Drip irrigation, a modification of the trickle system of water distribution, was introduced in 1950s as a method for dispensing irrigation water at a specific point. Its application to hydroponic growing came into common use beginning in the 1990s. With plants rooted in either a container of media or block of rockwool or coir, a dripper head is placed at the base of the plant for the dispensing of either water or nutrient solution. The system is fairly easy to install, relatively free of malfunction, and is cost effective in its delivery of water and nutrient solution to the plant.
Since drippers do clog or occasionally malfunction, they need to be monitored during their period of use. To minimize clogging, a filter is placed at the head of the delivery line to remove suspended material from the water or nutrient solution to be delivered. The flow volume from a dripper head is determined by the size of its opening. Flow from the dripper requires that the water or nutrient solution from its source must be under pressure.
The combination of frequency and length of time for each irrigation as well as volume dispensed, based on the size of the dripper opening, will determine how much of the rooting media becomes saturated at each irrigation. The amount dispensed and frequency of irrigations is determined by crop demand usually a calculated value based on stage of crop growth and the atmospheric demand of the plant for water. Normally the amount of water and nutrient elements applied is in excess of that needed by the plant.
With the drip system, what portion of the applied water or nutrient solution retained in an inorganic substrate, such as course sand, perlite, or rockwool, is determined by their physical properties. Initially, these rooting media are essentially “nutrient element free,” but with time, the rooting media will become “fertile” from the accumulation of unabsorbed nutrient elements as soluble ions, to be then followed by the accumulation of precipitates. These precipitates are a mix of calcium sulfate and calcium phosphate that initially formed acting as a “seed” to keep the process continuing with each application of nutrient solution. Co-precipitation of other elements, such as copper, iron, magnesium, manganese, and zinc, also occurs. The rate at which these processes occur depends on the rooting media, rate of root absorption, and the elemental composition and frequency of application of the nutrient solution. Eventually the plant will have three sources of nutrient elements, that being supplied by the nutrient solution that retained in the rooting media as soluble ions, and that in the form of precipitates. Since root surfaces are highly acidic, there contact with these precipitates will dissolve elements that can then be root absorbed. At this point, the grower has lost control of the nutrient element status of his plants.
With time, the electrical conductivity (EC) of the retained nutrient solution in the rooting media will reach the point that it adversely affects root uptake of water and nutrient elements. Therefore, growers are advised to periodically determine the EC of either an aliquot of the effluent from the rooting media, or of a drawn a sample from a cavity in the rooting media. Depending on the EC of the retained solution, the rooting media may require leaching with pure water to reduce the EC. Unfortunately, leaching will not remove all of the soluble ions and will not remove the accumulating precipitates.
With each change in stage of crop growth, it is common practice to adjust the elemental composition of the nutrient solution to meet changing demands, due to the formation and/or maturing of fruit, or the production of flowers. However, depending on the reservoir of elements in the rooting media and the nature of the change in the nutrient solution formulation and irrigation schedule (volume and frequency of application), a favorable plant response may be problematic.
If the rooting media is organic, such as pineback, course peat, saw dust or coir, their physical and chemical properties complicate the interaction among that of the nutrient solution, retained nutrient solution, and the plant roots. Precipitate formation is less likely to occur since these organic substrates have both physical and chemical exchange properties that provide some degree of “buffer capacity” that would keep the accumulating precipitating elements, such as the calcium cation and the sulfate and phosphate anions, from concentrating sufficiently to form precipitates.
Allan Cooper (Copper, 1996), the inventor of the Nutrient Film Technique (NFT), suggested when using the “W” form of the nutrient trough, that a concentrated form of the nutrient solution be periodically flowed down one side of the trough and only water down the other. Using this same concept with the drip irrigation technique, one can apply a concentrated nutrient solution once a day, either in the early evening or before dawn, sufficient in volume to saturate the rooting media, and then apply only water when necessary to keep the plant fully turgid during the daylight hours. To make such a system work, the elemental composition of the nutrient solution needs to be carefully formulated.
Another concept that needs to be explored is to limit the application of nutrient solution as well as water to that just sufficient to saturate the rooting media at each irrigation.
Another means for utilizing the drip irrigation technique would be to place the rooting media in a water-tight vessel with the nutrient solution being released at the base of the rooting media, sufficient to maintain a constant inch depth of nutrient solution in the bottom of the container. I have found that a dilute modified formulation of the Hoagland-Arnon (Hoagland and Arnon, 1930) nutrient solution works best with this procedure. The advantage of this procedure is that all of the applied nutrient elements and water are totally utilized by the growing plant. Therefore, there is no runoff, no accumulation of unabsorbed nutrient elements, or EC changes that would affect water and nutrient element uptake by the plant.
If following a season of growing using the drip irrigation procedure with an inorganic rooting media that will be re-used (not recommended but practiced by some growers), the rooting media will have a substantial nutrient element charge. Some growers will steam leach the media to remove root material, and in the process will also remove some of the accumulated nutrient elements, both that soluble and in precipitate form.
The drip irrigation procedure is currently the most commonly used procedure for dispensing water and nutrient solution to hydroponically-grown crops rooted in either an inorganic or organic substrate. Far from being an “ideal” method, unfortunately little is being researched to make this method more efficient in its use of water and plant nutrient elements, as well as eliminating the need to periodically leach the substrate to reduce the EC due to retention of accumulating ions in solution.
Cooper, A. 1996. The ABC of NFT. Casper Publications, Narranbeen, Australia.
Hoagland, D.R. and I. Arnon. 1950. The Water Culture Method for Growing Plants without Soil. Circular 347. California Agricultural Experiment Station, University of California, Berkeley, CA.
J. Benton Jones, Jr. has a PhD in Agronomy and is the author of several books including Hydroponics: A Practical Guide for the Soilless Grower. It is available at crcpress.com.