In the recirculating hydroponics, pH of nutrient solution was gradually decreased until the middle of growing-period, and then began to go up and reached the initial level at the latter of growing-period regardless of Cu·Zn content. Changes of pH was gently down for supply of humus in nutrient solution. The growth of lettuce was good with supply of humus, but it was decreased as Cu·Zn content was increased. Although leaf chlorosis was caused by excessive content of Cu·Zn as 5, 10 ppm, it was controlled by adding humus in nutrient solution, and fresh weight of lettuce was obviously improved by adding humus.