Consequently, some plants were over-watered and the excess water and fertilizer were wasted on the ground underneath the benches providing a source of ground water contamination. It was quite common to have growers provide two to four times the volume of solution required to bring the substrate to container capacity. Then came the low volume drip systems set at 1-3 litres (0.26-0.78 gallons) / hour / emitter, which required lower operating pressures and provided a more uniform output among the emitters. The result was that smaller volumes could be applied, resulting in less solution being wasted and less leaching of the media occurred. With drip irrigation, the droplets of nutrient solution usually drain through a certain pathway within the substrate, saturating the bottom of the substrate first, and then the remainder of the upper substrate is wetted through capillary action. This is somewhat different from the conventional high rate, large volume irrigation where the water flows through the substrate more in a complete frontal action.

During the last five to 10 years, subirrigation has been reintroduced due to the many advantages of subirrigation over top irrigation such as:
* Water and nutrient savings as the irrigation solution can be recirculated;
* Greater uniformity of watering;
* More flexibility in pot size and spacing;
* Labour savings. Spacing, disbudding, and harvesting requires less time. No more plugged and/or maintenance of the emitters is required. The use of robotics (containers) can be more easily implemented if concrete floors or mobile trays are utilised.

With subirrigation, a large volume of solution is provided to the production area where the pots are sitting. However the substrate will only absorb moisture through capillary action until equilibrium (container capacity) is reached. The remainder of the solution will return to the holding tank. The extent of the capillary action depends on pore size distribution, the moisture content of the substrate as well as the surface tension of the solution. Pots with dry media theoretically may absorb more moisture than pots with moist media but it will take longer. Sphagnum peat moss becomes hydrophobic -- water repellant -- when it dries out and thus may not be able to absorb the same amount of water as was lost through evapo-transpiration since the previous watering. Total time for the solution uptake may take five minutes when the substrate is moist or more than two hours when the substrate is dry. Therefore subirrigated pots will require a higher frequency of irrigation than top-watered pots. If the substrate is too dry, two or more irrigations a few hours apart may be appropriate to achieve the desired rewetting.

There are different forms of subirrigation such as watering matt, ebb and flow and trough benches and flood (concrete) floors. The basic principle of subirrigation is that the solution (water + nutrients) is dispensed to the bottom of the pot through a number of holes in the container and allowed to be absorbed within a given period through capillary action against gravity by the substrate. The absorbed volume hardly ever exceeds the amount lost through evapo-transpiration and therefore depends on the size of the plant, amount of radiation, relative humidity and time period.

With top irrigation, the solution is applied to the upper surface of the substrate and allowed to penetrate by gravity into the substrate. Volume can be varied.

With subirrigation, there are two major reasons why closer monitoring of nutrient levels is required. Firstly, the water uptake by the substrate is small (due to small water losses from the small plants) and therefore the means by which recharging of the substrate with nutrients is limited. With overhead irrigation, one can quickly recharge the substrate with nutrients, while excessive subirrigation does not add any more nutrients. Growers should be knowledgeable about the nutritional status of the medium, when the subirrigation period starts. Experience has shown that direct-stick propagation programmes with overhead misting often leach out all nutrients before pots are moved to subirrigation.

Secondly, due to salt accumulation at the media surface, there are often very few roots in the top 3-5 cm (1.2-2 inches) of the substrate, and therefore a smaller rootzone volume is utilized by the plant with subirrigation. Experience has shown that subirrigation works better with smaller pot sizes. Capillary action will normally create a 10-13 cm (4-5.2 inches) water column and therefore pot sizes of 15 cm (6 inches) in diameter or less provide good uptake.

Types of subirrigation
Troughs. There are many different types and models but the roll-formed troughs function well. The troughs are usually 13-15 cm (5.2-6 inches) wide at the base and the length depends on the size of the greenhouse. The troughs are usually roll formed at the site so that there are no seams within the troughs and thus no leakage. Troughs are usually sloped (0.5%). The troughs are generally made of marine aluminum alloy and epoxy coated (optional) for some added protection against solutions with a pH < 5. It is important that the troughs are adequately supported to prevent sagging along the length of the troughs and stagnant water standing in the troughs between irrigations. The troughs can be stacked for wider spacing or can be turned over for the production of bigger pots (>18 cm (7.2 inches) diameter) or bedding plants (flats). The troughs allow for easy shipping (boxes can slide over the troughs) and can easily be cleaned between crops. Flow rate of the irrigation solution should allow for an irrigation of about five to 10 minutes. Outside rows may require a bit longer. It usually takes another 10 to 20 minutes for the troughs to drain completely. Another rule of thumb is, that the water front must reach the end of the trough before the supply is shut off.

Ebb and Flow. There are various manufacturers on the market (plastics or aluminum). This system is often used with movable containers. The latter are generally about 6 m (19.8 feet)long x 1.5-2 m (4.95-6.6 feet) wide. The important criteria are that the tables/containers are positioned horizontally to insure the supply inlet and drain outlet are even, and that no solution remains on the surface of the bench between irrigations. Filling time is generally five to 15 minutes, while draining often happens simultaneously. Cleaning of the tables/containers is important between crops to reduce carryover of diseases.

Concrete floors. These systems are becoming very popular with large growers with single crops (e.g. poinsettias, mums, lilies, foliage). The floors are made with steel reinforced concrete and floor heating. Supply and drainage are usually the same and located at the lowest point of the floor. Due to the relatively large areas (30-40m x 6m (99-132 feet x 19.8 feet)), large capacity pumps are required to raise the water level about 3-4 cm (1.2-1.6 inches) on these floors in about five to 10 minutes (1-2 m³ / min (3.3-6.6 feet³ / min)). Drain pipes must be larger diameter than in-flow pipes to allow draining within a similar time period. The system allows for easy shipping and cleaning of the floor. Wheeled seats make movement for personnel easier and less strenuous on their backs when working in the crop.

