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William R. Argo

Acceptable physical properties are an integral part of root-media quality. However, there is no one growing medium that works best in all situations because root-media physical properties are not constant, but rather can be affected by the grower. Understanding the root environment under production conditions requires an understanding of the dynamic nature of air : water : solid ratio in the medium. The objective of this review is to consider key aspects of root-medium physical properties, which include bulk density and particle size, container capacity, media settling, water absorption, rewettability, moisture release characteristics, and water loss due to evaporation from the root-medium surface.

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Paolo Sambo, Franco Sannazzaro, and Michael R. Evans

, whereas RH1, RH2, and RH4 had similar water content at this pressure. The RH6 had the lowest water content at 10 kPa pressure. Fig. 2. Water release curves for sphagnum peat, whole fresh rice hulls, and various grind sizes of ground fresh rice hulls. RH1

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E. Carmona, J. Ordovás, M.T. Moreno, M. Avilés, M.T. Aguado, and M.C. Ortega

Static hydrological properties [aeration capacity, easily available water, reserve water, water release curves: θvm), and specific humidity curves] and dynamic hydrological properties (saturated and unsaturated hydraulic conductivity) of sub strates based on industrial cork residue (the bark of Quercus suber L.) and cork compost were studied. Samples of similar granulometry have been used to establish the effect of cork composting on the afore mentioned physical properties. Different models were tested to describe the mechanism of water release from these materials. Van Genuchtens model (Van Genuchten, 1978) was the best fit and produced specific humidity curves that revealed slight differences in the ratio of water capacity function. When cork residues were composted for 7 months, important changes occurred in hydrological properties of the material as it became more wettable. Water retention significantly increased from 45% to 54%, at a potential of 5 kPa, although this did not necessarily result in increased water available to plants. A study of the unsaturated hydraulic conductivity (Kunsat) of these materials revealed a significant de crease in the Kunsat water potential at 0-5 kPa, which corresponds to the range in which the irrigation with these substrates was usually carried out. The long composting process resulted in increased Kunsat between 4 and 5 times that of uncomposted material, which would improve the water supply to the plant.

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B. Dehgan, T.H. Yeager, and F.C. Almira

other products or vendors that may also be suitable. We thank Steve Linda for statistical assistance; Mary Collins for water-release curve determinations; and Stockhausen, Greensboro, N.C., for financial assistance. The cost of publishing this paper was

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Michael D. Frost, Janet C. Cole, and John M. Dole

Improving the quality of water released from containerized production nurseries and greenhouse operations is an increasing concern in many areas of the United States. The potential pollution threat to our ground and potable water reservoirs via the horticultural industry needs to receive attention from growers and researchers alike. `Orbit Red' geraniums were grown in 3:1 peat:perlite medium with microtube irrigation to study the effect of fertilizer source on geranium growth, micronutrient leaching, and nutrient distribution. Manufacturer's recommended rates of controlled-release (CRF) and water-soluble fertilizers (WSF) were used to fulfill the micronutrient requirement of the plants. Minimal differences in all growth parameters measured between WSF and CRF were determined. A greater percentage of Fe was leached from the WSF than CRF. In contrast, CRF had a greater percentage of Mn leached from the system than WRF during the experiment. Also, regardless of treatment, the upper and middle regions of the growing medium had a higher nutrient concentration than the lower region of medium.

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Jeb S. Fields, William C. Fonteno, Brian E. Jackson, Joshua L. Heitman, and James S. Owen Jr.

Pine tree substrates (PTSs) may provide growers with sustainable substrate component options. Improved processing of PTS components has provided new materials with little scientific evaluation or understanding of their hydrophysical behavior and properties. Moisture retention characteristics were developed for two PTSs and four traditional greenhouse components: sphagnum peat, coconut coir, perlite, pine bark, shredded-pine-wood (SPW), and pine-wood-chips (PWC). Mixtures of peat containing 10%, 20%, 30%, 40%, and 50% of perlite, SPW, or PWC were also characterized. Hydrophysical properties were measured, allowing for comparison of the PTS components to the more traditional substrate components (peat, coir, perlite, and pine bark). The SPW was constructed to retain water similarly to peat and pine bark, whereas the PWC was made to increase drainage like perlite. Shredded pine wood had higher total porosity and more easily available water than did PWC components. Total porosities of SPW and PWC were similar to pine bark and coir; air space and drainage were higher than peat and coir because of the lower percentage of fine particles in the PTS components. The two PTS components had a greater influence on water drainage and retention dynamics than did perlite when amended with peat as an aggregate. Water release patterns of SPW or PWC components at low tensions were lower than peat and greater than pine bark; drainage was similar to perlite at higher tensions. Equilibrium capacity variable models predicted similar physical properties (and trends) across multiple container sizes for peat mixes amended with perlite, SPW, or PWC. The impact of PWC on drainage and aeration was similar to perlite in all containers, but these effects were greater in smaller containers.

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Michael Raviv, J. Heinrich Lieth, David W. Burger, and Rony Wallach

Physical characteristics of two media were studied concerning water availability to roots, as reflected in specific transpiration rate, stomatal conductance, and specific growth rate of very young leaflets of `Kardinal' rose (Rosa ×hybrida L.), grafted on Rosa canina L. `Natal Brier'. Plants were grown in UC mix [42% composted fir bark, 33% peat, and 25% sand (by volume)] or in coconut coir. Water release curves of the media were developed and hydraulic conductivities were calculated. Irrigation pulses were actuated according to predetermined media moisture tensions. Transpiration rate of plants was measured gravimetrically using load cells. Specific transpiration rate (STR) was calculated from these data and leaf area. STR and stomatal conductance were also determined using a steady-state porometer. Specific growth rate (RSG) of young leaflets was calculated from the difference between metabolic heat rate and respiration rate, which served as an indicator for growth potential. Low STR values found at tensions between 0 and 1.5 kPa in UC mix suggest this medium has insufficient free air space for proper root activity within this range. Above 2.3 kPa, unsaturated hydraulic conductivity of UC mix was lower than that of coir, possibly lowering STR values of UC mix-grown plants. As a result of these two factors, STR of plants grown in coir was 20% to 30% higher than that of plants grown in UC mix. STR of coir-grown plants started to decline only at tensions around 4.5 kPa. Yield (number of flowers produced) by coir-grown plants was 19% higher than UC mix-grown plants. This study demonstrated the crucial role of reaching sufficient air-filled porosity in the medium shortly after irrigation. It also suggests that hydraulic conductivity is a more representative measure of water availability than tension.

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Judith Pozo, Juan E. Álvaro, Isidro Morales, Josefa Requena, Tommaso La Malfa, Pilar C. Mazuela, and Miguel Urrestarazu

as described by Rodríguez et al. (2015) . The duration of each irrigation event was selected by adjusting the volume to be supplied to each cultivation unit depending on the obtained soil water release curve ( Fig. 1 ). To generate the water release

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Johann S. Buck and Michael R. Evans

in a pressure plate system. In this test, the water released from 0 to 1 kPa of pressure was considered to be the AFP space and the water held at a pressure of 1 kPa was considered to be total WHC. Water released from 1 to 10 kPa was considered to be

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Isidro Morales and Miguel Urrestarazu

selected by adjusting the volume to be supplied to each cultivation unit depending on the soil water release curve obtained ( Fig. 1 ). To obtain the water release curve of the coir substrate, the following volumes were calculated (vol:vol): total porosity