Hydroponic Techniques    Media    Hydroponic Fertilization    Dissolved Oxygen    PH    TDS, PPM, EC

The word “hydroponic” is derived from the Greek terms "hydro" = “water” and "ponos" = "labor", or working water. Hydroponic gardening utilizes nutrients present in water solutions to attain growth. Herein lies the essence of hydroponic gardening: to provide the plant with the ideal water and nutrient ratios and optimum environmental conditions for growth to achieve maximum yields.

Think of a plant as its own architect and construction crew. The responsibility of the grower is simply to drop the building materials off at the worksite at the right time and in the right amounts. To take the analogy a step further, lets compare soil to hydroponics. In a soil-based scenario, the construction crew continually begins construction from existing job sites. There are leftover materials everywhere, making the possibility for contamination far greater than if they were permitted to start from scratch. Because of the nature of the leftover materials and the state in which the new materials exist on the site, the architect is forced to deal with unwanted materials, in unwanted quantities and the grower has no reliable or immediate way to determine what is there and what isn’t. In other words, the construction process is not streamlined. In a hydroponic scenario, the architect and construction crew begin design from a clean slate and the grower has a better and more immediate grasp on what is available and what is not available. This allows construction to focus on the task at hand, instead of being forced to sift through unwanted or unnecessary materials making the entire operation more efficient. The energy of the construction crew is therefore focused, resulting in a reorientation of energy from preventative and wasteful practice to quantifiable construction or growth. In short, hydroponics eliminates the barriers and stresses associated with plant growth and allows a much smoother and straightforward method of growing resulting in overall faster and higher yields.

Maintenance
Maintaining a hydroponic system is different from traditional soil-based growing. However, once the grower gets a crop under their belt and realizes the potential of hydroponic gardening the light bulb comes on. People ask us all the time, “why haven’t I been doing this all along?” The grower will also find that once the basics are mastered, a hydroponic garden is actually less maintenance than a soil garden- no weeds, no hand watering, fewer pests and diseases, higher yields. You can essentially take the whole “green thumb” argument and throw it out the window. A hydroponic garden waters itself!

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Hydroponic Techniques

Wick systems are passive systems, meaning it has no moving parts and the nutrient solution remains in one place. Plants are fed through capillary action from a wick drawing nutrient solution into the growing medium from the reservoir. The biggest draw back of this system is that plants that are large or use large amounts of water and nutrient may use up the nutrient solution faster than the wick(s) can supply it. In this case additional wicks may be used as a supplement, or another technique can be utilized.

Of all soilless methods, water culture, by definition, is true hydroponics. It is also the simplest active hydroponics system to set up on a small scale. In this system the plant roots are totally immersed in a nutrient solution. In a water culture system the roots grow directly into the reservoir as opposed to having a remote reservoir. The actual design of the system is limited only by the imagination of the builder. The system must provide means to (1) support the plant above the solution, (2) aerate the solution, and (3) prevent light from reaching the solution (to prevent the growth of algae).

Drip systems are probably the most widely used type of hydroponic system in the world. They are similar to drip irrigation systems popular with commercial farmers for their ability to conserve water through direct feeding. Operation is simple; a timer controls a submersed pump, which turns the pump on and off. Nutrient solution is dripped onto the base of each plant by a small drip line. In a recirculating drip system the excess nutrient solution that runs off is collected back in the reservoir for re-distribution.

The Ebb and Flow (or flood and drain) system works by temporarily flooding the grow tray with nutrient solution and then letting the solution drain back into the reservoir. This action is normally done with a submerged pump that is connected to a timer. The timer is set to come on several times a day, depending on the size and type of plants, temperature and humidity and the type of growing medium used. The drain cycle improves the oxygen contact with the plants roots. Using the right medium will ensure that moisture will be available for the roots so that they do not dry out between cycles. One of the main attractions of an Ebb and Flow system is the ability to containerize your plants and physically move them around. This aids in continuous production scenarios and enhances the control of a grower utilizing a veg (blue) and bloom (red) room scenario. The main disadvantage of this type of system is that unless your medium ensures moisture retention there is a vulnerability to power outages, since the only way for your plants to access food is through the action of the pump.

