Agriculture Reference
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properly encapsulate the urea particles to produce the controlled release of the fertilizer from
the bead matrix (these reasons have not been provided here). The lowest rate of urea released
from the matrix was found in the sample with the highest crosslinked density
(Tangboriboonrat and Sirichaiwat, 1996).
Fertilizers were entrapped in styrene-butadiene rubber as the polymer matrix for the
controlled release of fertilizers (Helaly et al., 1993). The rate of releasing fertilizers from the
capsule was controlled by the concentration of fertilizer, the temperature of the enviroment
and the pH of the medium. The urea released from the capsule was prolonged for two months.
After the addition of clay to the capsules, there was a decrease in the release rate of the
nitrogen. The effect of the filler on the apparent activation energy of the amount of
ammonium nitrate released into the water was also reported. After the urea was encapsulated
in the rubber matrix, the release of urea from the capsules increased with temperature. This
was due to diffusion, which allowed for a temperature-dependence of both the diffusion
coefficient in water and the saturated concentration of urea. The rubber matrix may prevent
both the high leaching losses and the seedling damage associated with a high concentration of
free urea. The effect of the size of the encapsulated urea bead on both their release
characteristics in moist soil and N-supply to plants was also reasonably well predicted by the
diffusion model. At the same time, the slow-release form of urea was apparently able to
supply nitrogen to plants over the span of a full growing season.
Figure 24. Photographic images of the urea capsule coated with different layers of modified PVA.
3.4. Encapsulated FertiliserFertilizer with the Modified PVA
The urea released from the beads was subjected to a number of physical and chemical
parameters including those related directly to the release from the beads into the water
medium, the release conditions (temperature) and those resulting from changes to the
characteristics of the controlled release device (beads). The photographic images of the urea
capsule coated with the modified PVA (DPVA) at different layers are provided in Figure 24.
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