Influence of the physical properties of pumice and biochar amendments on the soil's mobile and immobile water: implications for use in saline environments

Context. Biochar and pumice have potential to improve soil water retention and mitigate salinity. However, little is known about their effect on salt transport in sandy soils. Aims. We investigated the influence of the porosity and pore size distribution of soil amendments with pumice and biochar on the mobile water content of a New Zealand sandy soil. Methods. Pumice and biochar (1.5-cm, 3-cm and 6-cm in diameter, [empty set]) were characterised using scanning electron microscope technology. The fraction of mobile water present in these amendments, previously added to a sandy soil at different application rates and particle sizes, was determined using a tracer ([Na.sup.+]) technique. Key results. (I) Pumice exhibited a wider pore-size span than biochar; and (2) both materials had a predominance of pores with [empty set] < 30 [micro]m; but (3) the total porosity in pumice and biochar was not significantly different; (4) pumice had a significantly larger (P < 0.05) mean absolute micro-scale porosity than biochar; and (5) a significantly greater (P < 0.05) relative resident [Na.sup.+] concentration than biochar, irrespective of the particle size. Conclusions. These results reflect a larger fraction of the mobile water in pumice than that of biochar under near-saturated conditions, irrespective of the biochar particle size; and this increased as the pumice particle size increased. Implications. While both materials are expected to contribute to water retention and thus might alleviate salt-stress by diluting salt concentration, pumice may perform better than this specific biochar on improving the retention of plant-available water. Keywords: dilution, miscible displacement, mobile-water fraction, particle size, physical properties, porosity, salinity, scanning electron microscopy.
Introduction The sustainability of agriculture in arid regions is challenged by the limited availability of water, and the need to manage salinity in soils and irrigation-water. This demands long-term interventions, [...]