RESEARCH Wood Products ROOT ZONE in the Researchers continue to study how wood as a substrate component can help growers reduce production costs. BY BRUCE BUGBEE AND ROYAL HEINS M ore than 30 years ago we started discussing nutrition and substrates for containerized production. Science is a ponderous enterprise. We are still optimizing. Our current challenge is to determine the modifications necessary to use high fractions of wood in peat-based substrates. Wood products can be 30 to 50 percent of the cost of peat, but the addition of wood affects both the physical characteristics (water holding capacity and aeration) and the chemical characteristics (pH, nutrient holding capacity and microbial stability) of substrates. Our studies have included treatments with up to 75 percent wood by volume because some useful modifications become more aparent in high wood treatments, and because we think wood substrates can ultimately be used at high percentages of the substrate. POTENTIAL VS. EFFECTIVE WATER HOLDING CAPACITY Water holding capacity is the difference between the maximum volume of water a container can hold after watering and the volume when the plant wilts (Figure 1). Like the gas gauge in a car, each time the pot is fully watered, it fills the tank. The potential water holding capacity of an 80/20 peat/perlite substrate is about 60 percent of the substrate volume. The exact potential depends on substrate settling (density) and pot height. Our data indicate substrates with either sawdust or shredded wood hold slightly less water; the potential capacity declines from 60 to 50 percent as the percentage wood increases to 75 percent (Figure 2). This potential capacity, however, can only be reached if the maximum water volume is achieved after watering. 22 FEBRUARY 2019 GPNMAG.COM Substrates become difficult to fully rehydrate when they get close to the minimum water holding capacity. Because of this, the effective water holding capacity can be less than half of the potential. Our studies indicate that adding a wetting agent can help to improve the effective water holding capacity - especially in substrates with higher fractions of wood. 50% 25% Bound water Solids 0% PERCENT BY VOLUME Figure 1. Tracking the water holding capacity of a container is similar to a gas gauge in a car. Potentially available water ≈ 60% Air ≈ 20% 75% wilting point container capacityhttp://www.GPNMAG.COM