It is generally believed that hydroponics as a method for growing plants is more efficient in its use of water and nutrient element reagents as compared to growing plants is soil – a fact that is not always true. Hydroponic-growing methods can be very wasteful in their use of water and nutrient element reagents.
There are two classifications for hydroponic systems, one being “open”, the other “closed.” An “open” hydroponic-growing system is one in which the nutrient solution is discarded after one pass through either the rooting medium or root mass. For the “closed” hydroponic-growing system, the nutrient solution after passing through either the rooting medium or root mass is recovered for reuse. Most hydroponic growing systems are operated as “closed” systems; although at some point, the nutrient solution is discarded depending on what hydroponic method is used. For example, Hoagland and Arnon (1950), in the description of their standing aerated nutrient solution technique, recommend that the nutrient solution be replaced after 7 to 10 days of use. Hunter (1977) states the following regarding the use of a nutrient solution, “The (nutrient) solution is commonly used for 1 to 2 weeks, discarded, and a fresh (nutrient) solution applied. Usually, additional fertilizer salts are added after the first week to assure that sufficient nutrients are available to the plants through the second week.” The basis for elemental supplementation of a nutrient solution is normally determined by monitoring its electrical conductivity (EC), measuring the initial nutrient solution EC, and then adding back elements, primarily the two major elements, nitrogen (N) and potassium (K), to bring the EC back to its initial level.
A greenhouse tomato grower following given instructions was discarding about 2,000 gallons of nutrient solution every 7 to 10 days. I calculated that over an 8 month growing period, he had discarded over 63,000 gallons of water. Included in this water (i.e. nutrient solution) were substantial quantities of plant nutrient elements. So by the end of the growing season, for example, it was like buying a 100 lb bag of potassium nitrate, bringing it to the greenhouse for use in formulating the nutrient solution, but putting the bag into the trash dumpster to be carried away!
By contrast, a greenhouse hydroponic lettuce grower employing both the standing and NFT hydroponic-growing systems, re-circulates the nutrient solution by first passing the nutrient solution through a 2-stage filtering system to remove all suspended material, and then, adding water to bring the solution back to its original volume. On a 3 to 5 day schedule, the elemental concentration in the nutrient solution is determined and elements brought back to their original concentration (Jones, 2014a). When the grower was asked when he first formulated this nutrient solution, he replied, “two years ago!”
What is frequently called “spent nutrient solution” will contain sizeable quantities of nitrogen (N) and phosphorus (P); therefore, the to-be-discarded solution could be classified as a “hazardous waste,” requiring special techniques for its disposal. Hunter (1977) discusses this issue in terms of the effect that repeated discarding of a nutrient solution can have on the disposal environment.
A hydroponic greenhouse tomato grower collected and stored the spent nutrient solution and leachate generated by the periodic leaching of the rooting medium (perlite) for application on his vegetable garden soil during the summer months. Initially, it seemed like a perfect means for handling the gathered solutions, providing both water and nutrient elements for use by the growing vegetable plants. After two seasons of following this routine, the grower discovered that there was an imbalance developing among some of the nutrient elements in his garden soil as well as an accumulation of phosphorus (P) to an excessive level. He stopped the practice and sought other means for disposing the spent nutrient solution and collected leachate.
Another aspect when collecting and re-circulating a nutrient solution is the accumulation of various elements, particularly the micronutrients as well as elements not specifically being added to the nutrient solution but existing in the surrounding environment that then end up in the collected and re-circulated nutrient solution. Hunter (1977) describes what can happen when collecting and re-circulating a nutrient solution over an extended period of time in terms of elemental accumulation. A NFT greenhouse tomato grower was using galvanized metal sheets as the support base for the NFT plastic sheeting that formed the rooting NFT trough. Asked how often he replaced the nutrient solution, replied, “when the zinc (Zn) level in the nutrient solution becomes potentially toxic.” Even though there was no direct physical contact between the galvanized metal sheets and the nutrient solution flowing down the NFT plastic-formed trough, somehow Zn found its way into the nutrient solution. Therefore, elements in the water formulating the nutrient solution, contaminates in the nutrient element reagents, contact between the nutrient solution and its surrounding environment and atmospheric deposition of dust can result in the accumulation of unwanted elements that may reach concentrations that could adversely affect plant growth and product yield.
So, what is the hydroponic grower to do? It is a good question that is not being addressed other than the recommendation to periodically discard the nutrient solution after a period of use.
The initial nutrient solution formulation in terms of elemental content and concentration needs to be such that when applied to the rooting medium or root mass, there is minimal accumulation of elements as both ions in solution and as precipitates with each application of the nutrient solution (Jones, 2014b). Therefore, it requires an ability to regulate the elemental formulation in terms of content and concentration as the use factors are adjusted in terms of application frequency and volume per application sufficient to meet the plant requirement for water and nutrient elements. Unfortunately, no such computer-guided program exists capable of regulating those factors that will result in the desired goal – minimization of the accumulation of elements as ions in solution and as precipitates in the rooting medium or root mass, while at the same time, meeting the water and elemental nutritional needs of the plant.
The author has offered a hydroponic growing system that solves the problem that currently exists, a growing system in which the plant utilizes all of the water and nutrient elements provided, therefore no residue accumulates (Jones, 2014c).
Hoagland, D.R. and D.I. Arnon. 1950. The Water Culture Method for Growing Plants Without Soil. Circular 347. University of California Agricultural Experiment Station, Berkeley, CA.
Hunter, Jr., Johnson. 1977. Hydroponics: A Guide to Soilless Culture Systems. Leaflet 2947. Division of Agricultural Sciences, University of California, Riverside, CA.
Jones, Jr., J. Benton. 2014a. Putting the Nutrient Film Technique to the Test. Maximum Yield 15(6):66-74 (June 2014).
Jones, Jr., J. Benton. 2014b. Complete Guide for Growing Plants Hydroponically. CRC Press, Inc., Boca Raton, FL.
Jones, Jr., J. Benton. 2014c. Vegetable Gardening Hydroponically: Complete Guide for the Home Gardener and Commercial Vegetable Grower. GroSystems, Inc., Anderson, SC. (available as an ebook at: Hydrogrosystems.com)