NITROGEN

Two distinctive leaf changes characterize this deficiency:

(1) When plants first become nitrogen-deficient, an overall yellowing of the leaves occurs. Prior to this time, these plants may have been high in nitrogen, and they may have grown vigorously to form large leaves of a good, green color. With nitrate depletion, however, their leaves become light green, turning to yellow. Yellowing continues as the plant ages, accompanied by wilting and an accelerated death rate of older leaves.

(2) While these changes are taking place in larger leaves, other symptoms are often overlooked. Newly formed leaves in the center of the plant become much smaller, lanceolate in shape, and the color becomes an intense green. Leaves often lie nearly parallel to the soil surface with petioles curved slightly upward. Actual size of the center leaves depends upon the rate at which nitrate is formed in the soil and absorbed by the plant. If this rate is slow, the leaves will be small, and the plant may develop a "pineapple" crown. When the rate of nitrate supply is fairly rapid, yet still insufficient in the petioles, the new leaves will be larger -perhaps almost large enough to cover the soil surface between the rows and make the field appear uniformly green. Only a close inspection of the plants and the diphenylamine test can reveal a true nitrogen deficiency.

Some knowledge of nitrate distribution in a sugar beet plant is helpful when interpreting the results of the diphenylamine test. After nitrate is absorbed by the plant, it moves to the petioles of the young leaves. From here, the nitrate moves to the corresponding blades or remains "trapped" in the petioles as the leaves become older. Usually, the petioles contain three to six times as much nitrate as do the blades, until the leaves become deficient in nitrogen. At this time, both the petioles and blades show a negative test for nitrate. Even though the older leaves on the same plant are high in nitrate, it fails to move out fast enough, causing a deficiency to occur in the young leaves. However, when new nitrate becomes available, it moves directly to the young leaves where rapid growth resumes.

The presence of nitrate in the storage root, as shown either by the diphenylamine test on cut surfaces of the storage root or in the pulp at the time of sugar analysis at the tare laboratory, indicates that nitrogen has not been sufficiently depleted to allow for rapid accumulation of sugar in the storage root. Delay in harvesting, whenever feasible, can result in a higher sugar content for the current crop.