It has become a common practice for many hydroponic enthusiasts, hobbyists and even commercial growers to dispose of the nutrient solutions they have that “are not adequate anymore” by throwing them down the drain. Most of them are not aware than hydroponic solutions are environmentally contaminating.

Hydroponic solutions contain all the basic nutrients necessary for the development of photosynthetic organism. This of course, means that hydroponic solutions are the ideal source of nutrients for algae growth. When they are disposed of down the drain, the solutions cause algae blooms which are terrible for the environment because when algae populations dye massively, they deprive water of oxygen and cause the death of aquatic organism.

The solution ? It is actually very simple. Have a hydroponic growing container in which plants that you “don’t care for” are grown. For example, have a hydroponic growing container in which grass, weeds or other type of “low nutrient requirement” plants are grown. This way, when your hydroponic crop has a solution it can no longer use, pour that solution into your other hydroponic container. Now leave the solution for those plants to take care off for two months. Once this happens, the solution should be very depleted of nutrients and unable to cause any algae bloom of importance.

Even better, the “waste solution” container, can have plants you can use. For example, you can cultivate herbs with low nutrient requirements that are for indoor use and have them as the “byproducts” of your main hydroponic system. It is also excellent for growing grass for any cattle or horses you may have. (below a picture of papyrus, a plant that naturally deals with solutions with low nutrient concentrations)
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Every hydroponic grower should know how to prepare his or her own solution in order to save money and improve crop results. Preparing a nutrient solution is an exact science involving some basic chemical knowledge that is in fact, not so hard to do. One of the main problems in preparing hydroponics solutions (besides knowing the formulation) is knowing how much to weight of each available nutrient salt in order to achieve the desired concentrations.

In order to solve this problem, you can use the hydroponic nutrient calculator I created called Hydroponic Buddy. The hydroponic calculator is located here.

It is fairly easy to use, the calculator includes links to tutorials you can find within this blog which will tell you how to do many things, from the calculation of simple formulations to the creation of A+B+C concentrated solutions, the copying of commercial nutrients, etc.

And that’s it ! Press the calculate button and you’ll be shown the final concentration results coupled with the amount of grams you should weight of each salt in order to obtain the concentration results you desire. Now you just need to go to your local supplier and buy each one of the salts in order to prepare your hydroponic solution. This let’s you experiment with different nutrient concentrations as well as with recommended and optimum nutrient concentrations for different plant species.

Amongst one of the few properties that hydroponic growers use to control their nutrient solutions is electrical conductivity (EC). The main problem with the measurement of the EC, is that few growers really understand it’s meaning and more often than not, grossly overestimate the amount of information it can give them. Therefore, I decided to create this FAQ in order to better explain electrical conductivity, it’s limitations and it’s uses.

So, what is electrical conductivity ?

Electrical conductivity measures the easiness in which an electrical charge can flow through a certain length of a certain material. It is usually measured in S/cm which just means that the material has a certain conductance in S (Siemens) per centimeter. A material with a higher electrical conductivity let’s charge flow more swiftly (it offers less resistance to the movement of charge).

Why is this useful in hydroponics ?

It is useful in hydroponics because the conductivity of a solution is directly proportional to the amount of salts (in this case, the salts are our nutrients) dissolved inside it; so, if a solution has more salts dissolved, it has a higher conductivity. Therefore, measuring EC can give you an idea of how many nutrients are left in your solution.

What are the limitations of EC in hydroponics ?

The first limitation arises because of the chemical character of the property we are measuring. Since EC is proportional to the amount of dissolved salts in each solution, you could suppose that measuring EC would always allow you to calculate nutrient concentrations within your nutrient solution. This is wrong ! Salts increase conductivity but each different ion present inside the solution has a different specific conductivity (they contribute differently to the overall EC) so you could in fact be deceived because you could just have a small amount of an ion that conducts a lot or too much of an ion with a small conductivity. Of paramount importance are the ions that determine pH which have conductivities hundreds of times larger than other ions.

What are some common mistakes when measuring EC ?

Given the above mentioned conditions, EC should always be measured at a constant pH. An EC measured at pH 5 and an EC measured at a pH of 7 will be completely different given that the ions which determine pH have a very large effect on the EC value. Another important fact is that the conductimeter should be calibrated using a solution of known conductivity. If it is not, comparison between measurements can be meaningless.

What is EC useful for ?

The electrical conductivity can tell you if your solution has lost nutrients or water due to evaporation, if measurements are done at the exact same pH value. The EC should be measured when the solution is prepared and three times each day after then. If your solution’s EC becomes too high, you can add water to lower it to the original value. If EC becomes too low (70% of original value), you should not add nutrients. This means that your solution has been substantially changed in composition by the plant and it needs to be disposed off and a fresh one needs to be prepared.

