Manganese and Boron Toxicities in Standard Culture Solutions

Recommended concentrations of manganese and boron in Hoagland culture solutions have been found to be toxic to barley, slightly toxic to lettuce and non‐toxic to tomato plants. Barley plants grown in standard Hoagland solutions developed dark brown spots on the older leaves. These spots varied from small spots to large blotchy areas. A decrease in yield and an increase in spotting were found to be associated with an increase in the manganese and boron coucentrations of the solutions. A correlation was observed between the manganese content of the leaf and the appearance of manganese toxicity symptoms. Decreasing the manganese concentration, increasing the macro‐salt concentration or adding sodium silicate to the culture solution repressed or prevented the appearance of manganese spots. Calcium, magnesium and potassium salts were found to be antagonistic to manganese, reducing the manganese content of the leaf tissue when supplementing the salts already present in the solutions. Silicon was found to repress manganese toxicity symptoms but had no effect upon the manganese content of the leaf tissue. Reducing the boron concentration or increasing the macro‐salt concentration eliminated the boron type spotting while the addition of silicon resulted in only a slight decrease in the necrosis due to boron. Barley plants grown in winter tolerated much higher manganese and boron concentrations before showing toxicity symptoms than plants grown in the summer. Manganese and boron toxicity and manganese deficiency symptoms are compared in a photograph.