# Evaluation of Conifer Wood Biochar as Growing Media Component for Citrus Nursery
## 3 Results and Discussions
### Chemical and Physical Properties of Growth Media
At the end of the trial, except for E and F, growing media containing biochar showed decreases in pH equal to 18% (C), 17% (D), and 24% (B), where the amounts of biochar were 12.5%, 25%, and 50%, respectively. These values reach the acceptable/optimum pH range of 5.3-6.5 [34] for most woody plants [25]. The growing media G (control) and A maintained lower pH values throughout the trial. In particular, at the beginning, the control was strongly acidic [36]; this may have reduced nutrient uptake. It is well known that pH has a strong effect on plant growth through its impact on the activity of beneficial bacteria. The soluble salt content, related to the EC of a saturated paste [37], assesses the ability of the soil water to carry an electric current. Thus, soil water EC is a good indicator of the quantity of nutrient mineral ions available for uptake. Initially, EC in G (control) was highest, followed by F and E. The EC in the other growing media was significantly lower (Table 1). The high levels of both pH and EC in compost-based growing media are reported [38]. Although the pH results agree with several studies [39; 40; 41], they are far from optimal for an ideal growing medium and could damage the growth of many acidophil and salt-sensitive species. The initial EC levels did not influence seedling growth, in accordance with previous results reporting good growth of a number of woody species in the presence of initial EC values exceeding 0.8 dS cm\({}^{-1}\)[42; 43]. Heavy rainfall (typical for a Mediterranean climate) at the end of the trial in autumn caused leaching of nutrients as denoted by the EC decrease. Except for A, which showed a significant increase in EC, a drastic reduction in EC was observed for all other growing media. In particular, B and C (biochar 50% and 12.5%) registered the lowest values with respect to optimum \(\leq\)0.5 dS cm\({}^{-1}\)[34], but these were still well above the 0.06-0.2 dS cm\({}^{-1}\) recommended for healthy, vigorous growth [44; 45]. The physical characteristics of the various growing media also changed based on their composition and the time of testing. The time-based changes occurred as a consequence of root growth as well as swelling and shrinkage associated with the cycling water status, and with decomposition of the organic components. Physical characterization of mixtures is very important to evaluate the percentage of solid material, water, and air capacity. Unlike natural soil, the volumes under investigation were very small (6.5 L) and very homogeneous, therefore, our sampling protocol provided relatively accurate estimates. The container weight (Table 2) was higher for F than for A, B, D, and G. A high bulk density increases transport costs and is associated with reductions in both porosity and air capacity [46]. Compaction increases a soil's bulk density, while reducing pore volume and water-holding capacity. At the start of the trial bulk densities ranged from 0.46 (G, control) to 0.84 (E), whereas the remaining mixtures were similar to the higher value (Table 2). At the end of the trial E had the lowest bulk density, comparable to G (control). At the end, the bulk density in the mixtures containing biochar decreased from 5% (B) to 44% (E), probably due to root growth and downward movement of fine particles to greater depths increasing from 7% (F) to 15% (D). For growing media C bulk density did not change significantly over the trial period, whereas for G (control) it increased by 19%. The value of bulk density in an ideal growing media should be \(<\)0.40 g cm\({}^{-3}\), with a value range between 0.1 and 0.3 g cm\({}^{-3}\) considered acceptable for seedling propagation [34; 47]. At the end of the trial the mixtures containing biochar had higher bulk densities than optimum (0.40 g cm\({}^{-3}\)), from 0.66 (B) to 0.58 (F) (Table 2). The high bulk density values in this trial are a result of their 50% soil component. The hydrological parameters are very important for water availability and also suitability for plant growth [28; 48].