The acidity of your substrate and watermix is very important for plants as it determines in large part the absorption rates of nutrients and trace elements. The value that is used to indicate the degree of acidity is called pH and this stands for the concentration of ions in the water. The lower the pH level, the more acidic the water or substrate is.
For most plants, the optimal pH varies on the plants in question, but a value between 5.2 and 6.2 works for most potted plants. The quality of the irrigation water and a stable pH level is therefore very important for plant growth.
When the pH value varies, the uptake of nutrients varies, which can be seen in an irregular growth of the plants.