The Nutrient Film Technique (NFT) is a water-cultural technique in which plants are grown with their root systems contained in a plastic trough through which nutrient solution is continuously circulated. Work on NFT cropping was pioneered by Allen Cooper at the Glasshouse Crops research Institute in Littlehampton, England, in 1965. The term nutrient film technique was coined to stress that the depth of liquid flowing past the roots of the plants should be very shallow in order to ensure that sufficient oxygen would be supplied to the plant roots.

Feel free to experiment by building your own hydroponic system. As long as you have oxygenated nutrified water at the proper pH it doesn’t matter how you feed them. The possibilities are endless! Be sure to enclose your reservoir so as to prevent evaporation and control feeding times via timers in the case of a top-drip or ebb and flow setup. Email PG or do a simple web search to find simple plans for construction.

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Media

The chemical, physical, and structural properties of a substrate, or media, can have major effects on plant growth, root health, yields, and produce quality. It is important to select the proper media for the type of system and crop being grown. A seasoned grower develops knowledge of crop specific logistics through experience. It is a good idea for a beginner grower to experiment with several media types in their specific growing situation to determine which media suits the plants being grown, the system being used, and the specific ambient environment. Substrate Properties This section is aimed at educating the grower about the specific characteristics of

Since all forms of media provide good general stability we will discuss the specific physical structure of media first. The physical structure of a substrate is made up of two major components: the solid particles and the pores between the particles, or the lattice. Of these two, the pore space between the solid particles is most important. Substrate porosity can be divided into three categories: large, small, and very small.

Large pores can be easily recognized in a substrate by saturating it and allowing it to drain. Pores that readily lose water by gravitational drainage are termed large pores and have a diameter larger than about 60 microns. These act as passages for the drainage of surplus water or nutrient, root growth, and pores for exchange of oxygen and carbon dioxide.

Small pores (0.3-60 microns) act as a reservoir for moisture that can be utilized by the plant between nutrient applications. These pores retain water and nutrients for plant growth.

Very small pores (less than 0.2 microns) retain water when plants growing in the substrate have reached the permanent wilting point. They retain water at suction levels higher than can be exerted by the plants is unavailable for plant growth. However, they do ensure capillary rise of water by conduction and therefore play a role in the spread of water through the substrate.

A good hydroponic substrate contains the right balance between large and small pores to provide sufficient moisture between nutrient applications, a high degree of aeration and capillary action to evenly spread moisture throughout the root zone, and sufficient large pore space to allow root outgrowth into the substrate. General recommendations for suitable hydroponic substrates are at least 35-50 percent water-holding capacity by volume and 25-40 percent air space after drainage.

A substrate can affect the composition of the nutrient solution and assimilation of elements by plants depending upon the size of the granules and their structural, physical, and chemical properties. Soilless media are selected based on having low levels of natural nutrients to prevent any alteration or imbalance of the nutrient solution. The ability of certain media to retain nutrients against leaching losses is related to its cation exchange capacity, or CEC. The CEC is the ability of the media to attract and hold various cations such as potassium, calcium, magnesium, and iron, for use by the plant’s roots. These positively charged ions are attracted to the negatively charged media particles and therefore aren’t leached as quickly from the media. A media with a high CEC will require less frequent applications of nutrients than a media with a low CEC. Zeolite is an example of a media with high CEC.

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Coconut Fiber

Perhaps the most important aspects of coir fiber as a growing medium are lack of initial nutrients and its ability to act as a pH buffer. Coir’s negligible initial nutrient composition and slightly acidic pH (pH 5.8-6.5) is ideal for plant growth and hydroponic use because it will not affect the carefully controlled nutrient and pH levels of the nutrient solution.
Coir fiber has the ability to absorb and retain large quantities of water and nutrient for plant use (typically between 80-88 percent) between irrigations. Air-filled porosity values of 23-29 percent have been measured, which is within the recommended range for a container mix but a little on the low side when compared to other free-draining media, such as expanded clay or perlite. Coir also resists decomposition, making it more desirable than other substrates, such as peat or sawdust, which have a tendency to break down and lose their free-draining structures resulting in root suffocation and rot.