You cannot do this because you don’t know which nutrients the plant took up. By adding nutrients to the solution you could be putting too much or too little of any given compound. Of course, you could always do some fancy atomic emission analysis to know the exact ionic composition of the solution but the safest (cheapest and easiest) thing would be to adequately dispose of your nutrient solution and start a fresh batch.

Most hobby and some commercial hydroponic growers often get their nutrient solutions from stores that sell “generic” hydroponic nutrients. In some cases, this standard solutions are “compensated” with the aid of some additional help coming from additive solutions that are formulated to add certain qualities that the original standard solution is missing.

Well, the sad thing here is that by not preparing his or her own nutrient solution, the hydroponic grower is wasting his main edge in soiless culture. The composition of the nutrient solution.

In the particular case of lettuce, different nutrient solution compositions have varying effects on the plant, this, of course, because the particular chemistry of your nutrient solution must be fitted for your plant in order to achieve optimum results. The use of generic solutions to feed your plants will more often than not end with some loss in product quality (dry weight, nutrient content, marketability, etc).

In a peer reviewed study published in 2004, M.S. Karimaei et al [1] compared different nutrient solutions for lettuce cultivation under hydroponic conditions. Amongst their conclusions, was that the Hoagland solution offers the best conditions for lettuce cultivation amongst the solutions they tested. They also found some important factors such as the electrical conductivity being inversely correlated to dry weight and K concentration. This shows the utmost importance that using the correct and not the “generic” nutrient solution has on your lettuce growing potential.
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[1] Karimaei, M.S., Massiha, S. and Mogaddam, M. 2004. COMPARISON OF TWO NUTRIENT SOLUTIONS’ EFFECT ON GROWTH AND NUTRIENT LEVELS OF LETTUCE (LACTUCA SATIVA L.) CULTIVARS. Acta Hort. (ISHS) 644:69-76

As I published earlier, the use of hydrogen peroxide is extensively known in hydroponic cultivation as a disinfectant. However, this little molecule has far more uses and some of them also pertain to plant grow. For example, as it was discussed in the fourth International Symposium on Seed, Transplant and Stand Establishment of Horticultural Crops; Translating Seed and Seedling Physiology into Technology in February 2008, the use of hydrogen peroxide has been studied as an agent to aid the germination of seed.

Indeed, the chemical qualities of hydrogen peroxide make it ideal for the replacement of the stratification of certain seeds. In a peer reviewed article published about the use of hydrogen peroxide for the germination of eastern gamagrass seeds, the authors discovered that the use of hydrogen peroxide 15% for 18 hours indeed helps the seeds germinate by “dissolving” the outer coat of the seeds and therefore facilitating water’s access to the seeds embryo.

In fact, hydrogen peroxide is so effective at this, that the above mentioned treatment substitutes a four week period of stratification at 4°C. The seed containing cupules treated with Hydrogen peroxide effectively germinated after only 2 weeks, when such a time was impossible before the treatment. The article mentions that hydrogen peroxide was the “most effective” mean of breaking up seed dormancy and effectively carry out germination. (Below a picture of eastern gamagrass)
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As we all know, hydroponic growers face the challenge of maintaining pure and innocuous solutions that should be free of any type of bacteria, algae or any other microorganism. The growth of any of these organisms inside the nutrient solution carries with it the imminent possibility of plant disease as well as an increased risk of malnutrition and of course, nutrient deficiencies.

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Algae, in particular, are one of the most troublesome organisms as they are found everywhere and they grow ecstatically inside a hydroponic nutrient solution (after all, they are photosynthetic organisms !). These little creatures love to colonize plant roots (depriving them of food) and also consume a large proportion of the nutrients present inside a hydroponic solution.
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So what should a concerned grower do about the incredible problem of algae growth ? Nature has given us part of the answer in the form of a powerful oxidant called hydrogen peroxide. This molecule, whose formula is basically H2O2 decomposes forming molecular oxygen and water. It also reacts with organic matter in a redox reaction to oxidize it. In other words, algae and hydrogen peroxide cannot coexist.
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However, there is an important problem that arises when using hydrogen peroxide. It does not discriminate between roots and algae so using more than the optimum amount leads to plant root death caused by the same quality that kills algae. So what is this optimum amount ?
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I would love to show you a peer reviewed article that studied this issue but, as a matter of fact, no one has actually studied the levels at which these conditions are right at a scientific level. Most of what we know is currently empirical. Nonetheless, I have – from personal experience- verified that the application of 1mL of hydrogen peroxide (3% v/v) per liter of nutrient solution every week does seem to prevent algae and does not damage plant roots.
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In this section I will answer some of the questions that I have already answered in mails that have been sent to me, and that I believe will be helpful to most of the people interested in developing hydroponic crops. This FAQ covers the essential part about the hydroponic growing media and some advice based on my experience and personal knowledge.