Water Culture systems is a very simple hydroponic systems. While technically simple, they are still very effective for growing plants. Not only do hydroponic growers like using water culture systems, but many commercial growers use this system on a large scale. Mainly because it is a simple and easy concept. It's also very inexpensive system to build, and a reason why it's popular with home growers Since it is so simple, there are hundreds of ways to build the system.
What you need to build a Water Culture system:
Large container to hold the nutrient solution A floating raft like 2” ridged Styrofoam insulation Air pump with air stone 2 or 3” Net pots and growing media (like coco coir, gravel, etc) to hold the plants How the systems works: The plant is suspended in the net cups through the Styrofoam right above the nutrient solution. The roots hang down from the cups directly into the nutrient solution. The roots remain and grow into the solution. The roots do not suffocate due to the dissolved oxygen emitted by the air pump via stones in the solution. The more air bubbles the better for water culture systems. The bubbles should rise from the bottom and making direct contact with the roots. This will provide adequate oxygen for the roots and plants to thrive. There are two ways of providing aeration and dissolved oxygen to the nutrient solution.
Types of aeration
An aquarium air pump and air stones are typically used to provide air bubbles to the nutrient solution for the systems (as well as other types of hydroponic systems). The air stones are made of a porous rock like material, the small pores creating small air bubbles. The smaller the bubble, the better uptake and dissolved oxygen concentration. A soaker hose can be used in place of air stones. The soaker hose creates even smaller air bubbles. Smaller air bubbles provide more contact surface with the water. The contact between the air bubbles and water helps replace the dissolved oxygen uptake by the plants roots.
Though not typical in water culture systems, surface agitation from falling water is another way of aerating the water and nutrient solution. The higher the falling water, the more downward force it creates. This downward force increases agitation and aeration of the solution. This method is more common in commercial systems because of the large volumes of water they need.
Recirculating Water Culture systems
Another variation of the typical water culture system is a recirculating system. This system works like a flood and drain system but never drains. You can have as many growing reservoirs as you want connected to one central reservoir. Each growing container has its own fill line, as well as a drain/overflow tube that drains back to the central reservoir. Some growers use buckets instead of wide or shallow containers. Each bucket has its own plant and filled with nutrient solution. Most have rows of these buckets. Using a fountain/pond pump to pump the nutrient solution up to each of the buckets. As the water fills the buckets, the excess water spills over into the overflow tube and flows back to the reservoir where it's recirculated back through the system again. Most growers that recirculate the nutrient solution and use a single air pump in the central reservoir, rather than in each individual bucket. They run this system 24 hours a day. Recirculating the water allows you to utilize falling water as a source of aeration. Also you don't need to constantly check the water level in each container. This is great for growing large, or numerous plants in the same system. Almost all commercial systems recirculate water this way.
Deep Water Culture or DWC
Deep water culture or DWC is basically the same as the water culture system. It uses “deep” water depth in order to grow its plants and roots. The depth can vary 8 or more inches. Regardless of the water depth, DWC systems are still water culture systems. Most of the time the water/nutrient solution depth doesn't need to be deeper than 8 inches. That's really only needed for larger plants that have larger root systems that need more space. These plants generally require more water and nutrients. There is no difference functionally between DWC and regular water culture systems. Water volume is different than water height. If you take a gallon of water and pour it in a wide bucket, the water height may only be a inch or two high, but you pour the same gallon of water in a 3 inch wide tube, the water height will be closer to 2 feet high. So water volume and height are two completely different things. Should the water level be above or below the net cups? There's often confusion and sometimes maybe even debate on where the water/nutrient solution level should be in water culture systems. Should the net cup touch the water, or hanging just above it? I personally have more luck having the very bottom of the net cup touching the top of the water. If you add an air pump and stones, the bubbles will usually splash water onto the bottom of the cups anyways.. As long as the growing media or roots remain in contact with the water/nutrient solution, you will have good growth in your system. One great advantage of this system is that deep water takes longer to heat on hot days and the roots remain cooler and grow better. On cold days, the water will take longer to cool.
The Kratky Method
The Kratky Method of deep water culture was named after B.A. Kratky at the University of Hawaii who teaches non-recirculating hydroponic methods. Non recirculating hydroponic systems (also referred to as "run to waste" systems) don't circulate water/nutrient solution from the reservoir to the plants and back again to the reservoir. They still pump water from the reservoir to the plants, but then allow the water/nutrient solution to drain off onto the ground, or into a drain system to discard any runoff. It sounds wasteful, but non recirculating systems can be very efficient and have very little runoff if done right. Water culture systems by definition are non-recirculating, but can be modified to be circulating systems as well. The hydroponic system sometimes referred to as the Kratky method is simply a water culture system without the air pump, as well as part NFT system. It's a water culture system because the plants hang above the water/nutrient reservoir the roots hang down into. It's also part NFT system because like NFT systems, there is a gap between the basket holding the plant and water the roots sit in. This gap is an air pocket and is supposed to replace the air pump in a standard water culture system. While the plants are small the basket is supposed to touch the water so the roots can begin growing out the bottom. As the plants grow and the roots get longer, the plant drinks up some of the water as well. That lowers the water level leaving a air gap. Without the air pump to replace the dissolved oxygen and oxygenate the water, the plant's need the air gap to be able to get the oxygen from. This type of system design is useful in places where electricity is non existent or unreliable. However this methods does have it's distinct drawbacks. The air pump does more than just supply dissolved oxygen in water culture systems. The rising bubbles also keep the water moving around. When the water/nutrient solution is stagnant, the mineral salts (nutrients) settle near the bottom. As a result the nutrient balance becomes uneven (very strong near the bottom, and very weak near the top). The rising air bubbles from the air pump create movement in the water that keeps the nutrient solution mixing all the time, and thus nutrients evenly distributed throughout the water as well. Also, while the plants roots are able to get oxygen when using the Kratky method, the roots above the water line cant get nutrients, and the roots below the water line cant get oxygen because they have already depleted the dissolved oxygen in the water early on, and there is nothing to replace it. That's a source of stress for the plant. Think of it like being in swimming pool and not being able to move while having your nose above water so you could breath, and having your mouth below the water line and able to drink water so you don't dehydrate. You can survive this way if you had to, but it would be very uncomfortable. Plants are adaptable and will always try to adapt to their environment and surroundings as best they can. But the conditions provided when using the Kratky method are far from ideal conditions. While they are far from ideal conditions, and the cost to run an air pump 24/7 and/or otherwise replace the dissolved oxygen is extremely low. In areas where the electricity is very unreliable or nonexistent, the Kratky method can be a beneficial and useful option.