Everything you wanted to know about fertilization and plant nutrition!
The hydroponic nutrient solution is the sole source of nutrients to the plant. Therefore, it is imperative to apply a balanced solution, that contains all plant nutrients, at the right balance.
Several important factors have to be considered when choosing fertilizers and preparing a hydroponic nutrient solutions:
There are various standard nutrient solutions, such as the Hoagland solution (1933), Steiner (1961), Bollard (1966) and others. These standard solutions are good as a general guideline, but are not adapted to specific growing conditions.
Even if you decide to use one of the standard nutrient solution, make sure to use the nutrient concentration as a guideline, and not the fertilizer recipe that corresponds to them. The initial composition of the raw water that you use will affect the actual nutrients that have to be added with fertilizers.
|Element||Ionic form absorbed by plants||Common range (ppm=mg/l)|
|100-250 ppm elemental N|
|Phosphorus||Dihydrogen phosphate (H2PO4-)
Monohydrogen phosphate (HPO42-)
|30-50 ppm elemental P|
|Potassium||Potassium (K+)||100-300 ppm|
|Calcium||Calcium (Ca2+)||80-140 ppm|
|Magnesium||Magnesium (Mg2+)||30-70 ppm|
|Sulfur||Sulfate (SO42-)||50-120 ppm elemental S|
|Iron||Ferrous ion (Fe2+)
Ferric ion (Fe3+)
|Copper||Copper (Cu2+)||0.04-0.2 ppm|
|Manganese||Manganese (Mn2+)||0.5-1.0 ppm|
|Zinc||Zinc (Zn2+)||0.3-0.6 ppm|
|Molybdenum||Molybdate (MoO42-)||0.04-0.08 ppm|
|Boron||Boric acid (H3BO3)
|0.2-0.5 ppm elemental B|
|Chloride||Chloride (Cl-)||<75 ppm|
|Sodium||Sodium (Na+)||<50 ppm TOXIC to plants|
Suggested Nutrient Solutions for Various Crops
The electrical conductivity is a measure of the total salts dissolved in the hydroponic nutrient solution. It is used for monitoring applications of fertilizers. Note that the EC reading doesn't provide you with information regarding the exact mineral content of the nutrient solution.
In closed hydroponics systems, the hydroponic nutrient solution is recirculated and elements which are not absorbed in high quantities by plants (such as sodium, chloride, fluoride etc.) or ions released by the plant, build up in the hydroponic nutrient solution.
In this case there is a need more information about the nutrient solution content, that EC cannot provide. Testing the hydroponic nutrient solution frequently will help you decide on the timing for replacing the nutrient solution or dilute it with fresh water.
The optimal pH range of the hydroponic nutrient solution is 5.8-6.3. Micronutrients are more available in lower pH, but when pH levels drop below 5.5, you run the risk of micronutrients 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 tends to fluctuate.
Appropriate products for acidifying the hydroponic nutrient solution are sulfuric acid, phosphoric acid and nitric acid.
Ammonium/nitrate is one of the major factors affecting the pH of the nutrient solution.
The hydroponic nutrient solution consists of minerals in the raw water and nutrients added with fertilizers.
The selection of fertilizers and their concentration in the hydroponic nutrient solution greatly depend on the quality of the raw water. Therefore, testing the raw water prior to deciding on a fertilizer formula is imperative.
Minerals such as calcium, magnesium, sulfur and trace elements such as boron, manganese, iron and zinc may be present in the source water. These elements must be factored in when adjusting the hydroponic nutrient solution.
Additionally, raw water might contain high concentrations of unwanted minerals, such as sodium, chloride or fluoride, rendering it unsuitable for hydroponics.
This can be solved by diluting the water a pure water supply or pre-treating the raw water with desalination or ion-exchange.
Author: Guy Sela, international expert in plant nutrition and irrigation.
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