Although it is perhaps one of the most overlooked aspects, pH is very important in hydroponic and organic as well as regular soil gardening. pH is measured on a scale of 1-14 with 7 being neutral. Acids are lower than 7 and alkalis (bases) are above 7.
This article deals with the pH of hydroponic gardening and the availability of nutrients at different pH levels in a soilless growing medium. Organic and soil gardening have different levels, so the following chart doesn’t pertain to them.