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

Abstract

The absorption and assimilation of 15N-labeled urea, (NH4)2 S O4, 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/m2. Absorption of the fertilizer-N over 48 hours, as measured by 15N analysis of tissue digests, amounted to 35%, 39%, and 40% for the urea, (NH4)2 S O4, and KNO3, 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 (NH4) 2SO4 and KNO3. 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 NH4 -N was assimilated by 48 hours, whereas only half of the NO3 -N was reduced during that period.