hi all, this study was posted elsewhere on the site but the thread sank without trace - i thought it was particularly relevant to Organic growers and wanted to see what you all thought.
http://www.usu.edu/cpl/PDF/Optimization_of_SoillessMedia_High_pH_Water_Sources.pdf
also worth bearing in mind that many organic P sources - such as rock phosphate and bonemeal - are also liming agents in their own right and may exacerbate the problem too.
i use citric acid to reduce the pH of my alkaline tapwater - and my plants seem to do great on it - but recently i wondered if this affect was temporary or permanent - afaik it depends on the type of alkalinity.
lastly, remember that pH and alkalinity are different.
http://www.usu.edu/cpl/PDF/Optimization_of_SoillessMedia_High_pH_Water_Sources.pdf
obviously the addition of compost, EWC and possibly topsoil would mitigate the effects of watering with alkaline water, but given the pretty fast and dramatic effects shown in the study, i doubt that it would negate the effects completely.High root zone pH reduces nutrient availability and high alkalinity water is strongly buffered around an alkaline pH. Soilless media can be altered to improve nutrient availability. This study was conducted to optimize the composition of soilless media for use with high alkalinity water. Mixes of peat and/or perlite or vermiculite in 50/50 and 33/33/33 volumetric ratios were tested. In some studies, mixes were also amended with up to 2.4 g/L of dolomite limestone to neutralize the initial acidity of the peat. Mixes containing vermiculite settled more, had higher water holding capacity (WHC) and percent plant available water (%PAW), and similar air filled porosity (AFP), compared to mixes containing perlite. Dry mass was measured in corn, peas, tomatoes, and soybeans, and chlorophyll content was measured in corn. The addition of dolomite increased pH and decreased dry mass in corn, soybean, and tomato, but peas were unaffected. Chlorophyll content in corn also declined with increased amounts of dolomite. After a week of daily irrigation, pH 7.8 nutrient solution neutralized the acidity of the peat, without the need for addition of dolomite. Mixes containing vermiculite improved growth and chlorophyll concentration compared to mixes with perlite. The higher cation exchange capacity (CEC) of vermiculite-containing mixes may have improved nutrient availability. A soilless mix of only peat and vermiculite, in approximately equal volumes, resulted in the greatest growth and chlorophyll content when watered with high alkalinity nutrient solution.
also worth bearing in mind that many organic P sources - such as rock phosphate and bonemeal - are also liming agents in their own right and may exacerbate the problem too.
i use citric acid to reduce the pH of my alkaline tapwater - and my plants seem to do great on it - but recently i wondered if this affect was temporary or permanent - afaik it depends on the type of alkalinity.
lastly, remember that pH and alkalinity are different.
VGpH is a logarithmic measure (that is, every 1 unit change is a factor of 10) of the concentration of hydrogen ion and that determines the acidity of the solution (acidic if pH < 7). In the opposite direction away from neutrality, at pH > 7, the solution is basic aka alkaline which would be the case for your solution with soda ash. TA is a measure of any chemical species that can accept hydrogen ions so if the TA is high such species combine with the hydrogen that is added from acids such that the effect on pH is far less. Soda ash not only increases pH, but it also increases TA a lot as well. This basically means that though the pH may be measured as high from the Soda Ash, you can't tell how much acid you need to add just from the pH -- you need to know the TA or more easily use the acid demand test as indicated above.