Subirrigation: Watering From The Ground Up
The goal for any irrigation system is to deliver water to the growing medium as effectively and efficiently as possible. Effectively means getting the right amount of water into the growing medium. Efficiently means minimizing the amount of water that is lost from the system.
In order to irrigate effectively, an irrigation system must deliver water uniformly to every pot in an irrigation zone. An efficient irrigation system will either deliver water with minimal leaching and runoff or capture and reuse all the water that is not retained in the growing medium.
Sub-irrigation systems are generally both more effective and efficient than top-down systems. These systems include capillary mats, troughs, flood and drain trays and flood floors.
The simplest form of sub-irrigation, in these systems water is delivered to a porous mat that is in contact with the bottom of the container. This allows water to move from the mat to the growing medium. The mat is placed on a plastic sheet to retain water and is usually covered with a perforated plastic sheet to slow the growth of algae.
A recent innovation places a layer of fluffy material between the mat and the surface cover. When a pot is placed on the cover, otherwise separated from the mat, it sinks down to the mat. Water is usually delivered to the mat by a drip tube, but sprinklers or even hand watering can be used. Because a mat can be difficult to rewet when dry, it is kept partially moist all the time.
Capillary mat systems are most often used for small containers and crops that need to be kept uniformly moist.
Flood & Drain Irrigation Systems
These systems use troughs, ebb and flow trays and flood floors to deliver water directly to the base of pots or containers. In these systems, the water (or fertilizer solution) is pumped from a reservoir, returning during or at the end of an irrigation cycle.
Capturing and re-using irrigation water maximizes efficiency. However, debris tends to accumulate in the systems. At minimum, a screen or fabric debris filter is an essential component in recycled water systems. Another serious concern is the potential for the spread of disease between containers by organisms in the recycled water. More elaborate filtration, sanitization and/or sterilization methods may be required to treat the water.
Trough systems are typically constructed from marine-grade aluminum, flat on the bottom with short sides. The trough is placed on supports and is pitched slightly from the inlet to the outlet end. This helps water flow by gravity. There can be an air gap between adjacent troughs, but this is not always the case. The irrigation head is very shallow, so relatively long contact times are required for thorough irrigation. The volume of nutrient solution required at a given time is smaller for troughs than for trays or floors because the water head is shallower and the flooded area is smaller. Troughs are sometimes lined with a capillary mat to counter the tendency for channeling in the shallow flow.
Flood & Drain Trays
Flood and drain trays are essentially bench tops that double as an irrigation system. They are usually constructed of plastic, although marine-grade aluminum is sometimes used. The plastic trays are assembled on site from modules, so very large trays can be constructed. The tray is flat and the bottoms have channels so the tray fills and drains uniformly.
Every container has the same contact time and irrigation head depth. In some systems, the water enters and returns through the same line. Water is pumped in, then it returns to the reservoir when the pump shuts off.
In others, the water enters through one line and returns by a separate drain, usually emptying into a gutter that returns it to the reservoir. Water is pumped in at a rate faster than it can drain. This type of system is more flexible and adapted to use with rolling benches. Since flood trays can be moved, they are specially suited to automated transport systems.
This system is constructed from special concrete using a precision screed. Traditional flood floors have to be pitched upward from center to edge for drainage. Typically the edge is an inch or two higher than the center, forming a shallow “V.” At the base of the “V,” holes are drilled in the concrete to the supply and return line. The “V” fills up and drains back to the center.
Fill time is typically five to 10 minutes, and drain time is slightly longer. The depth of the irrigation head and the fill time are linked, and depend on the rate that water can be pumped onto the floor. The greater the depth, the longer it takes to drain as well. Containers in the center of the floor have both a longer contact time and a slightly higher head than those at the edge.
A recent innovation is the development of flood floors that fill and drain more rapidly. These systems typically have a wide, shallow “W”-shaped profile instead of the “V”-shape in traditional flood floors. A row of fill and drain ports are located at the base of each low point in the “W,” and the length of the run from the low to high point in the profile is shorter than in the traditional “V” floor. The depth of water at maximum flood is minimized, and flood and drainage are consistent with minimal contact time. The concrete surface is smoothed so very little water remains on the surface after a watering cycle. These systems can operate with an ebb and flood cycle as short as four minutes, compared to 20 minutes for a conventional system.
By manipulating the duration of flooding, the uptake of water during one complete irrigation cycle can be varied from as little as 30 percent of water-holding capacity to a level approaching full saturation. If the medium is only partially saturated, it is likely that very little solution drains from the newly-watered pots back into the reservoir, which may reduce spread of root-borne pathogens.
Some other benefits of rapid ebb and flow watering area more efficient use of water and fertilizer and a greater ability to impose water stress to control plant height growth. The benefit of a flooded floor, as opposed to a bench system, has to do with economies of scale and eliminating a substantial amount of hardware involved in bench support and plumbing.
Another recent innovation is the “cascading” flood floor. In this design, the floor is pitched slightly in one direction like a trough. Similar to troughs, water runs from the inlet at the high side to the outlet at the low side. The depth is shallow, less than half an inch, and the area can be flooded very rapidly.
When water is delivered by sub-irrigation, it contacts the growing medium at the bottom of the pot and is then drawn up by capillary action. The amount of water taken up will depend on the irrigation head, the size and placement of the drainage holes on the bottom of the pot, the height of the pot, the porosity of the growing medium and the duration of contact time between the growing medium and the water.
Different types of sub-irrigation systems allow varying degrees of regulating irrigation. In trough systems and cascading flood floors, the irrigation head is limited to a shallow depth. With trays and conventional flood floors, depth and duration are linked. By regulating depth and duration, especially with rapid-cycling floors, growers can control the amount of water taken up by the growing medium.