# Evaluation of Conifer Wood Biochar as Growing Media Component for Citrus Nursery
## 1 Introduction  
Citrus tristeza virus (CTV) is a major pathogen of citirculture and is responsible for what are sometimes catastrophic economic losses wherever citrus is grown [1]. Citrus replanting is not well recorded, but it is estimated that over the next decade some 20,000 citrus hectares will need replanting, and the only chance of success is if sour orange rootstocks are replaced by pathogen tolerant ones [1]. Examples of the latter are the trifoliate orange (_Poncirus trifoliata_ L.) and its hybrids. The citrus nursery industry must now implement strict CTV prevention strategies including routine analyses of plant material [2]. Obviously, this significantly raises nursery production costs. The commercial sustainability of tree production cycles also involves cutting costs by using alternative planting substrates to take the place of the conventional black peat [3], which is a limited resource becoming increasingly scarce and thus expensive. Among the alternatives to black peat is biochar, a solid material derived from the thermochemical combustion of biomass in the absence of oxygen [4]. Biochar application improves the structure and fertility of agricultural soils as a good amendment and is also an alternative product for use as a nursery substrate both for _Citrus_ spp. and ornamental plants [5; 6]. The biomass used in the biochar pyrolysis process is variable [7]. Many of the originalstructures of the biomass persist in the biochar and these influence its structural features [8]. The physical and chemical characteristics as well as the yield of biochar, depend on the pyrolytic process [9] and, in particular, the pyrolysis temperature which greatly affects the main biochar properties [10]. Pyrolysis conditions, together with the feedstock used, seem to significantly influence the biochar particle size distribution [11]. The original cellular structure of the biomass materials contributes to the material's macroporosity [12]. Macropores are crucial in the soil environment, affecting root penetration and improving soil microbial condition [13]. These structural aspects of biochar are the starting points for the formation of mesopores and micropores in the soil [14]. Moreover, it affects topsoil depth and texture through changes in the adsorption surface, pore-size distribution, soil density, and biological properties [15]. The associated reductions in N\({}_{2}\)O emission from biochar-amended soils and reduced nutrient leaching allow reductions in the amount of fertilizer needed [16]. Furthermore, biochar improves soil particle aggregation, workability, swelling and dynamic shrinking, permeability, and cation exchange capacity [17]. Water storage capacity is also enhanced [18]. Several studies have recently been conducted on the effectiveness of biochar as an alternative to peat compared to as an alternative thermal treatment of organic wastes (hydrochar) [19]. Recent researches have focused on the benefits of biochar on seed development, plant growth, and crop yield [20; 21]. Enhanced growing conditions are also associated with improved plant health and lower incidences of disease [22]. An additional benefit is the reduced bioavailability of heavy metals, important in some soils [23]. The research described here is based on the idea that the characteristics making biochar an excellent soil improver in field and forest environments may also be useful in nursery production systems when added as a component in growth media used in small-volume pots [24]. Although it is plausible that biochar will be useful in this situation, since growing conditions in pots in a nursery are extremely different from those in the field, it is necessary to confirm the usefulness of biochar to potting mixes under nursery conditions [25]. It is also necessary to determine biochar's usefulness with particular plants, species, and varieties.