Expanded Clay Expanded clay- also called hydroton, or “growrocks”- has a physical structure that is porous, allowing good entry of both water and air. The pebbles or balls can range in size grades from 1-18 millimeters. All types of expanded clay are sterile, inert, and well suited to many hydroponic systems due to its free-draining nature and attractive appearance. However, expanded clay does not hold a great deal of moisture between nutrient applications and salt accumulation and drying out can be common problems in improperly managed systems.

Expanded clay has the advantage of being reusable for many years, provided it is cleaned and sterilized between crops to kill any pathogens that may be present inside the structure of the clay particles. Heat, steam, boiling water, hydrogen peroxide, or chlorine can all be used to safely sterilize expanded clay between crops.

Vermiculite

Vermiculite is a porous, spongelike, sterile media. It is a natural mineral, which expands with the application of heat. It is formed by hydration of certain basaltic minerals. It’s lightweight and has a high water absorption capacity- holding up to five times its weight in water. It also has a relatively high cation exchange capacity, holding nutrients in reserve and later releasing them. Care needs to be taken in some systems when using vermiculite as a stand-alone since it is prone to over saturation when nutrients are applied frequently, often resulting in root rot.

Perlite

Perlite is a siliceous, sterile, spongelike, amorphous glass mineral of volcanic origin. When it reaches temperatures of 850-900C, perlite softens (since it is a glass) and water trapped in the structure escapes and this causes the expansion of the material at 7-15 times its original volume. The expanded material is a brilliant white, due to the reflectivity of the trapped bubbles. It is ideal for soilless culture as a stand-alone and as an additive to a soil or media that tends to get waterlogged. Perlite is a free-draining media that does not have the high water-retentive properties of many other substrates. It is essentially neutral with a pH of 6-7 but without any buffering capacity and, unlike vermiculite, doesn’t have any cation exchange capacity. While perlite does not decay, the particle size does become smaller through fracturing as it’s handled. Perlite is often mixed at a ratio of 1:1 with vermiculite, which improves the moisture-holding and cation exchange capacity of the media while still remaining free draining.

Rockwool

Rockwool is probably the most widely used substrate in soilless growing worldwide. It is popular with commercial and hobbyist growers since it is sterile, lightweight (when dry), convenient, and has excellent physical and chemical properties. It has a high water-holding capacity (80 percent), and good aeration (17 percent air holding capacity), but does not have cation exchange or buffering capacities.

Rockwool is nontoxic but can be an irritant to the skin or via inhalation when dry, so care should be taken when handling. There is also an increasing concern over the problem of disposal once its useful growing life is over. Finding uses for spent rockwool has been the focus of some research, but most ends up in a landfill. Because it does not break down and decompose in the soil, significant buildup can be a liability.

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Hydroponic Fertilization
Nutrients are one of the basics of any hydroponic system. In order for a fertilizer to be incorporated into a hydroponic system it must be soluble in water. If not, the plant cannot access it. The beauty of hydroponics is that the grower has complete control over the implementation of fertilizer, regarding type and concentration. They also have the ability to immediately monitor and maintain a relative consistency, provided a nutrient meter is available. Because of this, it is important for the grower to know what you are supplying plants and what can go wrong. With any nutrient solution there are three factors to keep in mind. First, the composition of your nutrient – does it contain all of the elements required for plant growth in the correct ratios? For the most part, this is taken care of with pre-formulated commercial nutrient lines. Check your labels! Secondly, with your balanced and complete nutrient solution, what strength (EC or ppm) should it be running at for your particular crop, stage of growth, and type of hydroponic system and how do we measure it? And finally, what are the differences regarding “synthetic” and “organic-based” hydroponic fertilizers?

