Solute Transport Through a Pine Bark Substrate

High fertilizer application rates, frequent irrigation and relatively low substrate nutrient retention can lead to increased nutrient leaching in containerized crop production. However, little is known about how individual mineral nutrient ions travel through pine bark (PB; Pinus taeda L.) substrate during irrigation events. Developing this knowledge may lead to improved management practices that reduce fertilizer leaching. A study was conducted to characterize the fundamental principles of how solutes are transported through a column of PB under saturated and unsaturated conditions. In this study, effluent was collected after being passed through the column of bark during the application of fertilizer solution and subsequent flush with deionized (DI) water. Effluent electrical conductivity (EC) and the concentration of individual ions (nitrate, phosphate, and potassium) were used to develop breakthrough curves (BTC), which depict the change in relative effluent solute concentration (C/C_o; where C = effluent concentration and C_o = input solution concentration). For both the saturated and unsaturated treatments, effluent EC increased during the application of fertilizer solution, and decreased during DI water application at lower effluent volumes than predicted by the piston flow model, which theorizes that effluent solute concentration will change upon the displacement of one pore volume (PV). This effect was more pronounced for the unsaturated treatment. For example, the center of the EC BTC (C/C_o= 0.5) during fertilizer application occurs at ≈ 0.85 PV and 0.65 PV for the saturated and unsaturated treatment, respectively. Furthermore, the BTC during the DI flush was centered at ≈ 0.80 PV and 0.60 PV after beginning DI water application for the saturated and unsaturated treatments, respectively. Nitrate and phosphate (anions) BTCs mimicked that of EC. However, the intensity of the potassium (cation) BTC during fertilizer application was considerably reduced apparently due to interaction with PB cation exchange sites and the calcium and magnesium residing on those sites from dolomitic lime additions. The speed in which the fertilizer solution moved through and was flushed from the columns was likely a result of the preference for solute to flow through macropores over micropores. These results demonstrate the relative quickness and ease which nutrients can move through and leach from a PB substrate under saturated and unsaturated conditions and demonstrate a potential source of inefficiency in fertilizer management. The research presented herein is a stepping-stone that will assist researchers to better understand and manage solute transport in a soilless substrate during irrigations.