For non-rigid porous materials like many soils and growing media, the ability to shrink during drying should be considered as it modifies the total pore volume and pore size distribution, which leads to changes in water holding capacity and air properties. Many studies have shown that the physical properties of growing media depend in part on their particle size. However, no study has been focused on the relationships between shrinkage and particle size for growing media to date. A milled peat (H3 – H6) was fractionated by wet sieving into six particle size fractions. Shrinkage curves using Hyprop systems coupled to a linear variable displacement transducer (LVDT) were performed to simultaneously measure water content, water potential, and total volume during the drying process. The results were expressed in void and water ratios as a function of water potential. The shrinkage curves suggested two types of physical behavior, differentiated by the presence or not of an initial shrinkage in the range of higher water potentials (from saturation to a maximum of -100 hPa). The initial shrinkage only concerned the finest particle size fractions and was characterized by little to no air entry in the porosity. It progressively decreased with increasing particle size and was not reported for >2 mm particle size fractions. Once the finest particle size fractions consolidated, but also for the other coarser particle size fractions, drying was accompanied by the absence or a very low shrinkage, with water potentials around -200 to -300 hPa. These previous drying phases refer to the drainage of interparticle porosity, whereas a last phase of shrinkage, corresponding to that of intraparticle porosity, was observed for lower water potentials. This study revealed significant shrinkage for fine particle size distribution materials over a range of water potentials which are of interest to irrigation management, confirming the importance of assessing shrinkage as one of the relevant indicators of physical properties.
Stan Durand won the ISHS Young Minds Award for the best oral presentation at the I International Symposium on Growing Media, Compost Utilization and Substrate Analysis for Soilless Cultivation in Canada in June 2023.
Stan Durand, L’Institut Agro, Angers, France, e-mail: stan.durand@institut-agro.fr
The article is available in Chronica Horticulturae