Solid‐phase organization and shrinkage properties of some growing media constituents according to particle size

Particle size is a key factor in shaping water and air retention properties and drainage capacity of growing media. Thus, manufactured growing media are made of screened, crushed, or sieved raw materials whose particle sizes are adapted to cropping objectives. The relationships between the particle size distribution of the growing media constituents and the resulting structure are, however, not well known, which requires better understanding of particle arrangement and its change with water upon shrinkage. A proper characterization of the structure would help to guide substrate manufacturing, which is inherently complex due to the use of various materials made up of heterogeneous particles in terms of size and shape. To this aim, we analyzed the shrinkage of white and black peats, coir, pine bark and wood fiber, raw material, and derived particle size fractions extracted by sieving. Hyprop systems coupled to linear vertical displacement transducers were used to determine the shrinkage curves. The dual porosity shrinkage XP model (XP model) was used to analyze the hydrostructural behavior of the different growing media constituents. The possible distinction of interparticle and intraparticle pores, based on the dual pore system assumption of the shrinkage model, was discussed. Interparticle porosity volume represented the major part of the total porosity, whatever the materials and particle size fractions. Greater volume shrinkage of interparticle porosity was observed for the smaller particle size fractions of materials. Conversely, intraparticle porosity volume shrinkage is of the same magnitude for all particle size fractions. The use of the XP model to study growing media is relevant, although no residual domain on the shrinkage curves was observed. This work revealed that particle arrangement and physical behaviors during drying of materials depend on the nature of constituents but also highly on particle size fraction. These results provide a complementary approach for characterizing the pore functional properties of growing media.