The Nutrient Solution – Composition
As mentioned above, there are many ‘pre-mixed’ nutrient solutions available that simply need to be diluted or dissolved in water before use. Often, these pre-made nutrients come in 1, 2, 3, or even more “parts” so the grower can change the ratio of the mineral elements to allow for either vegetative or fruiting and flowering growth, or for different crops. There are many excellent brands of these pre-mixed nutrients on the market, however, many growers have come across major problems when they try to use some of the ‘indoor plant food’ or other nutrients that have been designed for plants growing in soil or a pre fertilized potting mix. Often these types of products are not suitable for hydroponics because they are not designed to be a “complete plant food” or they are not water-soluble. For example, Nitrogen in the form of urea is not immediately available to a plant in hydroponics because urea is not soluble in water. For this reason Nitrogen must be delivered in its Nitrate form in order to be utilized in hydroponics. It is always preferable to buy a nutrient mix that is sold especially for hydroponic use, and is a “complete” plant food. To be “complete” a hydroponic nutrient needs to have the essential elements for plant growth, these are:

Nitrogen (N), Potassium (K), Phosphorous (P), Calcium (Ca), Magnesium (Mg), Sulphur (S), Iron (Fe), Manganese (Mn), Copper (Cu), Zinc (Zn), Molydenum (Mo), Boron (B), Chlorine (Cl)

{Hydrogen(H),Oxygen(O),and Carbon(C)------> come from air(CO2) and water(H2O)}

Solution Strength – Use and Measurement
Provided the nutrient you are using is complete and balanced, the concentration or strength of the solution has a major effect on plant growth and development. This is why it is essential to measure your solution concentration. Running the correct ppm or EC for your particular crop and system is important. The soil is an “everybody’s got to grow” environment. You can grow a head of lettuce next to a tomato plant and the respective plants take what they need from the general soil. You can imagine in this scenario, that the tomato must develop a much more extensive root system relative to the lettuce, because the tomato requires a higher level of fertilization to reach maturity and produce fruit. In a hydroponic scenario a grower would not use the same concentration of nutrient solution (ideally) to grow a head of lettuce and a tomato plant. You would top out the nutrient solution for the lettuce around 600 ppm and the tomato upwards of 1500-2000 ppm. If the lettuce were in the presence of the tomato concentration it would shrivel up and burn due to water stress. With the ability to control the level of fertilization in a hydroponic system the level of fertilization

It is sufficient to use nutrients based on manufacturers labeling. However, these guidelines are very general in nature. As we discussed above, the threshold for respective plants can vary greatly from plant to plant, even within genetic strains of the same plant. A nutrient meter, which relays the relative ppm, or EC is ideal for determining nutrient concentration. By pushing the plant during growth and noting the nutrient level at their respective threshold the grower gains valuable knowledge towards idealizing their growing experience.

Synthetic and Organic Based Nutrients
There are two kinds of formulations for hydroponic nutrients – synthetic (or refined mineral, or salt-based) and organic based. A synthetic nutrient is in the form of soluble salts formulated by humans for plant consumption. Similar to the way table salt (NaCl) disassociates in water to form Na+ (cation) and Cl- (anion), the pre-formulated fertilizer salts disassociate into the correct spectrum concentrations of necessary ion components needed for plant growth.

100% Organic fertilizer components are dependent upon organisms in the soil to convert the "organic" materials into an inorganic useable form for plants. Because of the non-soluble of many natural sources of nutrition, organic based hydroponic nutrients have 20-30% fertilizer salts with the rest being soluble “organic” components, such as guanos, plant extracts, worm castings, potash, kelps, etc. Because all of the components are not similar in structure and properties they disassociate at different rates in the “universal solvent” creating a slight pH fluctuation. This is the major difference between synthetic and organic based nutrients, but is easily overcome with patience and practice.

Having said this, there is absolutely no difference in the final ion product with respect to synthetic nutrients and organic based nutrients. An ion is an ion. It is simply a different way of delivering the food to the plant. As has been stated, plants “eat” ions in an inorganic form in the end anyway. In other words, plants do not eat guano ions, or kelp ions; they eat the inorganic constituents of these materials after they have been broken down or dissolved in water. A 100% hydroponic nutrient has not been formulated because in nature microorganisms and specific processes break down organic compounds to make them available to plants (i.e. “slow release” fertilizers). Since many organic materials are not soluble in water, they cannot be utilized in a hydroponic system, yet. There is great potential in the ability of scientists to locate unique plant extracts and formulations conducive to this idea. There is currently much energy being devoted to the technology.

