pH and Hydroponics
Most plants generally prefer a slight to mildly acidic nutrient solution with a pH between 6.5 and 5.5. In part this is because at in alkaline conditions, certain nutrients become less available to the plant. At a pH of 7.5 or higher, iron availability decreases, and Potassium (K) also becomes more stable and less available in alkaline environments. Nitrogen (N) has the best uptake around neutral (7), but the micronutrients manganese (Mn), iron (Fe), copper (Cu), zinc (Zn), and boron (B) are more available under slightly acidic conditions. Conditions with a pH of less than 5.5 can hinder beneficial microbial growth. In very acidic conditions (4.0-5.0) the manganese (Mn) and iron (Fe) can become so available that they become toxic to the plant since with nutrients, more is not always better.
In hydroponics, the pH range of 6.5 to 5.5 is best for allowing for the plants to take up the full range of nutrients.
pH and Soil
Many fertilizers will tend to decrease pH, so lime is often added to soil to “sweeten” or raise pH. Sulfur can be added to lower the pH to “sour” it, but so do many nutrients and organic materials, so it is often not needed. In general the processes are slower and gentler in organic gardening, and to some extent are self correcting, so pH is not as critical of an issue unless it is well out of the normal range.