Nitrogen Release Characteristics of Isobutylidene Diurea and its Effectiveness as a Source of N for Flue-Cured Tobacco

Leaching in coarse-textured soils contributes to inefficient N utilization by flue-cured tobacco (Nicotiana tabacum L.). Slow-release N materials reduce N leaching but those with rate of release dependent on biological degraduation have not been successful in furnishing the high proportion of total N required by tobacco during early growth. The objectives of this study were to investigate the release characteristics of isobutylidene diurea (IBDU), with N-release characteristics dependent primarily upon the effects of particle size on chemical dissolution rather than the effects of environmental variation on biological activity, in laboratory and greenhouse studies and to evaluate it as a source of N for flue-cured tobacco in the field. Urea, NaNO₃, and four particle sizes of IBDU (1.05, 0.75, 0.55, and 0.40 mm) were applied at rates of 200 and 400 mg N per kg of soil to a Norfolk sandy loam (Typic paleudult) in lysimeters and subjected to leaching at 10 intervals over a 109-day period. All applied N from NaNO₃ and 60% from urea were removed by the first leaching (0 days) at both rates of application. Only 5% of the applied N from IBDU was removed in the first leaching. At the 200 mg N application rate, 50 to 60% of applied N from the 1.05 to 025 mm IBDU particle sizes were removed after 10 leachings (109 days) but only four leachings (32 days) were required to remove equal amounts from the 0.40 mm particle size. At the 400 mg N application rate, 10 leachings were required for 55 to 60% removal of applied N from all particle sizes. The effects of particle size on the ability of IBDU to release N fast enough for maximum growth, but not so fast as to be subject to leaching, were evaluated in a greenhouse with two successive. crops of corn (Zea mays L.) grown under leached and non-leached conditions. IBDU particle sizes of 1.05, 0.55, and 0.40 mm were applied to Norfolk soil in pots at rates of 214, 321, and 428 mg per 5 kg of soil. Dry matter for the first crop was not affected by particle size, but it was greatest for the 1.05 mm IBDU in the second crop. Within a particle size, dry matter was not altered by lezching but total N uptake was greatest from 0.40 mm IBDU. This suggests that N release from 0.40 mm IBDU was rapid enough to support maximum growth but was not so rapid as to be subject to leaching under imposed conditions. In a field experiment on Norfolk soil, I BDU particle sizes of 1.05 and 0.40 nun were compared with NaNO₃ and urea as N sources for flue-cured tobacco. Materials were banded at 67 kg N/ha: (a) days after planting, (b) as a split application of a single source 12 and 26 days after planting, and (c) as a split application of one source at 12 days after planting and NaNO₃ at 26 days after planting. NaNO~ gave the highest yield of cured leaves when applied 12 days after planting, followed by urea, 0.4 mm IBDU, and 1.05 mm IBDU, respectively. Yields from both IBDU materials were improved when one-half of the total N was applied 2 weeks later as NaNO₃. Chemical composition of cured leaf varied little with treatment but urea gave higher levels of N and total alkaloids and lower sugars than the other materials. IBDU was effective in preventing loss of N by leaching in laboratory and greenhouse studies. As with other slow-release N sources, however, it was inferior to water-soluble N sources for flue-cured tobacco in the field.