Response of Potatoes to Nitrogen Form and to Change in Nitrogen Form at Tuber Initiation

A controlled environment study was conducted to determine if ‘Russet Burbank’ potatoes ( Solanum tuberosum L.) were more tolerant of NH 4 -N as established plants than as young plants. When grown on one N source for 86 days, root and shoot growth were best with NO 3 -N, intermediate with NH 4 + NO 3 , and least with NH 4 -N. Changing the N source from NO 3 -N or NH 4 + NO 3 to NH 4 -N at tuber initiation (58 days after planting) reduced both shoot and root growth. Conversely, changing the N source from NH 4 -N to NH 4 + NO 3 improved growth. The highest tuber weights were obtained with continuous NH 4 + NO 3 or when NH 4 -N was applied at tuber initiation to plants previously grown on NO 3 -N alone. Nitrogen source also influenced absorption of other nutrients. Changing the N source once plants were established, however, did not have a consistent effect on mineral composition. We conclude that when NH 4 -N is the sole form of N available to the plant, it is detrimental to potato growth regardless of stage of development.