The concentration of
nutrients in water is measured on a numerical scale of parts per million (PPM). This number basically tells you how much fertilizer is dissolved in your water. By closely monitoring this concentration you can maintain the best level of nutrients to meet your crop's requirements. You will need a TDS (total dissolved solids) meter that measures PPM. You also use the this meter to gauge when it's time to fill your
hydroponic system with a fresh mix. Generally, young plants prefer concentrations around 400 to 600 PPM, while mid-growth would be around 700 to 1000, and established or fast-growing plants need levels around 1100 to 1500.