With all of the subirrigation systems, the irrigation water is provided to the bottom of the pot. Generally, the maximum irrigation time should be about 20 to 30 minutes. This time period is generally sufficient to reach container capacity. Only when the media is extremely dry, do the pots require a longer irrigation period, or a split application.

Another misconception is regarding whether or not the nutrient solution will change. Our experience is that with a water depth of 2-3 cm (0.8-1.2 inches), there is virtually no drain coming from the pots. If it drains, then the solution is likely close to what went into the pot. Therefore the electrical conductivity (EC) of the recirculating solution does not change during an irrigation cycle. The pH does not change except in the case of a new concrete floor. In the latter case, pH may increase depending on the gravel/sand source used for the concrete. A sealant may help to reduce this problem.

Watering is an all or nothing process. It is impossible to provide light waterings. This can make growing bedding plants such as impatiens, petunias, salvia and pansy difficult. These crops if grown on flood floors should be watered by hand or with booms. Some other bedding plants such as fibrous begonia and ageratum will perform satisfactorily on the floor.

Water quality and solution management
Tank management. Many growers are working with three tanks, namely a starting solution (X), a maintenance solution (1/2X), and clear water. The piping system is made in such a way that the solution can be pumped to any section and back to the tank. As solution is absorbed, freshly mixed solution is added to the existing tank. The solution make-up of the fresh solution is similar to what is already in the tank.

Aeration. Although very little information is published on this topic, it is advisable to aerate the tanks. Normal air appears to be sufficient to maintain aerobic conditions.

Cleaning. Accumulation of organic matter may cause anaerobic conditions within the tank and an organic source for any pathogenic organism. Return water should be filtered before entering holding tanks. Cleaning the tanks at least twice per year is a good management practice.

Advantages of troughs versus ebb and flow versus concrete floor
Air movement. The spacing between troughs of 1-5 cm (0.4-2 inches) provides for air movement especially when there are heating lines below the bench. This will provide some plant species a better environment to encourage transpiration and reduce the incidence of disease. On the other hand, plants with non-pliable leaves such as African Violets and Gloxinia may get brittle leaves which may easily break during shipping.

Flexibility in pot size. Troughs are generally suitable for up to 16-18 cm (6.5-7.0 inches) diameter pots only. Ebb and flow tables and concrete floors can handle any pot size and/or trays (shuttle or bedding plant).

Cost of installation. Troughs are generally the cheapest route especially when retrofitting. Ebb and flow tables and/or container systems are the more expensive alternative.

Retrofit. Troughs can be easily installed on top of existing wooden or expanded metal tables. Care has to be taken for the additional weight. Existing tables should have adjustable legs to alter the slope or any dips in the trough.

Tank size. Tanks for concrete floors are generally large in order to handle the large volumes of water required, while ebb and flow tables and troughs can be smaller. Make-up fertilizer solution can be quite simple.

Environmentally friendly. It can be argued that concrete floors may be both environmentally friendly as well as unfriendly. The benefit of recirculation of the nutrient solution has to be weighed against the addition of concrete, steel and plastic to the greenhouse. If the greenhouse operation ceases to exist then one may have to remove about 4,000 tonnes/ha of concrete and steel.

Need for alternate substrate requirements with subirrigation?
This subject area has not been extensively researched. Surprisingly, very few growers, who have made the changeover to subirrigation, have indicated that the physical characteristics of their substrate in use before the changeover were inadequate. It should be understood that a proper substrate should have the right proportion of macro and micro pores so that capillary action can take place while still having enough air space after watering. Most substrates contain 50-75% sphagnum peat with the remainder consisting of some form of aggregate (perlite, vermiculite, rockwool, calcined clay, coarse gravel or Styrofoam pellets.

Disease concerns
One of the main concerns with subirrigation has been the presence of diseases within the substrate and the potential to spread the disease throughout the system. Although disease inoculum (fungal and/or bacterial) can spread through the system, this is not generally the experience growers producing holiday crops have had. Some crops are generally not grown on subirrigation benches due to this factor (cyclamen, zygocactus). An option would be to sterilize the return water. However due to the relatively large volumes of water returning with subirrigation (75-90% is returned) the available techniques (heating, ozone, UV-radiation, slow infiltration etc.) are very expensive to install and operate. Some growers are using composted materials to increase the microbiological activity within the media or disease suppressive organisms in order to retard or eliminate the disease organisms. However, this trend may be independent of the irrigation system employed.

Conclusions
Subirrigation is a labour-saving technique, while also reducing the waste of water and nutrients into the environment. Although suitable for many potted crops, the use for bedding plants is still somewhat limited. Despite principally being different, subirrigation is similar to low volume drip irrigation. Salt accumulation at the top surface of the substrate takes place under both regimes to a similar degree due to water loss at the top of the substrate and the upward capillary flow. Nutrition management should be primarily focused on the initial nutrient status when pots move onto a subirrigation system while fertilizing at a lower concentration during the middle phase of the crop and clear water being applied at flowering/shipping. The greenhouse environment will have a lower humidity (especially during cold winter months) with most subirrigation systems compared to most overhead watering systems as the areas underneath the benches will remain dry. This is of particular concern when establishing young, recently planted crops with a small leaf area.