Water hardness – Hydroponics

Water hardness is a combination of chemical and physical properties associated with the content of dissolved salts of alkaline earth metals, mainly calcium and magnesium – the so-called “hardness salts”.

Water containing 100-150 mg of calcium per liter is acceptable for hydroponic use and, in principle, for most plants, there is no need to add additional calcium.

Soft water contains less than 50 mg calcium per liter. For successful plant growth, it is necessary to additionally add calcium and magnesium salts.

Hydroponic fertilizer manufacturers take into account the presence of calcium and magnesium in their formulations, and produce fertilizer variations for both hard and soft water.

Water hardness is distinguished between temporary (carbonate) and permanent.

 

Temporary water hardness

Temporary hardness is a type of water hardness caused by the presence of dissolved calcium carbonate and magnesium carbonate. When these minerals dissolve in solution, calcium and magnesium cations (Ca2+, Mg2+) and carbonate and bicarbonate anions32-HCO3). The optimum pH for hydroponics is 5,5. The pH of the water will be too high if the irrigation water contains significant amounts of carbonate and bicarbonate (CO32-HCO3). This is often the case when using well water. Therefore, this water must be acid treated to neutralize the HCO3 and to lower the pH of the nutrient solution. The amount of acid to be added is determined by the HCO content3… When acid is added to water, the bicarbonate is neutralized by the proton of the acid and the pH of the solution will decrease. Calcium (or magnesium) will remain available for absorption by plants and the acid anion will remain dissolved in the water. For example, when adding nitric acid, the following reaction will take place:

Ca2+ + 2HCO3 + 2HNO3 ⇋ Ca2+ + 2CO2 + 2H2O + 2NO3

Nitric acid is widely used for this purpose, but phosphoric acid and its derivatives such as urea phosphate can also be used. By adding more acid, the concentration of associated anions in solution, such as nitrate and phosphate, will increase. These values ​​should not exceed the required concentration for the nutrient solution. This limits the amount of acid that can be added to neutralize HCO3… Therefore, the initial concentration of HCO3 in water is a major quality problem.

The neutralization of bicarbonate with an acid initiates the evolution of carbon dioxide (CO2) and water. CO2 must freely leave the nutrient solution; otherwise, the pH of the solution will not be lowered and will fluctuate. This means that the reaction of acid and bicarbonate must take place in open systems such as an open mixing vessel.

 

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Anna Evans

Author ✓ Farmer

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