pH affects the palnt growth because it affects the availability of nutrients to the plants. If the pH is higher than 7.5 then ions such as iron, Manganese, Copper, Boron and zinc become less available. When there is a deficiency then the plant’s growth will slow down. It also affects the absorption of nutrients by the plants because when the pH is lower then 6, Phosphoric acid, calcium and mangnesium become less soluble in water and it becomes difficult for the plant to absorb them. When the pH is between 3 and 5 and the temperature above 26 degrees centigrade then fungal diseases can develop that will affect plant growth.
It is richer in the nutrients the plants need for growth,anything less would not be healthy for plants and the plants would not grow they would be stunted and would not produce a good and healthy crop....good luck