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Polymer coated controlled release agrichemicals as mitigation tools in pastoral farming : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University, Palmerston North, New Zealand
Controlled release coating technology and nitrification inhibitors offer potential mitigation options, for the reduction of pastoral nitrate leaching. Previous published research on this topic was reviewed indicating two potential areas of new research and development around two main hypotheses:
• That polymer coated urea can allow high urea N applications in winter reducing application costs, nitrate leaching, herbage N content and urine N return to pasture.
• That polymer coated nitrification inhibitor dicyandiamide (DCD) can increase the longevity of DCD in soil and effectively inhibit nitrification of dairy urine affected soils.
To facilitate this research a range of coated urea and nitrification inhibitor dicyandiamide (DCD) products were produced using low cost, reactive layer, polyurethane (RLP) and were assessed in laboratory and field studies.
The mechanism of urea release from modified RLP coated urea was investigated, leading to the development of a comprehensive model of release, based on the porous water repellent nature of the RLP coating. The “hydraulic convection” model was validated using water extraction and under field conditions for modified RLP coated urea.
In, field trials (June-Nov 2007) using Italian ryegrass, a single application of 150 kgN ha-1 of palmitic acid modified RLP coated urea (5UCU) reduced winter nitrate leaching by 7 kgN ha-1 compared to uncoated urea and reduced peak herbage N levels over this period (150 days). Using an empirical N partitioning model for grazing cows, the reduction in herbage N was predicted to reduce urine N return by 5 to 10 kgN ha-1 over the 150 day trial.
The effectiveness of laboratory prepared controlled release nitrification inhibitor dicyandiamide (PDCD) was tested as a surface application in repacked core studies on two soils contrasting in organic matter content and anion sorption capacity, Manawatu fine sandy silt and Dannevirke silt loam. The data from this trial was used to develop a model to explain DCD movement and degradation soils, which predicted that PDCD can potentially increase DCD longevity by 120 days at 20 oC over uncoated DCD.