Previously known as flood and drain and currently ebb and flow. This poetic name accurately describes the mechanism of the system. Initially, the substrate is completely filled with a nutrient solution, and then the entire solution is discharged. For the first time in history, Alice and Robert Whitrow used Purdue University on an industrial scale. Their system consisted of containers with a brick or cement frame and bottom, filled with gravel. The supply of nutrients took place by gravity. After filling the tray with nutrient solution, it flowed into another tray or simply merged through the door at one of the ends of the container. Brilliant and simple idea! The system performs oxygenation! When water is drained, a draft occurs, which brings fresh air, saturated with oxygen, into the root zone and at the same time carries gaseous root secretions from it. This is an outdated system. But at its base modern industrial and compact versions for growing plants in rooms have been developed.
More successful it is really an unpretentious old system of periodic flooding. It originates not from the greenhouse industry but was developed by inhabitants of the Californian hinterland in huts that did not know electricity. It does not have a pump, and it works by gravity. It can be included in passive systems, but it also belongs to the true systems of periodic flooding.
Imagine two buckets (one bigger than the other) connected by a plastic hose. The hose is connected to the bottom of each bucket with a simple connection. A bigger bucket is a seedling pot, a smaller one is a tank. A seedling pot is filled with an inert substrate, plants are planted in it, then it is set to an average height, say, for a couple of bricks. The tank has two positions: one on the surface of the earth, the other on the shelf above the bucket. The tank is filled with nutrient solution. Then it is enough to set the tank on the shelf, and the liquid will fill the seedling pot. When the entire solution has flowed into the seedling pot, the tank must be lowered to the ground and the solution from the seedling pot will leak back into the tank. You need to repeat this action several times a day in your free time! This system does not require any special expenses and works well. But, as with any system without a pump, it requires a constant presence to perform the work of the pump and the timer. Of course, this system is not so common in our time, despite the fact that it gives remarkable results.
Nowadays people give pumps and timers. Therefore, the most common online system of periodic flooding is a plastic seedling tray mounted on a plastic tank or elevated above the ground on a frame. To save space, the tank is always placed at the bottom. The system is designed so that access to the tank is always open for maintenance. In this system, the nutrient solution is pumped from the bottom of the tank, from the tank under the tray, through a pipe connection. When the pump is turned on, the nutrient solution slowly fills the tray. This movement displaces the air, creating an upward flow, refreshing the root zone and squeezing out the gases accumulated in it. Another pipe connection, approximately 15 cm above the bottom of the tray, fixes the upper fluid level. It is also a precautionary measure in case the pump will work too long to prevent the tray from overfilling. If the pump does not turn off, water returns to the tank through the upper drain pipe connection (normal nipple).
When the pump is turned off, the nutrient solution begins to flow by gravity into the tank through the same opening through which the solution was rising. It was then that fresh, oxygen-enriched air re-enters the substrate. This, combined with the high humidity in this area, makes it the ideal environment for the roots. The lower nozzle is 0.5 cm above the bottom of the tray so that there is always water between the irrigation cycles. This is a good distinguishing feature of the system, as it constantly maintains moisture throughout the root felt.
Flooding occurs at regular intervals. How often? It depends on many factors, and the most important is the choice of substrate. Substrates have an important property to retain water, which predetermines the frequency of irrigation. Obviously, the faster the substrate is dried, the more often it has to be irrigated.
The simplest periodic flooding system is very similar to that described above. The seedling area is often square in order to optimally fit under the light source. To fix the plants, the tray can be filled with mineral wool, coconut fiber, expanded clay pellets or their mixtures. There are also deviations from the main scheme. First, you can put potted plants on the tray. This will help to save the substrate and make the system easier, but the most important is to allow the plants to be moved if necessary, in order to avoid shading of some plants with others. This method is called a subirrigation. Plants can be planted in cubes of mineral wool. In this case, a plastic film is applied to the tray to protect the roots from light. Obviously, you need to cut holes in the film so that the plants grow through them. In another, more convenient version, the tray remains empty. A hard cap with planting nests is put on it. This device has proven itself very well, but, like any method with bare roots, it is not suitable for beginners. These are the main varieties, but if you search the Internet for periodic flooding systems, it turns out that people were creative and invented many ingenious homegrown systems. Unfortunately, they often forget about the main thing ? simplicity!
Ultimately, the periodic flooding system is a great technology that can work wonders. The system meets the first law of hydroponics ? enrich the root zone with oxygen. Slowly raising and lowering the water level, the ?tidal? system provides excellent oxygenation ? the main prerequisite for a good system. The rise of water displaces the exhaust air from the root zone, and lowering the water ensures the supply of new fresh air.
The disadvantage of the system is its size limit. Tray area of 1 sq. M. it may already be cumbersome to move around the confined space of the greenhouse. Larger devices are practically unusable. Some ingenious systems combine two or three trays on one tank; so you can get 2 sq.m. seating area, convenient to maintain. Another problem is the irrigation cycle. It is important to find the optimal irrigation cycle, and it can vary depending on the time of year (even indoors!) And on the size of the plants. Irregular irrigation can damage the roots due to flooding (it occurs when oxygen dries out in the root zone) or from drying, or leads to growth below the optimum. It takes experience and time to select the proper irrigation cycle. According to modern research, frequent short-term watering provides better plant growth than long and less frequent. For a system with an area of 1 sq. M. It is worth using a well-drained substrate and a pump that is turned on for 10 minutes and turned off for 15 minutes. A total of 25 minutes of a cycle is obtained, which can be repeated continuously while the lighting is on, and about two times during a dark cycle. Using such a substrate as expanded clay, and following this simple irrigation schedule, you can successfully operate a system of periodic flooding.
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