Response of Irish potatoes to phosphorus and potassium on soils having different levels of these nutrients in Maine and North Carolina

Field experiments were conducted in Maine and in North Carolina to determine the effects of rates of phosphorus and potash applied to Irish potatoes grown on soils having a wide range in readily soluble phosphorus and exchangeable potassium. An average of the 1945 and 1946 yield data from North Carolina shows that applied phosphorus gave significant increases in yield at all six locations. Significant increases in yield were obtained on eight of the nine experiments in Maine. The degree of yield response to applications of P2O5 is related to the amount of readily soluble phosphorus in the soil. The increase in number of pounds of potatoes resulting from the first 80 pounds of P2O5 applied decreases as the amount of readily soluble phosphorus in the soil increases. Data from North Carolina reveal that the phosphorus content of the leaves is related to the amount of readily soluble phosphorus in the soil and to the amount of phosphorus applied. In the experiments in Maine the phosphorus content of the rachises sampled during the early bud stage was related to the amount of phosphorus applied. It was revealed that phosphorus was particularly important in influencing the number of tubers per hill on soils low in readily soluble phosphorus. Significant increases in yield from applied potash were obtained in all experiments in North Carolina and in five out of eight experiments in Maine. The number of pounds of potatoes resulting from the first 60 pounds of K2O applied tends to decrease as the amount of exchangeable potash in the soil increases. Data from North Carolina reveal that the potash content of the leaves is related to the amount of potash in the soil and to applied potash up to 120 pounds of K2O per acre. Similarly, in the Maine experiments, the amount of potassium extracted from rachises of the potato plant is related to the exchangeable potash content of the soil and to the amount of potash applied. The results of the potash tests in Maine indicate that considerably more potash is applied by most growers than is necessary for maximum yields. On the other hand, the results obtained in North Carolina indicate that increased yields may be obtained with application of potash greater than the 120 pounds of K2O per acre normally applied to potatoes in that state.