Micronutrients/Microelements in Hydroponic Solutions
The application of micronutrients differs from the application of macronutrients because of the much smaller quantities required for an optimal plant development and the dominant role of the pH with uptake. The dominant role of the pH with the uptake of micronutrients often involves that sometimes the uptake is more affected by the level of the pH than by the concentrations of the element applied.
Besides this, another very important fact for formulators (and growers) to be aware of is a high concentration of one metal micronutrient can reduce the uptake of other metal micronutrients so much so that a deficiency can occur. For example, high quantities of manganese usually induce a reduced uptake of iron because these two ions compete for proteins transporting them through the plasma membrane of plants. Because, the micronutrients are found in solution at very low levels this creates a very small margin of error where supplying micronutrients to plants is concerned. A fraction too much or little either way can easily result in subpar plant nutrition.
For this reason a great deal of care should be taken to apply ideal levels and ratios of micronutrients/microelements to solution. Further, it is important to understand that no matter what micronutrients/microelements are in solution (i.e. chelated or unchelated) pH is critical where the availability of micronutrients/microelements in hydroponics is concerned. For this reason, maintaining pH between 5.5 – 6.0 is imperative (ideal = pH 5.5 – 5.8). Read more about this here…..
Overview of Micronutrients/Microelements
The essential micronutrients required for plant growth are iron, boron, chloride, copper, manganese, molybdenum and zinc. Certain plant species may need other micronutrients for healthy growth: silica (Si), aluminum (Al), cobalt (Co), nickel (Ni), vanadium (V), and selenium (Se). For example, while silica isn’t classed as an essential nutrient it is a quasi-essential nutrient that has been shown in numerous studies to be beneficial to plant health and yields in various crops.
For now though we’ll approach things from a formulation perspective and discuss the essential micronutrients that are found in all hydroponic solutions. These being iron (Fe), boron (B), chloride (Cl), copper (Cu), manganese (Mn), molybdenum (Mo) and zinc (Zn).
To this point we have largely focused on iron in hydroponic solutions where discussing chelators in hydroponics (here). That is, iron needs to be supplied in chelated form to ensure availability for plant uptake. However, the other micronutrients are often supplied in non-chelated form. E.g. copper is often added to solution as copper sulphate, zinc as zinc sulphate, molybdenum as sodium molybdate etc.
This said, chelated forms of zinc, manganese and copper are available and some research shows that there are benefits to using chelated forms of some of the micronutrients in hydroponic solutions. We’ll look at this research shortly.
For now, let’s look at what various authors/experts specify as optimum ppm in solution where the micronutrients are concerned. Most of these numbers relate to tomato studies; however, these numbers (ppm in solution) also apply to several other commercially grown crops such as pepper, eggplant, cucumber and zucchini (i.e. tomato, pepper, eggplant, cucumber and zucchini have the same or very similar micronutrient requirements as tomato). This one, it should be noted. comes down to author. For example, when looking at what Pardossi et al (2011) recommend re micro element requirements in soilless growing you find a few differences between the ideals of the various noted crops. See following.
Source: Pardossi A., Carmassi G., Diara C ., Incrocci L., Maggini R., Massa D (2011) Fertigation and Substrate Management in Closed Soilless Culture
Conversion of mmol m-3 (chem geek speak) to ppm in solution (mere mortal language)
15 mmol m-3 0.837675 ppm
20 mmol m-3 1.1169 ppm
25 mmol m-3 1.396125 ppm
25 mmol m-3 0.270275ppm
30 mmol m-3 0.32433 ppm
1 mmol m-3 0.063546ppm
5 mmol m-3 0.32695 ppm
7 mmol m-3 0.45773 ppm
10 mmol m-3 0.54938 ppm
0.5 mmol m-3 0.04797 ppm
Okay, that cluster of mmol m-3 to ppm conversion aside, let’s keep things simple for now and look at what various reputable sources recommend as ideals for tomato crops grown in hydroponic situations. With keeping things simple in mind, all numbers refer to ppm in solution.
Kolota, E. Chohura, P. and Komosa, A
Fe–1.40, Mn–0.80, B–0.45, Zn–0.48, Cu–0.08, Mo–0.08 @ EC-3.20 mmhos cm–1 
Jarosz, Z. and Dzida, K.
Fe – 1.25; Mn – 0.55; B – 0.30; Cu – 0.05; Zn – 0.30; Mo – 0.03 
Boron 0.44ppm, Copper 0.05ppm, Manganese 0.62ppm, Molybdenum 0.06ppm, Zinc 0.09ppm, and Iron 2.5 ppm.
Benton Jones, Jr.
Fe as EDTA 4.5 ppm Fe, Mn as manganese sulphate 2.2 ppm Mn, B as Boric acid 0.32 ppm B, Cu as copper sulphate 0.065, Mo as ammonium molybdate 0.007 ppm Mo.
