Effect of pH on plant assimilation of elements, varies by soil type (and/or if soilless).

Hydroponic nutrient solutions are NOT organic – repeat NOT and never can/will be – so the above chart applies more in hydroponics (and likely DWC) than does the ‘organic soil’ chart below.

An iAVs biofilter prior to the inclusion of detritus/organics would be considered as mineral.  With the addition of organic solids (detritus) and with a developed soil microbial community, the biofilter becomes (is) ‘organic’ – aka includes Carbon-based biomolecules – (except for the ammoniacal Nitrogen fraction (TAN)).   However, TAN is not the only N source in iAVs.  Nitrogen is also sourced from amines, amino and nucleic acids (proteins), chlorophyl, peptides, some enzymes (hydrolases), ureides, et al. – each (with rare exception) being made plant available via microbial transformations (often  in a sequence involving multiple microbe species, as too with most other elements when sourced from complex organic biomolecules).

IMO, so-called ‘aquaponicists’ need to loose their apparent ‘fixation’ on (obsession with) Nitrification (‘seriously’) and start to include active consideration of the availability of ALL the plant essential elements.

The recommended pH for iAVs (water) is 6.4 +/- 0.4.    pH will not only influence nutrient availability for the plants but also influences microbial metabolism (activity, efficiency and even community/species composition).  Nitrification is most efficient at pH 7.0.  In iAVs, there is such a vast surface area available for biofilms that more than adequate nitrification occurs at even as low as pH 5.0 (at least over intervals of several weeks).  The toxicity of True Free Ammonia for fresh water fish rises above pH 7.0 and goes exponentially more toxic above pH 8.0.  In other words, TAN is far more toxic at higher pH and than at lower pH.

NOTE:  Calcium toxicity rarely occurs. HOWEVER, high levels of Calcium can compete with (inhibit) both Magnesium and Potassium (and possibly Phosphorus) uptake by plants, resulting in assimilation deficiencies of Mg and K regardless of how much of these elements are in the media/solution.  In animals (vertebrates), excess Ca is antagonistic with Phosphorus metabolism.

Additional Note:  I’ve seen several submitted sand composition analysis recently indicating a percentage (1 to 2% by volume range ) of Calcium oxide (CaO).  This compound is highly water soluble and extremely basic (WILL strongly raise pH in a proverbial heart-beat).  I suggest avoiding media that includes CaO as well as similarly significant levels of Calcium carbonate (CaCO3) as well as Ca(OH)2 and/or  CaO2.

I can’t stress strongly enough the importance of pH on biofilter performance and on plant growth (yield).  If your opinion is of a contrary nature,  please keep it to yourself.

Charts sourced from “Knott’s Handbook for Vegetable Growers, 5th Edition”, an Industry Standard reference for many decades.

PDF version  KnottsHandbook2012

Hardcopy version available on Amazon https://www.amazon.com/Knotts-Handbook-Vegetable-Growers-Maynard/dp/047173828X/ref

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