Hydroponics Systems

Hydroponics literally means "working water" (hydro=water, ponics=work). Practically, it means growing plants without using soil, delivering nutrients only by a aerated nutrient solution.

Hydroponics systems can be either closed or open systems. In closed systems the same hydroponic nutrient solution is recirculated and the nutrient content is monitored and adjusted.

Keeping the nutrient balance in such systems is a challenge and the hydroponic nutrient solution has to be sampled and analyzed every week. The nutrient solution composition has to be adjusted according to the results.

In open systems a fresh nutrient solution is introduced for each irrigation cycle.

Inert growing media are usually used in hydroponics. Unlike soil, that stores nutrients and directly interacts with the plant, the growing media used in hydroponics have little effect, if any, on the plant nutrition. As a result, the only source of nutrients is the nutrient solution, and therefore you have total control over your plant nutrition.

While soil allows more tolerance for inaccuracy, hydroponics leave very little room for errors. Because changes are rapid and mistakes can be very costly, hydroponics growers should make highly educated and accurate decisions.

Close monitoring of Water content is extremely important in hydroponics

Naturally, water plays a major role in hydroponics, making their quality and content - paramount.

There are several key questions that should be addressed when dealing with this issue:

Which nutrients are required? Are they all present in the correct concentration in your hydroponic solution?

What are the ratios between the nutrients? Do some nutrients affect the availability of others?

What is the total concentration of minerals in the solution?

Are there any harmful elements in the water? What is their concentration?

What is the pH of the hydroponic solution and how does it affect nutrient uptake by the plant?

First Step - Source Water Analysis

Hydroponics solution consists of minerals in the source water and the nutrients added with fertilizers. The choice of fertilizers type and amount added greatly depend on the initial content of source water. Therefore, testing the source water prior to preparing a fertilizer formula is imperative.

For example, your source water may contain an sufficient amount of calcium for your crop nutrition. In this case, you should not use calcium nitrate, not only because it is redundant, but also because any calcium addition might precipitate with other elements in the hydroponic solution, such as phosphorus, or interfere with uptake of others.

Additionally, source water may contain too large amounts of harmful elements, such as sodium, chloride, fluoride or excess of boron, rendering it unsuitable for hydroponics. This can be solved by pre-treating the source water with desalination or ion-exchange.

Source water analysis must contain at least the following information:

EC, pH, calcium, magnesium, chloride, sodium, sulfur and bicarbonate levels.

If your water source is a bore hole/well, it would also be wise to test for boron, manganese and fluoride levels.

The Essential Plant Nutrients

There are 13 mineral nutrients that are essential for completion of the plant's life cycle. Macro-elements are required in large quantities: nitrogen, potassium, phosphor, calcium, magnesium, sulfur. Micro-nutrients are required in very low concentration: iron, manganese, zinc, copper, molybdenum, boron, chlorine.

All of these nutrients should be provided in the hydroponics solution, in the right concentrations, and in adequate ratios.

According to the law of "limiting factor" , if one nutrient is deficient, other nutrients cannot compensate for the deficiency, and the crop may suffer, resulting in decreased quality and/or yield.

Nitrogen, phosphorus and potassium

Most water sources contain only very small amount of these nutrients, if at all, therefore they must be provided using fertilizers.

Commonly used soluble fertilizers are MAP, potassium sulfate, ammonium nitrate, potassium nitrate.

Calcium and magnesium

These elements are usually found in source water, sometimes in adequate concentration for our needs, especially in well-water. If the concentration is higher than required, the source water should be pre-treated.

Calcium nitrate is the only fertilizer appropriate for calcium addition to hydroponics irrigation water. Magnesium nitrate and magnesium sulfate are both appropriate sources for magnesium addition. Note that calcium nitrate and magnesium nitrate also contribute nitrogen to the solution.

Sulfur

Sulfur is present in a wide range of concentrations in various water sources, and plants growing in hydroponics can tolerate relatively high concentration. But sulfur excess might have untoward effects and even limit nitrate uptake.

Micro-nutrients

Iron, manganese, zinc and copper can be provided in the sulfuric form, but their availability is greatly decrease in pH greater than 6.5. The chelated forms, may also be used, because they are available for uptake in a wider range of pH. Some growers regard EDTA as harmful for plants, and avoid its use.

Molybdenum is usually provided using sodium molybdate. The presence of sodium in this fertilizer should not be a cause for alarm. Because molybdenum is needed in minute quantities, very small amounts of this fertilizer are usually used, and the sodium addition is negligible.

Boron can be provided through boric acid or solubor. Solubor also contains sodium, but again, quantities are small enough as to not have a significant effect on sodium concentration in the solution. Range for adequate boron levels is very narrow (0.2-0.5 ppm) and can easily be missed, resulting in either deficiency or toxicity. Therefore boron supplements should be carefully added. Well water often contain sufficient boron levels, so no boron addition is needed.

Harmful elements - Sodium - Chloride

Chloride is required by plants in minute quantities and most water sources contain chloride concentration well above and beyond plants require, so chloride deficiency is extremely rare. Chloride related problems are more commonly those of toxicities rather than of deficiencies. Therefore, using fertilizers which contain chloride is uncommon in hydroponics.

Sodium can be very harmful in recirculating systems, since it builds up with time in the hydroponic solution. Threshold concentration of sodium and chloride for most hydroponics grown plants is 75 ppm.

Electrical Conductivity (EC)

Electrical conductivity is a measure of the total salts dissolved in the solution (learn more about EC). It is used for monitoring daily applications fertilizers. Note that the EC reading doesn't provide you with information regarding the ratios between nutrients.

In closed hydroponics systems, with recirculating solution, nutrients which are not absorbed by the plants (such as sodium, chloride, fluoride etc.) or ions released by the plant, build up in the hydroponic solution. In this case you need more information about the solution content, that EC cannot provide. Frequent water analysis tests will help you decide on the timing for replacing the nutrient solution or dilute it with good quality water.

pH

The optimal pH range for the nutrient solution is 5.8-6.3. micro-nutrients are more available in lower pH, but when pH levels drop below 5.5, you run the risk of micro-nutrients toxicity, as well as impaired availability of calcium and magnesium. In hydroponics, especially in closed systems, the roots readily affect the hydroponic solution pH, so pH tend to fluctuate.

Appropriate products for acidifying the hydroponic solution are sulfuric acid, phosphoric acid and nitric acid. The preferred one is sulfuric acid because the EC control and pH control are kept separate. This makes the grower's job much easier.

Ammonium/nitrate is one of the major factors affecting the pH of the nutrient solution

Nutrient balance

Several nutrients compete with each other over uptake by the plant, so keeping adequate ratios is important for avoiding deficiency. For example excess of potassium compete with calcium and magnesium absorption. A high iron/manganese ratio can result in manganese deficiency, and high sulfur concentration might decrease the uptake of nitrate.

As mentioned in the beginning of this article, the nutrient balance in a closed hydroponics system changes frequently and has to be closely monitored and managed. Harmful minerals like sodium, chloride and fluoride build up in the solution and might reach toxic levels.

Guy Sela is an agronomist and a water specialist. In 2005 Guy developed a unique software for fertilizer management that helps growers reach optimal yields, while saving time, money and frustration.

Read more of Guy's articles, tips & info at: http://www.smart-fertilizer.com/index2.php?id=3