The pH value provides a measure of acidity and alkalinity of an aqueous solution and it is proportional to the concentration of hydrogen ions. A pH neutral (=7), water is formed by ion OH- and H+ in equal measures. By adding a solution to a neutral or acid is a basic cause of an increase in the concentration of H + ions or OH- respectively. When the value is below pH 7 we are describing the acidity of the solution, but beyond(or above ph7)we are describing the alkalinity . Strong acids have pH values ranging from 1 to 3, weak acids between 4 to 6 pH, while weak bases from 8 to 10 pH and finally strong bases from 11 to 13 pH.The easiest and cheapest method for measurement of pH is by one type of colour metric: comparing the colour obtained from the chemical reaction pH liquid test mixed with the fertilized solution to a metric shade, which then gives the corresponding values concentration. In hydroponic crops it is recommended to maintain the value between 5.5 and 6.8. Values outside of this range are preventing the proper absorption of macro factors including (N-P-K etc.).see ph meters, combo ph meters and monitor.