Size Guide Number and Rosin–Rammler Approaches to describe Particle Size Distribution of Granular Organic-based Fertilisers

The physical properties of organic-based fertilisers are not well known. The main objective of this study was to evaluate the range of granule size distributions of organic-based fertilisers and compare five approaches used to describe their size distribution: (1) the size guide number NSG and uniformity index; (2) the size range variation coefficient; (3) the uniformity coefficient; (4) the geometric standard deviation of the particle size diameter; and (5) the Rosin–Rammler function. Six mineral (MF), four bulk-blended organic-based (BOF), and ten compound organic-based fertilisers (COF) were studied. Standard NSG boxes and Tyler sieves were used to measure their granule size distribution. The size guide number ranged from 200 to 246 for MF, from 204 to 216 for BOF and from 99 to 255 for COF. The uniformity index was higher for slow release MF and for COF with NSG>210. All five distribution parameters separated the fertilisers into similar groups. The coefficient of uniformity was the least discriminating approach in grouping the fertilisers, while the geometric standard deviation and size range variation provided similar results and the Rosin–Rammler approach was the most discriminating. The NSG and uniformity index were estimated with the Rosin–Rammler function. Comparisons between estimated and measured size guide numbers indicated that the Rosin–Rammler function performs better for MF and for spherical granule shapes. Differences between the uniformity index values were greater for BOF, COF, and for cylindrical-shaped granules. The Rosin–Rammler function was not accurate for homogeneous and binomial particle size distributions, or for cylindrical granule shapes. However, in all other cases, this function was more precise and presented many advantages over the other methods, including the fact that it covers the entire range of particle sizes and can be used to estimate other parameters.