The primary basis for selecting a nutrient solution formulation is its elemental content.
The trick is to match a nutrient solution formulation to the plant species and growing system so that all the essential plant nutrient elements are being supplied to meet the requirements for sufficiency. For those growing systems not using a rooting medium, standing aerated, Nutrient Film Technique (NFT) and aeroponics, the nutrient solution formulation must contain all the plant essential elements. For those growing systems that use a rooting medium, those elements that are available in the rooting medium must be known or determined, and those content elements adjusted or eliminated from the nutrient solution formulation to prevent an imbalance, excess or toxicity from occurring. If the rooting medium is a natural product, such as an organic soilless mix, composted milled pinebark, a composted wood by-product, rockwool, or coir, these substances can contain substantial quantities of some of the plant essential elements, particularly the micronutrients.
What elements are present in a rooting medium and at what concentration may be known information or can be determined by means of a water-equilibrium test (Jones, 2001), a test that is available at some soil testing laboratories. When the grower knows what elemental contribution the rooting medium will provide, a nutrient solution formulation can then be selected to provide the other essential elements. An important cautionary fact is that those nutrient solution formulations identified as being “complete” in that they contain most or all the essential plant nutrient elements, have limited use and not suited for use with rooting medium growing systems.
It is important that all the essential elements are in their proper concentration and ratio to maintain plant nutrient element sufficiency. This may require the mixing of 2 or more reagents or a mix of reagents or concentrate solutions to obtain the proper elemental concentration and ratio.
Some nutrient solution formulations are promoted based on their correlation to plant species and stage of development from vegetative to flowering to fruit set and then on to maturity. Such formulations can have a place if the elemental content is properly matched to the plant species advancing development, providing that element or those elements that will meet a particular requirement at each stage of growth. The decision to use such formulations requires care to ensure that with each stage of plant development, there remains nutrient element sufficiency.
The form of the element is equally important as to their effect on plant growth, such as the ratio between nitrate-nitrogen (NO3-N) and ammonium-nitrogen (NH4-N) as the presence of low levels of ammonium-nitrogen enhance nitrate-N absorption and plant utilization. However, at high levels of ammonium-N (greater than 30% of the total nitrogen), this cation can be toxic to some plant species.
The use of chelated forms of an element, primarily for iron (Fe) and/or the other mineral micronutrients is not recommended as the chelated element will not always remain in its original form when in a multi-element nutrient solution formulation, and in addition, the chelate EDTA has been found to be toxic to some plants.
The final aspect of nutrient solution formulation selection is the associated use factors, such as volume of nutrient solution per plant for the standing-aerated hydroponic growing system, and for rooting medium systems, volume of nutrient solution per application, and the frequency and the timing of each application. For the rooting medium systems, these decisions are usually made based on the water needs of the plant. The water-holding capacity of the rooting medium and volume of rooting medium per plant are the factors that would determine the schedule for nutrient solution irrigations. With more frequent application of a nutrient solution, the diluter the nutrient solution should be. Alternating the application of a nutrient solution with only water will be a determining influence on what the use factors should be. The objective is to supply just that quantity of essential element needed by the plant, no more or no less.
Many of the commonly marketed nutrient solution formulations are based on the “one size fits all” philosophy which can lead to elemental insufficiencies. However, plants will grow quite well under a range of growing conditions, even when the nutrient solution formulation is not matched to the plant species/growing system. This is because plants can adjust, to some degree, to unmatched conditions. However, plant growth will be less than optimal, sometimes without showing visual stress and/or nutrient element insufficiency symptoms. My experience has been that stress and/or nutrient element insufficiency symptoms will usually begin to appear when the plant enters its flowering and/or fruiting stages, and particularly when under less than optimum light, temperature and/or moisture conditions. I have witnessed instances of crop failure due to the use of unmatched nutrient solution formulations and their associated use factors, creating a nutrient element insufficiency that begins to appear as the plant enters its flowering or fruiting stages, or when entering its early mature stage of development. Once such insufficiencies occur, they cannot be corrected. Therefore, to avoid such eventualities, match the nutrient solution formulation with the requirements associated with the plant species and the hydroponic/soilless growing system used.
There are numerous nutrient solution formulations that have been developed and used with good success, their reagent base and elemental contents can be found in the books by Jones (2005, 2011) and Resh (2001), along with instructions as to their use.
J. Benton Jones, Jr. has a PhD in Agronomy and is the author of several books including Hydropopnics: A Practical Guide for the Soilless Grower.
Jones, Jr., J. Benton. 2001. Laboratory Guide for Conducting Soil Tests and Plant Analysis. CRC Press, Inc., Boca Raton, FL.
Jones, Jr., J. Benton. 2005, Hydroponics: A Practical Guide for the Soilless Grower, 2nd Edition. CRC Press, Inc., Boca Raton, FL.
Jones, Jr., J. Benton, 2011.. Hydroponic Handbook: How Hydroponic Growing Systems Work. GroSystems, Inc., Anderson, SC.
Resh, H.M. 2001. Hydroponic Crop Production, 6th Edition. New Concepts Press, Mahwah, NJ.