Growth Technology Buffer 4 & 7 - 300ml or 1 Litre With Essentials pH Meter Pack
300ml or 1 Litre of each. For hydroponics, this is often done at pH 7 with Buffer 7 and then at pH 4 with Buffer 4. Note that the optimum pH value for soil is different for hydroponics (usually pH 6.0 to pH 6.5).