As discussed earlier, if you have extremely high water salinity, you may have to fertilize at well below the sub optimal levels to avoid salt build up problems for your plants roots. Various elements are available at different levels depending on the pH of the solution they are dissolved in, with some becoming virtually unavailable if the pH goes too far out of range. We use a base line pH in the 6.6 range for our irrigation mix, and use calcium supplements on the mix of strongly calcicolous plants and those associated with ultrabasic rock. This is explained in more detail in Use of Calcium Based Supplements for Paphs. Be aware that most municipal water supplies have artificially raised pH s to reduce leaching of toxic substances into the water.
Rain water has both the lack of buffering and the possibility of very acid pH to start with.
Fertilizers differ in their alkalinity or acidity depending on composition. It is important to know the pH of your final irrigation mix, and make adjustments if necessary to get it into an appropriate range. It is also possible to use a limited amount of potassium silicate to raise your pH, follow label directions to avoid using too high a level. Do not try to raise the pH levels of fertilizer concentrates that contain calcium or magnesium and phosphorus, you may cause precipitates to form. The pH adjustment of these fertilizers must be made in the diluted form.