by J Benton Jones, Jr.

Nutrient Solutions: Historical Background, Formulations and Use, Part 2*

What needs to be changed?

From my experience when using the Hoagland/Arnon formulations (Hoagland and Arnon, 1950), I recommend the following changes:

• Reduce the P concentration to 15 ppm, the ppm ratio of K to Ca should be 1:1 by either increasing or decreasing either element – the important requirement being that K and Ca be equal in concentration in solution
• Increase the Mg concentration to 60 ppm
• Increase the Zn concentration 0.80 ppm, iron chelate (either FeEDTA or FeDTPA) should not be used as the Fe source

Chelates

It is well known that the EDTA (ethyl­enediaminetetrataacetic acid) chelate is toxic to plants even though FeEDTA is a common form of Fe included in many nutrient solution formulas. In the past, iron (fer­rous) sulfate (FeSO4.7H2O), iron (ferric) sulfate [Fe(SO4), iron (ferric) chloride (FeCl3.6H2O), and iron ammonium sulfate [FeSO4(NH4) 2SO4.6H2O] have been found to be reliable plant-­available sources of Fe in a nutrient solution.

Recently Rengel (2002) found that the inclusion of EDTA (at 100 ppm) in a nutrient solution decreased the growth of young wheat plants. Iron was accumulating in the roots of the wheat plants when FeEDTA was in the nutrient solution compared to when an EDTA-free nutrient solution was used. In addition, the uptake and transport of both Cu and Zn from roots to plant tops was significantly reduced when EDTA was present in the nutrient solution.

The DTPA (diethylenetriaminepen­taacetic) chelate, thought not be toxic to plants, is currently being recommended as the Fe chelate of choice as FeDTPA. As was observed by Rengel (2002), DPTA may act like the chelate EDTA, restricting the uptake and translocation of Cu and Zn, something that needs to be investigated. This may partially explain why frequently I have observed low Cu and particularly low Zn concentrations have been observed in assayed leaf tissue when evaluating the nutrient element status of tomato plants. Also, chelated forms for the micronutrients, Cu, Mn, and Zn, should not be used in the formulation of a nutrient solution.

The Beneficial Elements

There are elements that have been identified as being potentially "beneficial" to plants (Asher, 1991; Morgan, 2000), but do not meet the requirements for essentiality established by Arnon and Stout (1939). Therefore, should these elements be included in a nutrient solution formulation?  Morgan (2000) has found that there are some elements that do enhance plant growth under certain circumstances. The early hydroponic researchers devised an "A-Z Micronutrient Solution" as a means of ensuring that potentially influ­encing elements at trace levels would be included in a nutrient solution formulation (Jones, 2005). Some have suggested that the elements essential for animals — arsenic (As), chromium (Cr), cobalt (Co), fluorine (F), iodine (I), nickel (Ni), selenium (Se), and vanadium (V) — but not for plants, would be good candidates for inclusion in a nutri­ent solution. The two elements where essentiality has been supported by research studies are nickel (Ni) (Brown et al., 1987) and silicon (Si) (Takahashi et al, 1990; Morgan, 2000), with Si being the element that some recommend for inclusion in a nutrient solution formulation as silicic acid (H4SiO4) at 100 ppm. Potassium silicate and sodium silicate have been suggested as equally suitable sources of Si for addition to a nutrient solution.

Since many of these so-called “beneficial elements” maybe found as "contaminates" in some of the major element source reagents, such as calcium nitrate, potassium nitrate and magnesium sulfate, there may not be the need to purposely add a mix of beneficial elements, such as the A-Z Micronutrient Solution, to ensure the presence of these elements in a nutrient solution. This would also suggest that selecting high purity reagents may not be the best choice. In addition, the rooting medium itself may contain trace levels of some of these same elements.

Nitrate and Ammonium

There is considerable research that indi­cates that the form of N supplied to the plant can have a significant effect on vegetative growth and fruit yield as well as related quality factors.

