GSEA y anotación

Sulphur coating of fertiliser was developed in the 1960s by the Tennessee Valley Authority, in this process ( Figure 1.1) the fertiliser, urea, is preheated to 80 oC and spray coated with molten sulphur heated to 148 oC in a rotary coating drum. The sulphur solidifies forming the initial coating. The coating , however, is prone to numerous flaws (fine cracks) produced by mechanical faults and transformation of the allotropic forms of sulphur during cooling and storage (McClellan and Scheib 1975). These faults are overcome by a secondary coating of 2% wax sealant and coal tar biocide applied to the hot product prior to cooling. On cooling the product may become tacky due to residual oil present in the wax, so a conditioner (2% diatomaceous earth) may be applied to prevent caking (Rindt et al. 1968).

4

Figure 1.1 Schematic of TVA pilot plant for sulphur coated urea, using twin

fluid sulphur spray. Redrawn from Rindt et al.,(1968).

The agronomic effectiveness of sulphur coated fertilisers is dependent on their release characteristics, the plant uptake rate and cropping duration. Urea release from sulphur coated urea (SCU) is the result of biological oxidation of the sulphur and sealant, which unblocks sealed pores and reduces the thickness of the coating structure until the

coating ruptures resulting in “catastrophic release” or diffusion release via pores. The rate of release is dependent on the granule coating thickness and distribution, which may vary from insufficient to excessive. Typically for a 20% S coated urea, one third of the urea releases immediately, a third over the crop growth period (90 to 150 days) and the remainder over a longer term (Shaviv 2001).The ideal coated product (9%S) produced by TVA using their pilot plant showed advantages over uncoated urea in terms of the dry matter yield of Bermuda grass in a 17 week green-house trial (Rindt et al. 1968). The result showed that a coated product with a release rate of 1% per day produced only 50% of the dry matter relative to uncoated urea in the first 3 weeks. However in the subsequent cuts over the following 14 weeks the cumulative dry matter yield increased to 120% of the uncoated urea yield (Figure 1.2).

Atomizing air 148oC Coal tar Perheating air 138oC Molten Sulphur 148oC Urea

Segmented coating drum Wax Conditioner Cooling air Final Product Screen Metering pump Preheater Coating Sealing Cooling drum Metering pumps

5 A similar yield pattern was observed in field trials of winter forage oat grown on a alkali (pH 8.4) sandy clay loam at the Indian Agricultural Research Institute, New Delhi, India (Joshi and Prasad 1977) during a mild winter (minimum monthly temperature 6.3-8.1oC). In these field trials treatments of 100 and 200 kgN ha-1 of uncoated urea and SCU with 16%S coating (supplied by TVA) were applied. The uncoated urea was applied in a split application, with 2/3 being drilled in at sowing followed by 1/3 after the first harvest, while all the SCU was drilled at sowing. During the mild winter there was no significant difference in dry matter yield at the first harvest 74 days after sowing, while at the second harvest, 80 days later, the SUC treatment produced a dry matter yield146% of the urea treatment. However, during the second cooler winter (minimum monthly temperature 4.9-5.9oC) the SCU treatment produced a significantly lower yield in the second harvest with a dry matter yield 56% of the urea treatment, which was attributed to the lower soil temperature.

SCU in New Zealand (NZ) has been evaluated in high country pasture (NZ) to improve the survival and vigour of ryegrass and clover following direct drilling of seed and SCU in weed infested soil (Pollock 1989; Pollock et al. 1994). In Australia, SCU was used as an annual N application (250 kgN ha-1) to annual ryegrass (Au) crops (Maschmedt and Cocks 1976). These trials showed that drilling SCU (25-75 kgN ha-1) with ryegrass seed increased seedling vigour, while a single application of SCU to annual ryegrass

increased herbage N recovery from 44% for urea to 78%. In contrast to these long term results, quick maturing vegetable crops such as potatoes, cantaloupes and tomatoes have shown significantly reduced yield with the application of SCU with 13.5% S coating in comparison to ammonium sulphate and urea (Lorenz et al. 1972). The low yield and N uptake that occurred in these cases was due to a large proportion of granules remaining intact for longer than the crop growing period (Raban 1994).

The studies reported by Rindt et al, (1968) and Joshi and Prasad (1977) showed, while SCU can be effectively produced with low sulphur coating (9%S), the fragile nature of the coating required commercial products to use higher coating levels (16%S) to allow bulk handling and storage. This increase in coating thickness results in a high

6 the cropping period, resulting in low yields and wastage of product. Sulphur coated urea with „long-tail‟ release characteristics has limited application for short term crops. The maximum benefit of SCU is obtained when it is applied in semi-permanent turf and long-term potting mixtures for ornamental plants allowing the full recovery of the applied N (Furuta et al. 1967; Maschmedt and Cocks 1976; Sharma et al. 1982; Sartain and Ingram 1984).

Figure 1.2 Comparative dry matter yields of Bumarda grass in glasshouse pot

trials grown with the addition of 160 kgNha-1of 9 %(■) and 15%

(▲)sulphur coated urea, urea(♦) and a blend of 40% urea and 60% SUC(x). Data from (Rindt et al. 1968)