University of Florida
Tomato in rockwool, perlite and NFT – transplant until termination.
Fe-DTPA 2.8 ppm, Cu 0.2 ppm, Mn 0.8 ppm, Zn 0.3 ppm, B 0.7 ppm, Mo 0.05
University of Arizona
The micronutrients should remain at the same concentration throughout the life of the crop. Optimum concentrations for tomatoes are:
Boron 0.44, Copper 0.05, Manganese 0.62, Molybdenum 0.06, Zinc 0.09, Iron 2.5 ppm
Galuku Pty. Ltd. Coco (Coir) Peat (Cucumber and Tomato in Coir)
Planting until harvest
For the most part, it is recommended to maintain the micronutrients/microelements in solution at the same rate/concentration from planting to harvest. This raises some questions where off-the-shelf hydroponic nutrients are concerned, where dilution rates (ml/L) change over the course of the crop cycle – meaning that at e.g. 2ml/L (half strength) the plants are getting half the micronutrients/microelements that they would be receiving when the same nutrient is applied to solution at e.g. 4ml/L (full strength).
Add to this that various growers apply nutrients to solution at different ECs and you pretty much end up with a situation where micronutrient numbers are all over the place when you consider the grow scene as a whole. It’s a less than perfect situation and one that can easily lead to deficiencies, or for that matter excesses, which results in less than optimal growth/yields. Other than this, there tends to be higher demand in some crops for micros such as boron, iron, and zinc when the plants are heavily flowering/fruiting. Therefore, there may be e.g. a week or so during the crop cycle where the plants benefit from a cautious and measured boost of these micronutrients in solution.
This is why I approach things very differently from 2-part nutrient suppliers/manufacturers where formulating nutrient concentrates for hydroponics is concerned. Essentially, the system I use means completely separating the microelements from the macroelements to ensure I can apply the ‘micros’ to solution at very precise levels throughout the entire crop cycle. I’ll discuss the way I do things a bit later.
Understanding the Importance of Source Water Quality re Micronutrients/Microelements in Formulation
Boron in Source Water Supplies: Overview
One microelement/micronutrient that formulators need to be cautious with in formulation is boron, which dependent on author is specified at ideals as low as 0.09 to as high as 0.8 ppm in the nutrient solution.
Boron is one of the essential plant nutrients required by plants for healthy growth but it is only needed in very small amounts and can therefore become toxic to plants even at very low concentrations.
Boron toxicity in plants is characterised by stunted growth, leaf malformation, browning and yellowing, chlorosis and necrosis.
The US Environmental Protection Agency developed three specific boron guidelines for irrigation waters since crops show different sensitivity to boron. For sensitive crops (e.g., citrus trees) the value is between 0.3 and 1.25 ppm. For semi-tolerant crops, such as cereals and grains, the value is 0.67 to 2.5 ppm and for tolerant crops, that includes most vegetables, the guideline is 1.0 to 4.0 ppm.
In Australia and New Zealand, it is recommended that the boron concentration in irrigation waters should not exceed 0.5 ppm. [6a]
Municipal/mains water rarely contains enough boron to be toxic to plants but well water or springs occasionally contain toxic amounts. Additionally, desalinated seawater can contain higher levels of boron than is found in typical mains/municipal/aquifer water supplies. Boron problems in agriculturally grown crops originating from the water are probably more frequent than those originating in the soil.
For example, while researching the levels on boron found in bore/well water supplies I came across this post on the Garden Web forum…..
“I just had my well water tested by a well-respected lab. The Chloride came in at 450 ppm (same as mg/L) and Boron at 16 ppm (mg/L).”
For the record, if you were to use this water supply in your hydroponic system your plants would undoubtedly suffer from a massive boron overdose and toxicity would quickly set in. Fortunately, these extreme levels of boron are uncommon in water supplies; however, mains/municipal water alone can often provide adequate – if not slightly excessive – levels of boron to the hydroponic nutrient solution. Therefore, caution needs to be taken regarding how much boron, if any, is added to a nutrient formulation and water testing (approximate $50.00) is highly recommended to ensure precision in formulation. That is, your source water could have 0.01 ppm (add some B via the nutrient) or 1 ppm of boron in it (don’t add B via the nutrient). Or, for that matter, your source water supply could contain 5 ppm of B, in which case action would be needed to reduce B in the source water supply. You will never know until you have your source water lab tested.
Naturally Occurring Levels of Boron found in Various Source Water Supplies
In nature, boron is found in the form of boric acid, borate, or as a borosilicate mineral. In water boron is predominantly found in boric acid form and hence it not a charged ion which can make it hard to remove via water filtration.
Different levels/concentrations of boron are found in mains/municipal water supplies, RO filtered seawater, stream, dam and bore water. In some cases these levels/concentrations can be excessive. What this means is that where formulating plant nutrition, the boron that is present in the water supply needs to be factored into the formulation equation.