A mixture of ammon­ium (NH4)-N and nitrate (NO3)-N frequently results in better plant growth, if that concentration ratio does not exceed 25 to 75, as compared to when NO3 is the only N source. For some crops, such as tomato, NH4 in the nutrient solution can increase the incidence of blossom-end rot (BER), a commonly occurring fruit disorder. Therefore, some recommend that NH4 not be included in the nutrient solution during the tomato plant’s fruiting period. I recommend that at least 5-10 percent of the total N in the formulation be as the NH4 form, even when growing tomato.

pH and Electrical Conductivity (EC)

It is essential that the pH of the nutrient solution as well as the rooting medium be maintained acidic, the optimum range being between pH 5.0 to 6.0. However, it isn't necessary to adjust the pH unless the nutrient solution and/or rooting medium becomes alkaline, greater than 7.0. Plants can grow quite well at a pH level as low as 4.5. Therefore, no adjustment is generally needed when both the nutrient solution and rooting media are acidic.

The electrical conductivity (EC) of a nutrient solution which is a measure of the accumulation of ions, frequently referred to as “salts,” in the rooting media is an important parameter. As the EC increases, the ability of plant roots to take up water and nutrient elements from a nutrient solution declines. Hydroponic growers are advised to monitor the runoff from the rooting media or the solution retained in the media for its EC and leach when it exceeds a certain level. An increasing EC in the rooting media suggests that the elemental concentra­tion in an applied nutrient solution is either too high, or the frequency of application greater than needed.

Element Accumulation in the Rooting Media

For most nutrient solution formulations including their use parameters, the total nutrient elements applied exceeds that required by the plant. Therefore with time, an accumulation of unused nutrient elements in the rooting media occurs, initially observed as an increase in the EC, or “salinity,” of the retained solution in the rooting media. What follows is not generally known, that is the formation of pre­cipitates of calcium phosphate and calcium sulfate. These precipitates are most easily observed as a grayish-white sludge, best seen in a flood-and-drain hydroponic gravel rooting media system. Inserting one’s hand into the gravel bed and removing it, the hand will be coated with this sludge. Included in this precipitate are the micronutrients, Cu, Fe, Mn, and Zn.

This same precipitation phenomenon occurs in all rooting media (gravel, sand, perlite, rockwool, coir, etc.) when a full-strength nutrient solution is repeatedly applied. There initial forma­tion creates the "seed" that keeps the precipitation process going with each nutrient solution application. The other driving force that enhances precipitation in the rooting media is the rate of water removal by plant transpiration that concentrates the elements in the retained nutrient solution. This precipitate cannot be leached from the rooting medium, and its accumulation will begin to significantly influence the nutrition of the plant. Since the immediate area around the root is strongly acidic, precipitate in contact with the root will be dissolved and the released elements root absorbed. The plant then has 3 nutrient element sources, that being supplied by the nutrient solution, that remaining in solution in the rooting media, and that being dissolved from the accumulated precipitates. At this point, the grower looses control of the nutritional status of his plants.

At the end of a growing season, perlite samples were taken from BATO buckets used in the production of greenhouse tomato. Assaying the perlite as soil, the perlite was found to contain sufficient nutrient element concentrations to be classed as a "very fertile" soil. What had begun as an inert, nutrient-free perlite, it now had a high nutrient content containing substantial quantities of Ca, Mg, P, S, Cu, Fe, Mn, and Zn, at levels greater than what was needed to meet the crop’s requirement for these elements. This meant that the plants during more than half of their growing cycle were being significantly influenced nutritionally by what had accumulated in the perlite rather than what was being applied by the nutrient solution. If reused, the perlite would start with a high nutrient element charge that would significantly affect the nutrition of a follow-up crop.

One procedure that can slow the precipitation process would be to apply only one or two aliquots of full-strength nutrient solution in one day’s cycle. For example, for a drip irrigation procedure, make one nutrient solution application at sunrise and another in mid-day, and then only water needed to maintain fully turgid plants. Another scheme would be to apply only what is specifically needed in terms of the amount and balance of nutrient elements, a procedure that would work but difficult to implement. The objective would be to apply only those nutrient elements as needed determined by the crop requirement at each stage of plant growth.

Although precipitate accumulation in the rooting media provides a source of some essential elements for plant utilization over time, precipitation can also reduce the immediate availability of elements being supplied by the nutrient solution, particularly the micronutrients Cu, Fe, and Zn. This may explain why low levels of these elements have been observed in plants, particularly from mid-season onward.

There is much more to be learned when it comes to the formulation of a nutrient solution and what its use parameters should be – no one yet has the complete answer.

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 final article in this two-part series. Part one was published in the Garden & Greenhouse November-December 2008 issue.