by J Benton Jones, Jr.

Probably no other aspect of the hydroponic growing technique is as misunderstood and misused is that associated with the formulation and use of a nutrient solution. At a Greenhouse Hydroponic System and Crop Workshop a number of years ago, the participants were given an 11-page handout entitled “Nutrient Solution Formulation for Hydroponic Tomatoes" in which there were five formulas using individual salts and four premixed formulations. Options were given employing a number of different reagents to achieve the desired elemental concentration in the final nutrient solution. Although the formulation descriptions were well pre­sented, numerous questions arose as to why there were so many different formulas and options related to their use. 

Historical Background

Most hydroponic nutrient solution formulas are based on the two for­mulas given in the California Agricultural Experiment Station Circular 347, authored by Hoagland and Arnon (1950). This circular has been the most widely cited publication in all crop science literature. In addition, the scientific literature is full of hydroponic formulas that are identified as "modified Hoagland nutrient solutions" with little given that describes what was modified.

What most may not know is that the Hoagland/Arnon nutrient solution formulations have a use component: one gallon of nutrient solution per plant with replacement on a weekly basis. If any of these use parameters are changed, i.e., the volume of solution, number of plants, and/or frequency of replacement, plant per­formance will be significantly affected, a factor that probably isn't fully understood or considered by those who use these formulations or some other.

Elemental Form

An element must be in solution as an ion in order to be absorbed by the plant root, although there is evidence that small molecules can be transported through root membranes. Two ions, nitrate (NO3-) and potassium (K+), present in most nutrient solutions in fairly high concentrations, move readily from the nutrient solution into the plant root; while all the other ions in the nutrient solution are selectively absorbed. 

Use Factors

When using the flood-and-drain hydroponic growing technique, referred to as a “closed” system since the nutrient solution is recovered and re-used, the decisions to be made are: (1) volume of nutrient solu­tion per plant, (2) frequency of flooding the rooting bed, and (3) schedule for either nutrient solution adjustment and/or replacement? These are critical factors that will influence plant performance irrespective of the initial nutrient solution elemental content.

For those hydroponic growing systems in which a nutrient solution is applied to a rooting medium (such as perlite, rockwool, coir, etc.), by means of drip irrigation, is not recovered, referred to as an “open” nutrient solution delivery system, the elemental composition of the nutrient solution and fre­quency of application will affect the residual elemental content in the rooting media. In addition, the volume of nutrient solution applied will determine the distribution of elemental concentrations within the rooting media. With time, the rooting media will require water leaching in order to remove what is referred to as “accumulated salts.”

Interacting Effects of Nutrient Solution Composition, Volume, and Replacement

Place a young vigorously growing tomato plant in 1 quart of full-strength aerated Hoagland/Arnon nutrient solution. In 48 to 76 hours most all of the N, K, and B in the nutrient solution will have been absorbed by the plant, while the other elements in solution will be only slightly reduced. In a short time, plant growth will slow. Place the same type of plant in 5 gallons of full-strength aerated Hoagland/Arnon nutrient solution, and it will be 5 to 7 days before plant growth will begin to slow, primarily due to an element imbalance in the nutrient solution.

Now place a young vigorously growing tomato plant in 1 quart of quarter-strength aerated Hoagland/Arnon nutrient solution. In less than 48 hours, all of the N, K, and B in the nutrient solution will have been absorbed by the plant, while the other elements will be slightly less than that initially in solution. Shortly, plant growth will slow. Place the same type of plant in 5 gallons of quarter-strength aerated Hoagland/Arnon nutrient solution and it will be 10 to 12 days, or longer, before plant growth will begin to slow.

Now repeat the 4 experiments described above, but replace the nutrient solution with fresh every day. What plant effects will occur and when? Under which of the 4 experimental conditions will the plants do best? The answer is “that system in which the plant has the least affect on the composition of the nutrient solution – that is the plant is essentially growing in a constant-composition nutrient solution.” One might ask, “Is there a hydroponic growing system in which the plant root has the least effect on the applied nutrient solution?” Indeed there is, it called aeroponics.

