A high pH buffering capacity is an advantage because it ensures minimal initial and ongoing pH maintenance – especially where high alkalinity make-up water is used.    This feature is extremely beneficial because lack of pH control is a common cause of nutrient failure.  As a rule-of-thumb, a high quality ‘bloom’ formulation should require virtually no initial pH adjustment, even when used with high alkalinity/hard water i.e. after dilution with a range of make-up waters, the pH should always fall well within the range 4.5 to 6.5.  However, many brands fail to offer this feature.

The significance of pH 5.0 to 6.0:  It is over this compromise pH range that all growth factors are catered for to produce optimal growth.  If the pH is allowed to rise much above 6.0, over half the essential nutrient species (especially calcium, sulphate and the trace elements copper, iron, manganese and zinc) can precipitate thus becoming immobile and unavailable for transport by the water flow to the roots (Fig 1.2).

The precise pH at which precipitation of macronutrients starts is determined by the combined concentrations of calcium and sulphate.  Except for fertilizers "low" in calcium and sulphate this problem commonly occurs at pH values of around 6.5 for concentrations which would yield conductivities of 2.5 mS/cm in distilled water and pH 7.0 for 1.5 mS/cm.  Hence, to avoid precipitation, higher nutrient concentrations generally must be held at lower pH values.

In spite of this precipitation problem, some references advocate pH values well above 6.5 for some plant varieties - conditions which risk depleted concentrations of the above mentioned elements.  This is incorrectly justified by quoting the chart in Fig 4 as proof.  As highlighted, it is important to realize that this data is based on soil culture. 

Comment on common recommendation of pH 6.2 (or 6.3):  Although this is a commonly recommended pH value, it has no scientific basis. It appears to have gained a sort of mythology status from the early days of hydroponics when the only cheap means for hobbyists to measure pH was by using the common bromothymol blue pH indicator sold by pet shops for pH maintenance of fish tank water. Because the lowest pH value able to be determined by that indicator is about 6.2, that value has unfortunately become an entrenched recommendation in some sections of the hydroponic industry.

Overview:  pH buffering capacity

Do not under-estimate the need to maintain pH under 6.5.  A high pH buffering capacity will make it easier to achieve that result by allowing less frequent checking of pH – especially where high alkalinity water is being used.  It will minimize the time spent maintaining pH and help prevent poor results from what may otherwise be a good quality nutrient.

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