Available Soil Calcium in Relation to "Damping Off" of Soy Bean Seedlings

The damping off disease has usually been associated with unfavorable temperature, moisture, and light conditions, but these experiments show that, in the case of soy bean seedlings, the disease is also closely associated with the nature of soil colloids, particularly with the nature of the adsorbed ions (calcium). 2. Hydrogen ions are not an important factor in determining the disease. Within a range of pH 3.8-6.94 it was found that heavy damping off occurred when the calcium supply was low, but none occurred when the calcium supply was high. 3. Calcium is an important factor in the control of the disease. With increasing calcium concentration the number of diseased plants decreased rapidly, both at pH 7 and at pH 4.4. At high calcium concentration no damping off occurred between pH 3.8 and 8.0. 4. In preventing the disease, calcium ions are superior to other monovalent or divalent ions at equal concentrations. For the chlorides, the importance of cations in checking the disease decreases according to the series Ca > Mg > K. 5. Availability of calcium is an important feature in controlling damping off. Free, diffusable calcium ions were more effective than adsorbed, exchangeable calcium ions, while calcium bound by the forces of the crystal lattice in the mineral anorthite was not able to prevent the disease.