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Dissolved Oxygen (DO)

A hydroponic nutrient solution is not just a mix of fertilizer salts and water. There are a number of organisms and compounds commonly found in our hydro systems that we need to be aware of. One of the most important of these is dissolved oxygen (DO), which is vital for the health and strength of the root system as well as being necessary for nutrient uptake. Plants breath just like all organisms via respiration. We are used to thinking that plants produce oxygen from CO2, which is true, but it just happens the overall amount of oxygen used is dwarfed by the amount produced by photosynthesis. Oxygen is an essential plant nutrient – plant root systems require oxygen for aerobic respiration, an essential plant process that releases energy for root growth and nutrient uptake. In many water-based hydroponic systems,the oxygen supplied for

Oxygen requirements for plants in flower tend to be more demanding in comparison to vegetative states. This is due to the size of the root system, temperature, and nutrient uptake rates, not the specific stage of growth.

Injury from low (or no) oxygen in the root zone can take several forms and these will differ in severity between plant types. Often the first sign of inadequate oxygen supply to the roots is wilting of the plant under warm conditions and high light levels. Insufficient oxygen reduces the permeability of the roots to water and there will be an accumulation of toxins, so that both water and minerals are not absorbed in sufficient amounts to support plant growth.

While it is possible to measure the levels of dissolved oxygen in a hydroponic nutrient solution, it is not carried out as often as EC/ppm or pH monitoring due to the cost of accurate DO meters. However, if an effective method of aeration is continually being used, and solution temperatures are not reaching excessively high levels, then good levels of oxygenation can be achieved without trouble.

Most growers are familiar with the need to have some sort of aeration in their nutrient solution due to waters high surface tension – whether they are in a recirculating water-based or media-based system. However, the effect of temperature of the solution on the DO levels and on root respiration rates also needs to be taken into account. As the temperature of your nutrient solution increases, the ability of that solution to hold DO decreases. For example, the oxygen content of a fully aerated solution at 50 F (10 C) is about 13 ppm, but as the solution warms up to 68 F (20 C) the ability of the liquid to ‘hold’ oxygen drops to 9-10 ppm. By the time the solution has reached 86 F (30 C) it is only 7 ppm. While this may not seem like a huge drop in the amount of DO, we have to remember that as the temperature of the root system warms, the rate of respiration of the root tissue also increases and more oxygen is required by the plant. For example, the respiration rate of the roots will double for each 10 C rise in temperature up to 86 F (30 C). So the situation can develop where the solution temperature increases from 68-86 F (20-30 C) during the day, with a mature crop, then the requirement for oxygen will double while the oxygen carrying capacity of the solution will drop by 25%. This means that the DO in solution will be much more rapidly depleted and then plants can suffer from oxygen starvation (root rot) for a period of time.

Perhaps one of the commonest problems in hydropnic systems is the Pythium pathogen. What many growers do not realize is that Pythium, being an “opportunist” fungi, often takes advantage of plants which have been stressed by a combination of high temperatures and oxygen starvation in the root zone. Pythium is usually described as a “secondary infection” meaning that the Pythium spores that are actually common in just about all hydroponic systems, don't actually attack the plant until it has been damaged in some way. Pythium is everywhere, so the best defense is a healthy plant. There are many products available that can help in your battle with root disease. Refer to the “roots” discussion in the Plant Nutrition section of this site for more info.

The variables to remember in regard to nutrient solutions are that aeration is vital to maintain the DO levels, temperatures should be kept within an optimum range, and a healthy plant is the best measure of protection against a disease outbreak.

