This is growing plants in a stream of continuously running water. The plants can be placed in a shallow container and the water containing the nutrients runs through the roots. This can be carried out in several different ways, either passing the water over the bare roots inside a tube or hollow container or passing the water through a mat of fibre into which the roots have grown or variations between the two. One of the main advantages of this system is that the nutrient water can be kept in a storage container and if any changes need to be made to the nutrient it is easy to do this to the storage container. The system usually runs be gravity, the storage container is placed in a higher position than the plants and the water trickles downwards until it has passed though the roots of the plants and into a lower storage container where it is pumped back up to the higher container. An advantage of this method is that it uses the nutrients efficiently, it allows oxygen to reach the roots (important for healthy growth) and the water can be adjusted very easily. The main disadvantages are that if any fault occurs with the circulation system the plants have little water to live on until the fault is corrected and the flow channels must be regular in their slope, if they are not then water can remain in the non-sloping areas and this can cause problems.