Making Isotonic Solutions For Draining : Preparing Your Own – and better – Clearex

When growing hydroponic products it is common in the industry to do a final treatment before picking up the crop in which nutrients are removed from the hydroponic solutions. While in most cases this is achieved by passing RO water through the system it is true that passing water with a very low osmotic pressure can make the plants absorb larger amounts of water than what we would ideally want, disturbing the osmotic equilibrium established by the roots with the nutrient solution. An approach that has been used to solve this problem is the use of isotonic cleaning solutions – such as Clearex – which drain the hydroponic media from nutrients without subjecting the roots to the stress of an hypo-tonic solution (such as RO or distilled water).
On today’s article I will teach you what the Clearex solution is supposed to achieve and how you can make your own (or even a better) solution to solve this final draining problem. First of all, removing nutrient from a hydroponic solutions is not so hard. Simply by running RO water through your system after draining the original solution you will remove most nutrients since these salts – contrary to what some companies tell you – are readily soluble and easily leave the media and roots when washed with RO water. The small problem when using RO water is that it is hypo-tonic with the roots, meaning that water will go into the roots to attempt to “lower” the concentration of the solutes within the plant’s cells.

Depending on what you want to achieve with this final draining solution you may have a problem when using such an hypo-tonic solution. In crops where there is fruit production, using such a solution can cause problems such as the rupture of fruits’ skin due to the higher rate of water absorption that takes place when plants are placed in a hypo-tonic media. In order to avoid these problems the best thing is to use an isotonic solution which has an osmotic pressure similar to the original nutrient solution.

Clearex achieves this simply by combining a few sugars to a concentration of about 4-6% in order to get to the point where the osmotic pressure of both solutions is similar. Getting regular table sugar an dissolving it in a ratio of 50g per liter of solution will achieve very similar results as those obtained with Clearex. However using sugars like this can have additional problems since sugars stimulate the development of fungi and bacteria within the root zones of the hydroponic plants.

In my opinion it would be possible to achieve better results by using an isotonic solution with a combination of salts and sugars in such a way that non-nutrient salts are used to provide an ionic content to the draining solution. Using a combination of NaCl, Sucrose and Sodium Hydrogen Carbonate to achieve a more balanced solution may provide better results when doing this type of draining procedures. Of course, this is based purely on my anecdotal evidence and an adequately controlled study would be needed to say anything conclusive for a particular plant specie.

In the end making these solutions is extremely simple and buying Clearex or such other solutions made for this purpose is an obvious waste of money. If you have obtained good results with solutions like these then you can simply make your own with simple sugars while it is possible that you could obtain results just as good as those by using RO water if your crop is not sensitive to hypo-tonic conditions. If you want to experiment a bit I would recommend using a solution with about 150 mg/L NaCl, 100 mg/L NaHCO3 (sodium bicarbonate) and 10g/L of glucose. Let me know if you get better, worse or similar results :o) (note that this is NOT a straight solution but a concentrated additive that should be used until the desired EC levels are reached)

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  • Charlier C
    October 14, 2010 @ 12:49 pm

    Dear Mr Fernandez;
    Thank you very much for this excellent fertlisers calculator. It is very usefull for me because I prepare myself my hydroponic fertiliser for the orchids I cultivate. Previously I use also an Excel sheet to prepare my solutions and actualy I am comparing my results using my Excel sheet with your calculator. I observe that it remain some bugs when I create my self some nutrient salts. As for exemple:If I wish that the new one created nutrient is used by the calculator I must change the “.” by a “,” and re-save the corresponding file text. Other question: is it a limit on the number of new nutrients created? I ask this question because sometimes the calculator don’t use all the new nutrients created. And finaly would you inform me about the the analysis results you have used to create the nutrient YARA Brand Calcium Nitrat (is it YARA Liva CalciNIT?).
    Thank you in advance for your comments.
    Best regards;
    Chemist Ing

  • Peter
    November 20, 2010 @ 3:48 pm

    Hi Daniel, What EC should I be aiming for when using this solution?

    • steve nieves
      December 27, 2014 @ 8:51 pm

      I think since the idea is to create an isotonic solution you would want to create a solution with the same concentration as the one you’re trying to replace.

  • spurr
    July 25, 2011 @ 8:27 pm

    Hey again Daniel,

    I wanted to let you know, I posted a message at a forum I visit that was promoted by this blog post of yours. My post discusses dissociating exchangeable cations from CEC sites (with protons) in media with high CEC, ex., soilless media such as s.peat moss or coconut pith/coir (i.e., hydroponic growing). My message was in response to a question someone posted about using isotonic solutions, such as Clear-X (I gave him the URL of your isotonic blog post) … the topic was so-called “flushing” prior to harvest.

    Here is the sum of what I wrote, in case you are interested and see any inaccurate info I wrote:

    Note: I do not practice so-called “flushing”, that is, irrigating with large volume of water, relative to volume of substrate. However, this post was written from the point of view that I would use flushing as a rule.


