Much research has been done  in hydroponics that correlates the availability of nutrients in the water to the pH. Many of the finding from these studies are applicable to aquaponics systems. The pH is a parameter that measures the acidity or alkalinity of a solution. This value indicates the relationship between the concentration of free ions H+ and OH- present in a solution and ranges between 0 and 14.

In soil, the Troug diagram illustrates the pH effect on the availability of nutrients to plants(Fig. 1). Similarly, changing the pH of a nutrient solution affects its composition, elemental speciation and bioavailability. The term “speciation” indicates the distribution of elements among their various chemical and physical forms like: free ions, soluble complexes, chelates, ion pairs, solid and gaseous phases and different oxidation states

An important feature of the nutrient solutions is that they must contain the ions in solution and in chemical forms that can be absorbed by plants, so in hydroponic systems the plant productivity is closely related with to nutrient uptake and the pH regulation. Each nutrient shows differential responses to changes in pH of the nutrient solution as described below.

In the nutrient solution, NH3 only forms a complex with H+. For a pH range between 2 and 7, NH3 is completely present as NH4+. Increasing the above 7 the concentration of NH4+ decreases, while the concentration of NH3 augments.The potential for increased levels of un-ionized ammonia, which reduced plant nutrient uptake from micronutrient precipitation, are additional problems associated with pH 8.5.

Phosphorus is an element which occurs informs that are strongly dependent on environment pH. In the root zone this element can be found as PO4 3-, HPO42-, and H2PO4-ions; the last two ions are the main forms of P taken by plants. On inert substrates, the largest amount of P available in a nutrient solution is presented when its pH is slightly acidic (pH 5). In alkaline and highly acidic solutions the concentration of P decreases in a significant way. Namely, with pH 5, 100% of P is present as H2PO4-; this form converts into HPO4-2 at pH 7.3 (pKa2), reaching 100% at pH 10. The pH range that dominates the ion H2PO4-2 on HPO4- is between 5 and 6. The pH-dependent speciation of P is showed in Fig. 3.

Potassium is almost completely present as a free ion in a nutrient solution with pH values from 2 to 9; only small amounts of K+ can form a soluble complex with SO4-2 or can be bound to Cl-. Like potassium, calcium and magnesium are available to plants in a wide range of pH; however, the presence of other ions interferes in their availability due to the formation of compounds with different grade of solubility.

As water naturally contains HCO3-, this anion turns into CO3-2 when the pH is higher than 8.3 or to H2CO3 when it is less than 3.5; the H2CO3 is in chemical equilibrium with the carbon dioxide in the atmosphere. Thus at a pH above 8.3, Ca2+ and Mg2+ ions easily precipitate as carbonates. Also, as mentioned above, when the pH of the nutrient solution increases, the HPO42- ion predominates, which precipitates with Ca2+ when the product of the concentration of these ions is greater than 2.2, expressed in mol m-3. Sulphate also forms relatively strong complexes with Ca2+ and Mg2+. As pH increases from 2 to 9, the amount of SO42-, forming soluble complexes with Mg2+ as MgSO4 and with K+ as KSO4-increases.

Iron, copper, zinc, boron, and manganese, become unavailable at pH higher than 6.5 nutrient availability for plant uptake at pH above 7 may be restricted due to precipitation of Fe2+, Mn2+, PO3−4, Ca2+ and Mg2+ to insoluble and unavailable salts. In conclusion, the proper pH values of nutrient solution for the development of crops, lies between 5.5 and 6.5.

Source: Nutrient Solutions for Hydroponic Systems Libia I. Trejo-Téllez and Fernando C. Gómez-Merino Colegio de Postgraduados, Montecillo, Texcoco, State of Mexico, Mexico