Natural and synthetic hydrogels as an amendment to overcome Radiata pine drought stress at the time of planting

Extending New Zealand’s planting season for Pinus radiata may increase the economic viability of mechanised planting. Doing this could necessitate adding hydrogels or water at planting to buffer dry soil conditions, however, optimizing amendment type and volume for seedling establishment is required. We compared how synthetic polyacrylamide hydrogel (PAM) and nanocellulose hydrogel (NCH) influenced water retention in different forest soils, and found NCH applied to intact soil cores at 0.5% could increase plant available soil water by up to 28%. However, results were inconsistent among the soil types tested. To couple plant response with amendment type, two nursery trials were conducted where P. radiata seedlings were planted on either shoulder of the traditional planting window (May to September) using PAM, NCH or water applied at different volumes. The results showed that during the pre-planting window trial (autumn: April-May), plant water reserves were no different between PAM and water treatments over 54 days in dry soil, but adding PAM to wet soil suppressed root growth. During the post-planting window (spring: November-December) PAM maintained plant water reserves higher than water over 39 days. The results suggest PAM is best deployed during the post-planting window, when plant water demands are higher. Field trials are needed to confirm these observations.