A magnesium deficiency induced by previous fertilizer treatments

In recent years potatoes growing on certain plats of the Rhode Island Agricultural Experiment Station have been found to be chlorotic and to make very poor growth. This correlation suggested a deficiency in some essential element. A small application of magnesium sulfate to some of the plants partially corrected the disorder. The investigations reported in this paper cover both field and greenhouse experiments designated to determine if a lack of magnesium in the soil was the chief cause of the disorder; and, if so, whether any element in the previous fertilization helped bring about the deficiency. With a fertilizer high in potash and on an acid soil, the yield was much reduced from that obtained with the standard fertilization. When magnesium sulfate was applied to the high-potash soil in 1931 and 1932, a significant increase in yield was obtained. However, supplementing the normal fertilization with magnesium sulfate gave a much larger relative increase. In 1933, a significant response was also obtained from the magnesium sulfate, but the depression in yield observed the two previous years with the high-potash soil did not occur. When lime is used the best yields of potatoes are usually obtained on those plats with a relatively high potash fertilizer. In experiments where different magnesium sources were used all were found effective when supplying a minimum of 20 pounds of MgO per acre. Larger quantities than this did not give further increases in yield. The greenhouse tests with a high potash soil showed a significant response to magnesium sulfate. While supplying magnesium was beneficial it did not give as large an increase with the high-potash fertilizer as was obtained from the low-potash and magnesium treatment. It was found that magnesium was the deficient element and that a high potash supply in the soil under acid conditions facilitates a magnesium deficiency. Probably, the cause of the magnesium deficiency is that the fertilizers applied have been relatively free from magnesium and that there has been a continual loss of this element by crop removal and leaching. Further, it is probable that the high concentration of potassium has largely displaced the available magnesium of the soil and that the excess concentration of potassium has interfered with magnesium absorption. For control of the disorder, under our soil conditions, it is recommended that magnesium limestone supplying 25 to 35 pounds of MgO per acre be applied.