Various surveys have been performed to determine the distribution of boron in United States waters. In 1969 the United States Public Health Service conducted a survey of 969 community water systems and found 99% of all waters tested had boron concentrations less than 1.0 ppm. The maximum level encountered was 3.28 ppm (Coughlin, 1998). In 1987 the National Inorganics and Radionuclides Survey reported a mean boron concentration of 0.15 ppm for 989 public water supplies tested. The maximum level encountered in this survey was 4.0 ppm . 
Boron was among the analytes in the USGS ground water monitoring in the Sacramento Valley in California in 1996 (Dawson, 2001) and the lower Illinois River Basin from 1984 to 1991 (Warner, 1999). In ground water from the Sacramento Valley aquifer, boron was detected in all thirty-one samples; concentrations ranged from 0.002 ppm to 1.1 ppm. The median concentration was 0.042 ppm. Two of the thirty-one samples had concentrations in excess of 0.6 ppm. 
Well/Ground Water and Boron
Boron concentration in well water/ground water really comes down to locale.
In Europe, the concentration of boron in drinking water does not usually exceed 1 ppm, but also higher concentrations are detected in water from natural sources. The concentration of boron in groundwater ranges from below 0.3 ppm to over 100 ppm. The average content of boron in groundwater in Italy and Spain is 0.5-1.5 ppm, and in the Netherlands and United Kingdom it amounts to 0.6 ppm. Approximately 90% of water samples collected in Denmark, France and Germany contained boron in concentrations below 0.3 to 0.1 ppm (WHO 1998, Haberer 1996). In Turkey, in areas where borax mining is concentrated, the content of boron in water ranged from 2.0 to 29.0 ppm (Cöl, Cöl 2003). In Eastern Europe, high concentrations of boron were detected in some highly mineralized natural waters in Romania (20 ppm), Georgia (to 10 ppm), Slovakia (up to 9.48 ppm) and Slovenia (5.5 ppm).
In Canada, a study of British Columbia groundwater, it was found that the total boron concentration ranged from 0.014 ppm to 4.05 ppm.
Levels of boron in US groundwater tend to average about 0.1 ppm; however, they are higher – between five and 15 ppm – in the western US. This is due to weathering of boron rich formations and deposits (Butterwick, et al., 1989).
More to be added to this material shortly re other potentially problematic micronutrients found in water supplies, chelation of micronutrients (to chelate or not to chelate?), optimums in research and crop specific ideals etc. Information about chelates in hydroponics can be found here…..
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Table 1. Pardossi A., Carmassi G., Diara C ., Incrocci L., Maggini R., Massa D (2011) Fertigation and Substrate Management in Closed Soilless Culture Retrieved 22/4/2016 http://www.wageningenur.nl/upload_mm/8/c/0/aa4b4486-a9db-429f-8b03-f19d4cec3ee6_Fertigation%20and%20Substrate%20Management%20in%20Closed%20Soilless%20Culture.pdf
 GUNES A., ALPASLAN M., INAL A. 1998. Critical nutrient concentrations and antagonistic and synergistic relationships among the nutrients of NFT-grown young tomato plants. J. Plant Nutr., 21: 2035-2047.
 Kolota, E. Chohura, P. and Komosa, A (2013) Efficiency of Chelate Forms of Micronutrients in Nutrition of Greenhouse Tomato Grown in Rockwool
 Jarosz, Z. and Dzida, K. Effect of Substratum and Nutrient Solution upon Yielding and Chemical Composition of Leaves and Fruits of Glasshouse Tomato Grown in Prolonged Cycle. Acta Sci. Pol., Hortorum Cultus 10(3) 2011, 247-258
 J. Benton Jones, Jr. Hydroponics: A Practical Guide for the Soilless Grower. Originally published in 1997.
 Hochmuth, G, J. and Hochmuth, R. C. Nutrient Solution Formulation for Hydroponic (Perlite, Rockwool, NFT) Tomatoes in Florida. University of Florida. Retrieved 21/4/16 http://edis.ifas.ufl.edu/pdffiles/cv/cv21600.pdf
 Galuku Coco Peat Australia Retrieved 23/4/16 http://www.cocopeat.com.au/technical/hydroponics/pdf/LiquidfeedTom_Cuc.pdf
[6a] Moss, S. A. (Sharon A.) Ambient water quality guidelines for boron [electronic resource. Retrieved http://www2.gov.bc.ca/assets/gov/environment/air-land-water/water/waterquality/waterqualityguidesobjs/approved-wat-qual-guides/boron/boron-tech-appnx.pdf
 Gupta, U.C. 1979. Boron nutrition of crops. Advances in Agron. 31:273 – 307.
9 and 10: Frey, M.M., C. Seidel, M. Edwards, J. Parks, and L. McNeill. 2004. Occurrence Survey for Boron and Hexavalent Chromium. AwwaRF Report 91044F.