Failure to understand and apply the following is one of the key
reasons for nutrient failure.
F
or
nutrients to remain dissolved and therefore available for uptake by roots,
it is critical to maintain the pH between 5.0 and 6.0 - with an absolute
maximum of 6.5 (Fig 1.2).
High pH values (i.e. above 6.0) are to be avoided more than low values
(e.g. pH 4.5 to 5.0). The effect of low pH’s upon the stability of
nutrients is relatively insignificant.
The precise pH at which precipitation of macro-nutrients starts is
determined by the combined concentrations of calcium and sulfate. Except
for fertilizers ‘low’ in calcium and sulfate this problem commonly occurs
at ~pH 6.5 where the net* EC is ~2.5 mS, or ~pH 7.0 for 1.5 mS solutions.
Hence, to avoid precipitation, higher nutrient concentrations generally
must be held at lower pH values.
*Assumes make-up water has nil EC.
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
1.1 as proof.
Before following such advice you may wish to test for
yourself whether or not this problem will occur with your water and
nutrient mixture by performing the "Jar test".
