PHYSICAL PROPERTIES OF PINE BARK SUBSTRATE AMENDED WITH INDUSTRIAL MINERAL AGGREGATE

Pine bark is the most common container substrate in the Southeastern United States nursery industry. Pine bark based substrates provide excellent aeration and a moderate amount of available water (AW), however, they have little water buffering capacity. Thus, frequent irrigation events are required to maintain adequate water. This often leads to low water use efficiency. Current studies have shown reduced water application needs and increased plant stomatal conductance and carbon assimilation when plants were grown in a mineral aggregate amended pine bark substrate compared to pine bark alone. An increase in water buffering capacity was reported which may explain the plant response. Our objective was to determine if increased substrate water buffering capacity could be explained as a function of substrate physical properties. To accomplish this objective, pine bark was amended with a calcined Georgiana palygorksite-bentonite aggregate (0.85–0.25 mm) at 0%, 4%, 8%, 12%, 16%, 20% and 24% (by vol.). Physical properties consisting of total porosity, container capacity, air space (AS), bulk density, AW, and unavailable water (UW) were determined. Soil moisture characteristic curves were determined for amendment rates of 0%, 8%, 12%, 16% and 20% (by vol.). Container capacity and AW increased linearly with increasing amendment rate, whereas UW and AS decreased linearly with increasing rate of mineral aggregate. Substrate moisture characteristic curves showed that more water was retained at greater substrate moisture tensions with increasing mineral aggregate rate, thereby increasing readily available water. Volumetric water content was initially greater at the 0% rate, however it quickly decreased at approximately 2 cm substrate moisture tension below those substrates amended with the mineral. The physical properties of the substrate in association with the inherent zeolitic and absorbed water of the mineral increased water content, resulting in increased buffering capacity which could reduce plant water stress. INTRODUCTION The United States nursery industry is a leading crop sector of U.S. agriculture with 3.97 billion dollars in gross sales in 2003 (USDA, 2004). Nursery inventory consisted of 73% containerized plants, with the Southeastern United States accounting for 41% of the over 7,000 national operations and 34% of the 186,000 ha in production area (USDA, 2004). Pine bark is the standard component of soilless substrate in containerized nursery production in the Southeastern United States. Pine bark was chosen due to its availability, favorable physical properties, and lack of detrimental chemical constituents. A salable Proc. IS on Growing Media Ed.: J.-C. Michel Acta Hort. 779, ISHS 2008 132 plant can be produced quickly in a pine bark substrate associated with high nutrient and water inputs. Pine bark is a relatively inert media with less water holding capacity than a mineral soil. Therefore, water use efficiency is a concern for growers due to increasing local, state, and federal intervention with water use and water availability. Best management practices for containerized plant production introduced in 1997 (Yeager et al., 1997) are becoming implemented widely in the United States, resulting in increased water use efficiency. Increased water use efficiency has been achieved by reducing water volume applied, adjusting water application timing, increasing water application efficiency, and amending soilless substrates. While many of these practices have been adopted by the nursery industry, there has been little change in substrate composition since the introduction of pine bark media due to cost, acceptance, and availability. Calcined or expanded clay and zeolite are alternatives to sand or other inorganic components used in peator pine bark-based soilless substrate (Handreck and Black, 2002; Reed, 1996). Clay mineral aggregate amendments have been studied primarily in peat-based substrates with little research being conducted with pine bark-based media. Warren and Bilderback (1992) compared rates (0, 27, 54, 67 and 81 kg m) of arcillite in a pine bark substrate, reporting curvilinear increases in available water (AW) and growth of Rhododendron sp. ‘Sunglow’ with increasing rates of arcillite. Cotoneaster dammeri C.K. Schneid. ‘Skogholm’ grown in a 14 L container with pine bark amended with 8% (by vol.) of calcined 0.85 to 0.25 mm Georgiana palygorksite-bentonite mineral aggregate required 0.4 liters day less water to grow an equivalent plant (Owen et al., 2003). In addition, stomatal conductance and net photosynthesis were significantly greater in the clay amended substrate compared to an unameded pine bark (Owen et al., 2006). This decrease in water use and increase in stomatal conductance without affecting plant growth was attributed to an increase in water buffering capacity. Clay offers the water buffering capacity found in soil, which is not typically present in soilless substrates due to their relatively inert components. Clays of interest as a soilless substrate amendment are the 2:1 layer phyllosillicate minerals: smectite, palygorskite, and illite. These phyllosillicate minerals are formed in layers composed of one octahedral sheet between two parallel tetrahedral sheets. Clays used in our research are a composite of the minerals: montmorillonite and palygorskite. Montmorillonite, a smectite, is a 2:1 layer mineral with a plate like surface or structure. The water associated with smectite is surface adsorbed or tightly bound interlayer water (Velde, 1992). Palygorskite (syn. attapulgite, Fuller’s Earth, hormite clay) ores occur in the Fuller’s Earth District in southern Georgia and northern Florida. Palygorskite is a silicon (Si) rich mineral that occurs as a 2:1 dioctahedral fibrous or chain-like aluminosilicate that appear as rods. This unique structure allows for the presence of zeolitic water (Velde, 1992). Adsorbed and crystalline water are also associated with this mineral. Crystalline water is a part of the mineral structure and zeolitic water occurs within the minerals capillary pores. The surface of the mineral can also adsorb water through electrostatic forces or hydrogen bonding, creating a hydration shell around an industrial mineral aggregate. Industrial clay minerals require processing before being used in industrial applications such as chemical carrier or barrier clay. The mineral is screened into various particle sizes for use with the most popular size for the agriculture industry falling between 0.85 and 0.25 mm (Moll and Goss, 1997). Industrial clay minerals are dried at approximately 121°C and described as regular volatile material (RVM) (Moll and Goss, 1997). RVM products are soft and have 8 to 12% water by weight. This dried product can be subjected to further heating (≤ 800°C) and classified as a low volatile material (LVM) which is calcined, or fixed, containing 0 to 1% water by weight (personal communication, Robert Goss, Oil-Dri RD Velde, 1992). At the completion of Ca-montmorillonite