Recirculating Systems
Traditionally, plant nutrients have been delivered as solid or liquid fertilizer sources wherein nutrients are solubilized into ionic form and carried down through the root zone by gravitational force in water. When soil was used, it allowed for potential storage of excessive nutrients as well as affording a system for buffering which stabilized the pH. With the advent of container growing and the use of soilless mixtures with greater aeration porosity and drainage capacity, much of the solution carrying nutrients are leached out of the containers. It has been suggested that nutrient solution be applied in excess (20% or more) as a leaching process to prevent the accumulation of excessive salts. Although a number of hydroponic systems (eg. nutrient film techniques) and substrates have been employed in the greenhouse industry in which nutrients are recirculated, it is typical to renew the solution periodically. There is at least a qualitative perception that extended (ie. over several cropping cycles) use of recirculated nutrient solution impacts negatively on production and quality. The practices of one-way circulation of nutrient fertilizers, leaching and periodic dumping of solutions contribute to waste and potential ground water pollution.  In order to reduce this leaching and dumping of nutrient solution, growers must reuse the solution for long periods of time. This is possible with subirrigation, in which the movement of liquid is unidirectional upwards into the pot by capillarity. This method does not change the solution returning to the storage tank so the solution can be used indefinitely. When solution is recycled through a substrate by overhead watering the components of the solution are significantly changed when they return to the storage tank. This can cause major imbalances in the nutrient solution which become difficult to restore. These imbalances make it necessary for growers to dump the storage tanks occasionally. Often before the tanks are dumped, salts accumulate in the solution which may impact negatively on crop yield and quality.
Brief notes on my research