    IIRC, in terms of pH, cations, percent base saturation (PBS) and CEC (cation exchange capacity), acidic pH will help dissociate exchangeable NH4, Ca, Mg, etc., from CEC sites by replacing them at some CEC sites with protons from hydrochloric acid, for example. HCL [1,2] acid or sulfuric acid [3] are good for low pH flush because they provide protons to reduce PBS; and do not provide a whole lot of cations we are trying to remove in the first place. After flushing with low pH (ex., 4.5), flushing with RO water is wise to remove residual ions from root zone and to reduce excess acidity. Thereby reducing the percent base saturation by removing some exchangeable cations (those from CEC sites) when flushing, which could have been taken up by the plant, had they been dissociated naturally.

    I recall reading some time ago, a study or two looking at alkaline pH effects on cations at CEC sites. IIRC, very high pH, ex., 9, can also lower percent base saturation by helping to dissociate some cations from CEC sites. But don’t quote me on that, maybe ask Mr. Fista, he’s studying organic chem.

    Good excerpt:

    Cation Exchange Capacity
    J.D. RHoades (1990)
    U.S. Salinity Laboratory
    Riverside, California


    METHODS OF SOIL ANALYSIS Part 2: Chemical and Microbiological Properties (Second Edition)

    A.L. Page, Editor
    R.H. Miller, Associate Editor
    D.R. Keeney, Associate Editor
    American Society of Agronomy, Inc.
    Soil Science Society of America, Inc.
    Madison, Wisconsin, USA

    “Cation exchange capacity (CEC), usually expressed in milliequivalents per 100 g of soil, is a measure of the quantity of readily exchangeable cations neutralizing negative charge in the soil. These charges may be viewed as being balanced by either (i) an excess of ions of opposite charge and a deficit (or negative adsorption) of ions of like charge, or (ii) the excess of ions of opposite charge over those of like charge. Methods of CEC determination based on either view (when they are correctly carried out) yield identical results, though different proportions of the various exchangeable cations are obtained when such are determined by methods based on the two views (Bolt et al., 1976).

    The negative charges [CEC sites] in soil constituents are derived from isomorphous substitution within the structures of layer silicate minerals, broken bonds at mineral edges and external surfaces, dissociation of acidic functional groups in organic compounds, and the preferential adsorption (by chemical reaction) of certain ions on the particle surfaces. The first of these four types of matrix charge is permanent charge and is independent of the pH value, the valence of the counter-ion, and the electrolyte level or composition of the bulk solution. The remaining three types of variable charge vary in magnitude depending on the pH value, electrolyte level, valence of the counter-ion, dielectric constant of the medium, and nature of the anion in the solution phase. Another source of variable charge in acid soils is that associated with the neutralization of permanent negative charge by strongly adsorbed aluminum-hydroxy polymers that carry positive charge. As the pH value rises, these polymers are precipitated as bulk Al(OH)3, thereby freeing the negative sites for participation in normal cation exchange reactions. Negative sites can be similarly neutralized by the adsorption of positively charged mineral particles, such as iron oxides. The positive charges on such particles originate from the specific adsorption of protons on the oxide/hydroxide surfaces, and their magnitude depends critically on the ionic strength and pH of the solution. Such charge is substantially neutralized at pH =>7. Another kind of neutralization of permanent charge is that caused by highly selective adsorption associated with the mica silicate minerals, such as biotite, vermiculite, and muscovite, which contain K and NH4 between the contracted platelets. These interlayer cations are not readily exchangeable, although they can be desorbed with certain chemical treatments and through weathering.

    Thus, it is obvious that CEC is not a soil property that is independent of the conditions under which it is measured. Different results will be obtained with different methods. Ideally the method to use is one that meas-ures the soil’s capacity to adsorb cations from an aqueous solution of the same pH, ionic strength, dielectric constant, and composition as that en-countered in the field, since CEC varies (especially in tropical soils) with these parameters. It is seldom practical to determine the CEC of each soil sample with reagents appropriate to its specific field solution conditions, since the latter information is not easily obtained and each CEC determination would require unique reagents. For this reason CEC determinations are generally based on reference solution conditions that must be standardized to obtain data that can be applied and interpreted universally. The method used should always be reported with the data.”


    [1] Note: Cl (chloride) ppm should be less than ~30-50 ppm in solution, lower is better. In nature, Cl is found upwards of 10 ppm in top soils near oceans, usually as NaCl. Cl is ubiquitous in nature and plants need (small amounts of) Cl. That is why flushing with low pH water, using HCL is fine. I think it’s better than flushing with sulfuric acid because HCL is only likely to provide Ca (in terms of cations we are trying to dissociate from CEC sites). But both should work just fine.

    [2] “Effect of nutrient solution pH regulated with hydrochloric acid on the concentration of Cl ions in the root zone in soilless culture of tomato”
    Waldemar Kowalczyk, Jacek Dysko and Stanislaw Kaniszewski
    J. Elemontal. 2008, 13(2): 245-254…/jurnal-10.pdf

    [3] See posts by Quantrill: h t t p :// www. gardens cure. com/420/plant-food-nutrients/113389-hydrochloric-acid-ph-down.html

  • February 23, 2014 @ 11:28 pm

    […] Check out this article. It's written by the guy who writes the hydrobuddy software:…r-clearex.html Reply With […]

  • […] check the pics. Check the tips. They’re a bit burned. I’ve been using this resource to mix my own isotonic solutions for a flush. Trials still ongoing. Thanks for joining us, catch us […]

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