Improving Water and Nutrient Retention Capacity of the Pine Bark Substrate Using Amendments

Significant amounts of water and nutrients are leached from the containers owing to the poor retention capacity of the substrates used in ornamental nursery production. This can result in wastage of fresh water and contamination of ground or surface water by nutrients leached from nurseries. The objective of the study is to asses the effect of amending a standard nursery substrate (8 parts pine bark + 1 part sand, PBS) with either calcined clay (PBSC) or diatomaceous earth (PBSD) at 10% v/v to improve water and nutrient retention. Measurements included soil moisture retention (SMR) curves for each substrate, and electrical conductivity (EC) and nitrate concentration of each substrate after sequential irrigations. The SMR curves indicated that container capacity of PBS substrate was lower than that of the PBSC substrate but not different to the PBSD substrate. This indicates that water retention capacity of PBS substrate can be improved by amending with calcined clay. Approximately 10, 28 and 38% of water drained as the water potential dropped from container capacity to -1.0 MPa (close to wilting point) in PBS, PBSD and PBSC substrates respectively. This indicates that more water can be available to plants when PBS substrate is amended with either diatomaceous earth or calcined clay. Moreover, amending PBS substrate with diatomaceous earth or calcined clay could decrease the risk of rapid exposure of plants to drought stress that may happen in PBS substrate. Although EC and nitrate concentration of all three substrates decreased during sequential irrigations, both measures remained higher for PBSC and PBSD than PBS substrate after any irrigation. This indicates that nutrient retention in general, and nitrate retention, in particular was improved by amending PBS with either diatomaceous earth or calcined clay. In general, PBSC appears to retain more water and nutrients than PBSD substrate.