for 30 inches of length [1:30]. A ratio of up 1 inch per every 40 inches [1:40] is acceptable. Growers should be able to adjust the slope as the plants mature to either expedite or slow the flow.
One common issue encountered with water flow is pooling around larger root systems. The roots in effect create a dam which causes excess water in one spot and an inadequate flow in others. This is one reason the channels should be adjustable. You'll also want to avoid uneven gullies / channels that sag. A sag interrupts the water flow. The channel should be kept as uniform as possible, any abnormalities easily hamper the uniform flow of water.
Channels should be limited to no more that 30 feet each. Longer channels have been known to contribute to nutrient deficiencies as the first plants hog up nutrients leaving a sparse feeding for those further down the channel. Commercial growers will sometimes add a second nutrient feed half way down the channel / growing tube, but for a home grower this only serves to befuddle the situation and is unnecessary when its just as simple to add a second grow tube / channel.
Nutrient film technique does not require an automatic timer, just a pump that runs continuously. As an interruption in the flow can be detrimental if not fatal to your plants, it is advisable to keep a backup pump on hand.
A gravity fed reservoir is sometimes used as a backup. It does not require a pump and hence is not subject to power outages or burnt out pumps. The reservoir is placed at a location elevated above the channel[s] and the solution is fed via gravity. Spent solution is dropped into a secondary reservoir [or the main reservoir that works off the pump] at a low point and either poured or pumped back up to the feed reservoir as needed.