The physical-hydraulic properties of eight substrate mixtures based on sphagnum peat and coir were determined and their effect on the growth of Begonia xelatior was studied. The particle size distribution, water retention curve, saturated hydraulic conductivity, and pore size distribution of the substrates were determined. All substrates exhibited high total porosity, satisfactory water retention capacity, and high saturated hydraulic conductivity. Increasing the percentage of perlite in the

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... tures contributed to the reduction of water retention capacity and the increase of large pores. Unsaturated hydraulic conductivity estimated by the Mualem-van Genuchten model showed a sharp decrease within a range of water pressure heads (0 to −50 cm) observed between two successive irrigations. To assess aeration and water retention capacity, total porosity; airspace; and easily, and nonavailable water, as well as the bulk density of the substrates, were determined and concomitantly compared with the "ideal substrates" determined by De Boodt and Verdonck. The comparative results showed that substrate porosity alone is not efficient to create ideal plant growth conditions, but dynamic parameters are also needed. Plants grown in a substrate classified as "nonideal" showed significantly greater growth than the plants grown in most of the other substrates studied. Coconut coir is an organic by-product derived from the mesocarp tissue or husk fiber treatment of the coconut (Cocos nucifera) fruit. The long fibers are used for industrial purposes, and the remaining materials, consisting of short fibers and dust, undergo aerobic composting. After composting, the stable material is dehydrated and compressed into a compact form (bricks) for easy transportation. With the addition of water, coir expands to 5 to 9 times its compressed volume. Coir can serve either as a stand-alone growing medium or as an ingredient in a mix for use in horticulture. Coir is used primarily as a peat alternative [10, 11] . The physical properties of coir and peat have been studied by many researchers [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] . Substrates such as peat and coir offer many advantages, such as low bulk density, being pathogen-and weed seed-free, high water retention capacity, and easy root penetration. The primary disadvantages of peat and coir are poor aeration and low pressure heads due to a low percentage of large particles. This creates a problem in the water-air balance and gas exchange under different watering regimes [1, 3, 14, 24] . To mitigate this disadvantage in pure substrates, various materials characterized by large particle sizes are added, with perlite being the most popular one used [2, 14, 20, 25 ]. The addition of coarse perlite to peat improves aeration [2, 14, 20, [25] [26] [27] [28] . However, the addition and mixture of different materials raises questions about the ratios of the substrate's mixture components to create a high-quality substrate for optimum plant growth. Standards for substrates were developed by De Boodt and Verdonck [24], determining appropriate physical values, such as total porosity, airspace, and water capacity for optimum plant growth in containers and in bed cultivation. De Boodt and Verdonck [24] recommend a limit of water availability in a water pressure range of −10 to −100 cm. This limit was later confirmed by other researchers [29, 30] . These standards have been implemented and used commercially at a large scale [31] [32] [33] and still form the basis for manufacturing growth substrates [34] . However, less attention was paid to dynamic parameters and processes such as water use and gas diffusivity [34] . In this direction, we try to point out the necessity of using, also, dynamic parameters in substrate evaluation. This paper presents a study of the physical-hydraulic characteristics of eight substrate mixes based on peat and coir and assessment of their effect on the growth of Begonia xelatior. The main objective was to determine the hydraulic characteristics of these substrates, widely used in floriculture, and the secondary, to evaluate them with the properties identified by De Boodt and Verdonck [24] in making the "ideal substrates". Also, it is investigated whether it is sufficient to assess the suitability of the substrates based only on their static hydraulic characteristics.