Determining the particle size range of organomineral fertilisers based on the spreading characteristics of the material

A. The aim of this work was to determine the particle size range of two organomineral fertilisers (OMF) that may enable broadcast application with standard fertiliser spreading equipment using conventional tramlines spacing. A theoretical model was developed which predicts the trajectory of individual fertiliser particles off-the-disc. The drag coefficient was estimated for small time steps (10-6 s) in the trajectory of the particle as a function of the Reynolds number. For the range of initial velocities (from 20 to 40 m s-1) and particle densities (from 1250 to 1500kg m-3) investigated, the model showed that the particle size range for OMF should be between 1.10 and 5.50 mm in diameter. Given the assumptions made in the analysis, this size range is expected to match, approximately, the minimum and maximum landing distances of individual particles of urea (size range: 1.00 to 5.00 mm). It was suggested that OMF should have about 80% of the particles in the range of 2.25 to 4.40mm in diameter. Due to the characteristics of the materials, spreading OMF with spinning discs applicators may be restricted to tramlines spaced at a maximum of 18m apart; especially, when some degree of overlapping is required between two adjacent bouts. The model presented herein requires validation with data obtained from spreading tests.