Manipulation of Soilless Substrate Hydraulic Properties to Optimize Container Crop Water-substrate Interactions to Reduce Water Requirements

The ability to utilize natural resources more efficiently in production settings will allow for sustainable containerized crop production, thus increasing economic viability while reducing environmental impact. Improved understanding of soilless substrate water availability will allow for improved water use and management decisions. Conventional measures and guidelines for soilless substrate based, in part, on static physical properties must be challenged as fresh water availability becomes limited, regulated, or scrutinized. Hydraulic properties are utilized less frequent, but may be manipulated to improve the ability of a substrate to retain, distribute, and deliver higher proportions of water to the plant, thus increasing water use efficiency. Manipulation of the hydraulic conductivity of a substrate can shift water to capillary (water held in pores and readily available for plant consumption) from gravitational (readily drained) and hygroscopic (adsorbed to substrate particles), both of which are considered to be less available for plant uptake. The goal of this research was to determine how modifying soilless substrate hydraulic conductivity via altering particle size distribution and subsequent pore size distribution and/or connectivity will influence containerized crop growth and water use. Aged pine bark was screened and/or blended with fibrous amendments to yield seven substrates, four with varying particle size and two coarse pine-bark substrates amended with peat or coir, and a control aged pine bark with varying hydraulic properties as determined using the evaporative method. Twenty-seven replicates of Hibiscus rosa-sinensis ‘Fort Myers’ plugs were planted in 3.9 L containers using each substrate and automatically irrigated to maintain a water potential of -50 to -100 hPa; termed water buffering capacity, with the use of lysimeters that monitored substrate water content every five minutes. Crops were grown until a salable crop was achieved, with growth index and water use efficiency measurements being recorded over time. At the completion of the study, plant biomass was partitioned and leaf samples collected for analyses. Growth and biomass measurements followed a similar trend; plants grown in substrates with finer sized particles and increased pore connectivity able achieved higher growth rates (P<0.0001). Crop water use efficiency as estimated via carbon isotope (¹³C) discrimination increased as substrate macropore volume decreased (P=0.0307). Modifying substrate hydraulics can increase the proportion of water that is available to plants by transitioning water from gravitational and hygroscopic water to capillary water, thus further extending water availability in bark based substrates used to produce containerized crops throughout the US.