In soils or growing media, pH strongly influences the availability of nutrients and the presence of micro-organisms and plants in the soil. Certain plants require a particular pH range to enable the required nutrients to be consistently available to the plant.
If the solution is too acidic or too alkaline it can cause "lock up" - a situation which restricts certain elements essential for growth from being absorbed by the root structure. This in turn reduces plant heath and performance. Deficiencies in the required elements become apparent in plant growth and can lead to crop failure.