C
Carl Carlson
There are other versions of this chart and I see them as a source of confusion for some folks.
Whether you are growing in soil, coco, perlite or a whole host of other mediums (other than water itself), this chart never applies to the water + fertilizer that you are feeding to the plants. It applies to the pH of the medium surrounding the roots. If you are trying to correct a single nutrient deficiency, you won't do it by simply altering the pH of the water + fertilizer that is fed to the plants.
If I may quote from one the best researches and publications on this subject:
pH Management and Plant Nutrition, Bill Argo
Part 2 Water Quality
When it comes to managing the pH of a substrate, the alkalinity concentration has a much greater effect than does water pH. Alkalinity (calcium bicarbonate, magnesium bicarbonate, and sodium bicarbonate) and limestone (calcium and magnesium carbonate) react very similarly when added to a substrate. And just like too much limestone, the use of irrigation water containing high levels of alkalinity can cause the pH of the substrate to increase above acceptable levels for healthy plant growth.
[..]
Alkalinity is a measure of how much acid it takes to lower the pH below a certain level, also called \ acid-buffering capacity. Alkalinity is usually measured with a test kit where dilute acid is added until a color change occurs at a specific pH. Alkalinity is not a specific ion, but rather includes the concentration of several ions that affect acid-buffering capacity. Under most conditions, the ions that have the greatest effect on alkalinity are bicarbonates like calcium, magnesium, or sodium bicarbonate and, to a lesser extent, carbonates like calcium or sodium. Several other ions including hydroxides, phosphates, ammonium, silicates, sulfides, borates, and arsenate also can contribute to alkalinity, but their concentration is usually so low that they can be ignored.
So before you start growing or if you already are, take a trip to the pet store or the pond supply and pickup an inexpensive multi test kit for water that includes alkalinity. [..]
Alkalinity is a measure of how much acid it takes to lower the pH below a certain level, also called \ acid-buffering capacity. Alkalinity is usually measured with a test kit where dilute acid is added until a color change occurs at a specific pH. Alkalinity is not a specific ion, but rather includes the concentration of several ions that affect acid-buffering capacity. Under most conditions, the ions that have the greatest effect on alkalinity are bicarbonates like calcium, magnesium, or sodium bicarbonate and, to a lesser extent, carbonates like calcium or sodium. Several other ions including hydroxides, phosphates, ammonium, silicates, sulfides, borates, and arsenate also can contribute to alkalinity, but their concentration is usually so low that they can be ignored.
Between 40 and 120 ppm of Total Alkalinity is generally recommended to maintain a stable medium pH over time. Lower TA tends to lead to low pH problems and high TA leads can lead to high pH problems.
The tendency of a fertilizer to change medium pH depends on it on the form of Nitrogen being used as well. Ammoniacal nitrogen can cause the medium pH to go down. Nitrate nitrogen can increase the medium pH. Urea nitrogen has to be converted to ammoniacal before it's used by the plants. Take a look at the label on your bottle or sack of fertilizer and you'll most likely find a very low percentage of ammoniacal and urea compared to nitrate. That's because these fertilizers are designed to be used with good water. So stop fucking around and test that water already!
The rest of Argo's series, including the section that goes into fertilizer and medium pH, is hosted here: http://www.staugorchidsociety.org/culturewater.htm
Water pH alone has very little impact on coco grow pH
