Fatman
wouldnt one expect the runoff PH to be higher if the input ppm was 650 PH5.3 and the runoff came out at ppm400 ph 5.8. just the lowering of the total ppm should cause the ph to rise. am i wrong on this.
The pH is a reflection of what is up taken by the roots and what balancing ions are released by the roots plus any effect caused by dying roots adding acids and also carbonates neutralizing acids. Only acids can neutralize hydroxides released by roots. There is no common acid equivalent of a buffer such as carbonates.
The fertilizer that is normally added to neutralize the pH rising effect is ammonical nitrogen. It just so happens that when conditions are such that the plants is up taking fertilizers that cause the roots to release hydroxides that the plant will uptake ammonical nitrogen instead of nitrates if they are present. In doing so it tends to balance the roots ions levels so that ions are not released in abundance in either direction H+ or OH-. If however you have too much ammonical nitrogen taken up your roots will put out more H+ ions causing a low pH. The denser the buds or the bushier the plants the more ammonical nitrogen that can be taken up if it is present. Combine that with the better the roots systems the better and larger the take up of phosphorus.
So with typical formulas there is not only a shortage of phosphorus once the plant bushes up, buds thicken and the root mass gets huge the higher demand for ammonical nitrogen to balance out the amounts of phosphorus that is taken up. Nearly all phosphoros in hydroponic fertilizers comes from mono potassium phosphate. Each phosphorus has tow hydroxides attached to it. ie thoase are hydroxides are dumped by the roots back into the water if there is not enough H+ taken into create a balance.
If a water analysis was run on the waste water I imagine you would find the chief nutrients being taken up at this time are phosphorus and any ammonical nitrogen available. The present uptake o phsophori us is fairly klarge as the phophorus in Lucas Formulas is 10%. However the ammonical nitrogen amount is small at 0.30% of the total formula analysis. While the percentage of ammonical nitrogen to nitrate may not seem small, you must also consider that the percentage of phosphorus to nitrgen is 1:2 If more ammonical nitrogen was available it would be taken up and your pH would drop. If both phosphorus and ammonical nitrogen was added the ppm output would be proportionally smaller as both would be taken up readily. Your pH would then depend upon your ration of phosphorus to ammonical nitrogen. That is if your carbonate levels are not high and therefore neutralizing the H+ released by the roots as a result of the ammonical nitrogen up take.
I really did try to make the answer simple.
Most commercial hydroponic nutrient formulas have ammonical nitrogen levels at minimal amounts (3% to 6% of total nitrogen or even lower). Even HW micro by GH is only about 8 percent. With high pressure aero or atomized aero with large root masses of hair roots I would mix formulas containing 15% ammonical nitrogen. That is the upper maximum level recommended for lesser efficient systems so as to not have large daily drops in pH.
Ammonium nitrate should be readily available at any garden shop in small boxes. (It can be used to make gun powder though so maybe not in Europe, in the US it just requires a permit for bulk loads) Just add a very small amount of it to your nutrients and your waste water pH will drop. However your ppm uptake will likely not increase anymore than the amount of ppm added by the ammonium nitrate.
A simple way to look at it is like this ammonical nitrogen is NH 4+ That 4+ means four hydrogen ions. ie acid Nitrate however as is typically present in calcium nitrate becomes 2HNO3 ie only 2 hydrogen and they combine with oxygen and become H2O (water) ie CaCO3 + 2 HNO3 → Ca(NO3)2 + CO2 + H2O so there is very little effect on the pH from nitrate uptake.
It is really sad that the administrators do not set the forum up to allow sub text and super text.
