Foliar Absorption of Urea, Ammonium, and Nitrate by Perennial Ryegrass Turf

The absorption and assimilation of 15 N-labeled urea, (NH4) 2 S O 4 , and KNO applied to the foliage of perennial ryegrass (Lolium perenne L.) turf were examined under a controlled environment. Each source of N was dissolved in deionized water to a final concentration of 25 g N/liter and spray-applied at a rate of 5 g N/m 2 . Absorption of the fertilizer-N over 48 hours, as measured by 15 N analysis of tissue digests, amounted to 35%, 39%, and 40% for the urea, (NH 4 ) 2 S O 4 , and KNO 3 , respectively. Absorption was also estimated by a washing procedure that measured the urea remaining on the foliage and by the increase in total N in the ryegrass tissue. There were no significant differences between the three methods for absorption of (NH 4 ) 2SO 4 and KNO 3 . The washing method, however, significantly overestimated absorption of urea. Partitioning of the absorbed N between tissues was similar at 48 hours for all three N sources, averaging 32% in new leaves, 52% in old leaves and shoot tissue, and 16% in the roots. Most of the absorbed urea- and NH 4 -N was assimilated by 48 hours, whereas only half of the NO 3 -N was reduced during that period.