For plants to be able to grow best it is important that the nutrient pH
be slightly acidic – the range of 5.5 to 6.5 is considered ideal for
most crops. Even when using quality hydroponic nutrients, the pH may
fall out of range when poor quality water is used to mix nutrient
solution, when pH unstable media is used such as many rockwool
and gravel products and when plant growth rates are so high that the
nutrient mix is thrown off due to rapid nutrient absorption by the crop.