The best growth I have ever obtained occurred with greenbean plants hydroponically grown in perlite with the nutrient solution replaced on a daily basis. The amount of water needed to flush the perlite free of nutrient solution was determined. The volume of nutrient solution needed to saturate the perlite was also determined. That volume of nutrient solution was added to the perlite when the greenbean plants were beginning their initial vegetative growth. Each morning, the perlite was flushed with water and after draining for an hour, that volume of nutrient solution needed to saturate the perlite was added. This routine was followed everyday during the entire season of plant growth and pod production. Pod yield was greater than what had been obtained using other systems of hydroponic growing. The reason for this unusually vigorous plant growth and high pod yield was due to the fact that the greenbean plants were essentially growing in a constant root-nutrient element environment.

Competitive and Synergistic Effects

There exists both competitive and synergistic effects among the ions in a nutrient solution. For example, the relative ratio of the cations, K+, Ca2+, and Mg2+ [as well as the ammonium cation (NH4+) if included in the nutrient solution], among themselves can significantly inhibit the absorption of Mg2+ if the concentration of the K+ and/or Ca2+ cations plus the NH4+ cation are high relative to Mg2+. This same effect applies to Ca2+ if low in concentration relative to the other cations. The absorption of the nitrate (NO3-) anion will enhance the absorption of the K+ cation, and to a lesser degree the Ca2+ and Mg2+ cations.

A high concentration of either cationic forms of iron (Fe2+ or Fe3+) will inhibit the absorption of Zn2+. High P in the nutrient solution will also inhibit the absorption of Zn2+, and to a limited extent, have the same effect on both Fe cations (Fe2+ and Fe3+) as well as Cu2+.  On the other hand, P tends to enhance the adsorption of Mn2+. As the elemental concentration in the nutrient solu­tion increases, the effect of the ratio of elements in the nutrient solution on the nutrition of the plant significantly changes. For example, the ratio between or among the elements N and S; Ca, Mg, and K to Ca and Mg; Fe and Zn; and K and Fe, may be more important than the concentration of anyone element alone. Making this observation, Steiner (1984) formulated a nutrient solution designed to minimize this effect by balancing the concentration of anions and cations in the nutrient solution (see Jones, 2005).

My solution to counter this effect is to use dilute nutrient solution formulations combined with a sub-irrigation growing technique, thereby mini­mizing the need to specifically balance the anions and cations (see: grosystems.com). Asher and Edwards (1978ab) discovered essentially the same thing when they were able to successfully grow plants hydroponically in very dilute nutrient solution formulations with plant roots suspended in a rapidly flowing constantly maintained elemental content nutrient solution.

Crop Requirement

Crop requirement is another factor that justifies some of the variations that exist among nutrient solution formulas. Not all of the essential elements have significant crop requirement aspects, but the major elements, N, Mg and P, and the micronutrients Cu, Fe, Mo, and Zn, can be specifically related to certain crops. In addition, nutrient element crop requirements change with each stage of plant development (i.e. vegetative to fruiting), a factor that could justify modifying a particular nutrient solution formulation and how it is to be used.

Plant Root Influence

The absorption of ions from a nutrient solution into the plant root is a complex physiological process influenced by temperature, aeration, root respiration rate, and rate of plant transpiration.  Root membranes selectively control the passage of ions from the surrounding nutrient solution into the root cells, with the transported ion passing into the xylem for movement into the upper portions of the plant. The size (surface area) of the root and its physical characteristics will influence ion absorption, although less a factor than that occurring in soil where root structure and size have a significant influence on ion absorption.

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. It is available at http://www.crcpress.com/. Dr. Jones may be contacted at This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

* This the first of a two-part series. Part two will be published in the Garden & Greenhouse January-February 2009 issue.

The following element symbols are used in the text: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), boron (B), copper (Cu), iron (Fe), manganese (Mn), zinc (Zn).