Variation in the physical properties of organo-mineral fertilisers with proportion of solid pig slurry compost

The use of organo-mineral fertilisers (OMFs) in agriculture is expected to increase in large part due to their advantages over mineral fertilisers and organic additives. However, the physical properties of pig slurry-based OMF required for storage, handling, and mechanical blending are still unknown. The objectives of this study were to determine the OMF physical properties in relation to 1) increasing the proportion of pig slurry compost and 2) changing the source of compost in the mixture. Twenty four mixtures of differing proportion of compost, monoammonium phosphate (MAP), and diammonium phosphate (DAP) were granulated. Increasing the compost proportion affected most OMF physical properties by increasing granule porosity, water content, and hygroscopicity and by decreasing the mean granule weight diameter, granule density, and crushing strength. Low-iron composts weakened the physical resistance of OMF in relation to storage whereas differences in OM content were not major factors affecting OMF physical properties. Increasing compost proportion increased water sorption from porous media, which might be beneficial for OMF field application. It is envisaged that this study could help commercial companies develop their own optimum OMF mixtures. However, determining the optimum compost proportion in OMF may depend on many other factors such as mineral fertiliser prices, compost availability, and crop response.