- Sulfur-coated urea (SCU), a slow release nitrogen fertilizer,
may be economically competitive with conventional fertilizers, but
little is known of the mechanism of urea release from SCU and the
environmental factors which affect the urea release rate. Objectives
of this thesis were: (1) to determine factors which affect the rate of
nitrogen release from SCU; (2) to compare nitrogen utilization by
tall fescue (Festuca arundinacea Schreb. ), fertilized with urea or
The rate of urea release from SCU surface-applied to soil was
determined during incubation for periods of up to five months under a
range of temperatures (5 to 35 C) and soil water potentials (-5 to -30
bars). Three SCU fertilizers with different seven-day urea release
percentages and sealants were studied: SCU 4 (wax sealant); SCU 25
(polyethylene-oil sealant, 30:70 w/w); and SCU 23 (no sealant). Ina four-year field experiment (1972-1976), the dry matter and
nitrogen yields of irrigated tall fescue fall-fertilized with 200, 400,
600, or 800 kg N/ha/yr as urea, SCU 30, or SCU 4, were measured.
Also, urea was applied in five equal increments in the fall and at the
first four cuttings each year at annual rates of 980 kg N/ha (1972-
1973), 700 kg N/ha (1973-1974), and 800 kg N/ha (1974-1975). Cumulative
nitrate leaching losses from tall fescue plots were estimated by
analysis of soil solution samples.
Urea release from sealed SCU was slowed by soil sterilization.
The rate of urea release from SCU without sealant was unaffected by
soil sterlization. After a lag period of 25 days, wax-sealed SCU
applied to non-sterile soil at 25 C and -5 bars water potential released
urea at a rate of 1.0%/day. In sterile soil, after an initial release of
about 10%, no further release occurred. At -15 and -30 bars water
potential in non-sterile soil, release took place at 0. 5% /day, with a
lag period of up to 60 days. SCU sealed with polyethylene-oil released
urea at a slightly faster rate at -30 bars that at -5 and -15 bars. Urea
release from wax-sealed SCU was more sensitive to temperature than
was urea release from the other SCU materials. Following an initial
release of 10%, urea release rates from wax-sealed SCU at 35, 25,
15, and 5 C were 1. 5, 0. 71, 0, and 0%/day, respectively.
The urea release pattern from multi-granule samples of SCU 4,
23, and 25 was shown to correspond to the coating thickness distribution. The coating thickness distribution can be rapidly determined
and is a better indicator of the long-term urea release pattern
than the conventional seven-day release test.
In the field experiment, over a four-year period, tall fescue
fall-fertilized with SCU produced about 20% more dry matter than
fescue fall-fertilized with urea, e.g. , 61 mtons /ha versus 50mtons /ha
at the 800-kg N /ha/yr rate. Fall application of SCU and split application
of urea resulted in the same dry matter yield. Residual nitrogen
resulted in higher yields by SCU-fertilized tall fescue during the second,
third, and fourth years. Four -year nitrogen recoveries by tall
fescue receiving fall applications of 800 kg N/ha/hr as urea, SCU 30,
or SCU 4 were 43, 58, and 61% respectively.
Over a two-year period nitrate-nitrogen leaching losses from
plots fertilized with urea (400 kg N/ha/yr), SCU 4 (400 or 800 kg
N/ha/yr) or urea in a split application (average of 800 kg N/ha/yr)
were estimated at 65, 20, 19, and 126 kg N/ha, respectively.
Nitrate-nitrogen leaching losses were < 10% of the total nitrogen