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pH
The pH scale is a way to measure the Acid or Basic (alkaline) quantities in water. The official definition of pH is: a unit of measure that describes the degree of acidity or alkalinity of a liquid solution. It is measured on a scale of 0 to 14. Acids are in a range from 0 to 7, with lower numbers being a stronger acid. Alkaline is in the range from 7 to 14, with the higher numbers being a stronger base. The term pH is derived from “p”, the mathematical symbol of the negative logarithm, and “H”, the elemental symbol for Hydrogen. The technical definition of pH is the negative logarithm of the Hydrogen ion activity {pH = -log[H+]}. pH expresses the degree of activity of an acid or base in terms of hydrogen ion activity. When substances with more hydrogen ions are added the pH gets more acidic, thus having more of a (+) positive charge. Similarly, when substances with more hydroxide ions are added the pH gets more alkaline, thus having more of a (-) negative charge. These charges are present everywhere in your solution and form the framework of nutrient uptake. The plant takes care of the hard part; all you have to do is supply it with the right materials. The charges surround the roots and exchange positive and negative charges allowing for absorption of nutrients into the roots via active transport. For this reason the pH must be monitored during the entire growth cycle of the plants to maintain the maximum healthy uptake of nutrients. The pH of the nutrient solution will affect how well each element can pass through the root cell wall and nourish the plant. However, once you have properly calibrated your fertilizer concentrations and the pH of that solution you can generally assume it will stay steady barring any unforeseen root disease. Having said that, it is always a good idea to monitor your system too much than too little.

A pH of 7 is considered to be neutral. Any substance between 3.0 and 10.0 can be handled fairly safe, from the standpoint that they will not harm exposed skin. Any chemical with a pH lower than 3.0 or higher than 10.0 should be handled with care.

When growing soilless it is very important to control the pH of the water. The recommended pH range for plants is 5.8 to 6.5, with 6.0 to 6.5 being ideal for vegetative growth. A slightly lower range, 5.8 to 6.2, is ideal for fruiting and flowering. However, it is much more important to be in the ballpark rather than on the decimal point in regards to pH. This idea will become second nature to the experienced grower. PH kits and drops and pH pens are available for maintenance. Adding pH UP or pH DOWN solutions to raise or lower your solution, respectively, will alter the pH of the solution.

The nutrient used in soilless gardening, which is added to the water to promote growth, can also affect the pH. When adjusting the pH of your solution, it is a good idea to add the nutrient first then measure the pH.

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TDS, PPM, or EC?
The two dominant forms of determining fertilizer concentration in a hydroponic system are Electrical Conductivity (EC) and Total Dissolved Solids (TDS). TDS is referred to as Parts Per Million (PPM), which many may be familiar with. In fact, TDS and PPM are actually conversions of EC.

Nutrients take the form of ions in solution. The same way NaCl table salt disassociates into Na+ and Cl- ions when dissolved in water, your nutrient solution breaks up into ions that represent the entire spectrum of minerals needed for plant growth. EC is determined by sending an electric pulse through your nutrient solution with a nutrient meter. The rate at which the pulse reaches its destination is relayed into the resulting EC. So a nutrient solution containing more nutrient (or ions) results in a higher EC because there are more ions there to carry the charge. An EC of 1is equal to different PPM readings depending on which conversion factor is used. Herein lies the problem with TDS and PPM. The so-called 442 conversion results in 700 PPM for every 1 EC. Conversely, the NaCl conversion is approximately 500 PPM per 1 EC. This situation is not dissimilar to the differences and confusion caused by the American system of measurement and the Metric system. The discontinuity between these forms of measurement has caused wasted energy and student frustration since their inventions. Similarly, by not having a universal standard for nutrient concentrations the possibility of universal recognition must wait on human conversion. The most reliable way to ensure your number means what it means is to utilize EC.

Having said all of this, in the end it is the plant that will tell you what it wants. The bottom line is, be consistent with your calibration. By ensuring that, at least, you calibrate to the same place every time you can develop knowledge of what number your plants desire. Treat your number as a benchmark for pushing your plants. Nutrient meters are not vital to a hydroponic growing operation, but represent an additional level of knowledge and control and can be extremely beneficial in acquiring specific understanding of plant responses to mineral and amendment additions.

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