# Biochar Amended Soils and Water Systems: Investigation of Physical and Structural Properties
## 3 Results and Discussion
### Bulk Density, Soil Water Retention and Plant Available Water
This suggests that in clay soils higher BC amounts might improve the soil water retention [29] by determining \(U_{\text{soil+BC}}/U_{\text{soil}}\) ratios higher than the \(\rho_{b,\text{soil}}/\rho_{b,\text{soil+BC}}\), and positive differences between the \(\theta\) values of the BC-amended soils and the \(\theta\) of the soils without BC. [PERSON] et al. [38] also reported more evident effects of BC on the total soil porosity when a higher difference occurred between the \(\rho_{b}\) of BC and the \(\rho_{b}\) of the soil. This suggests that an excessive reduction in the \(\rho_{b}\) of the soil mixtures is not always beneficial in terms of water retention, confirming that positive effects of BC application are to be expected in soils having high initial \(\rho_{b}\) values, such as the coarse textures soils, and not in soils with lower initial \(\rho_{b}\), such as the fine textured ones. In conclusion, BC certainly improved water retention of the loamy sand, confirming the results of previous investigations on coarse textured soils [31; 33; 34] partially improved water retention of the loam, increasing macroporosity, as also reported by [PERSON] et al. [27], but did not determine positive effects on water retention of the clay soil, as also found in previous investigations [22; 37; 38]. The differences obtained in our soil water retention curves obtained before and after BC addition by expressing the soil water content in terms of \(U\) or in terms of \(\theta\) values, also explain why some contrasting results have been found when the effect of BC on fine textured soils has been investigated. Of the previously mentioned papers, those reporting positive effect of BC based their conclusions considering the measured \(U\) values [19; 28], whereas those reporting no effects, or negative BC effects, based their analyses and conclusions considering the \(\theta\) values [36; 37; 38; 39]. Both \(\theta_{fc}\) and \(\theta_{app}\) significantly increased in the loamy sand, but significantly decreased in the loam and in the clay after BC addition. Values of the saturated water content, \(\theta_{s}\), the measured field capacity, \(\theta_{fc}\), the measured wilting point, \(\theta_{app}\), obtained for the three soils amended and not amended with BC, are reported in Table 4.  
\(D\) values (\(\upmu\)m) pore diameter corresponding to the \(|h|\) values at which \(\theta_{fc}\) and \(\theta_{app}\) were measured, i.e., \(|h|=330\) cm and 15,300 cm, respectively, were equal to 9.09 \(\upmu\)m and 0.2 \(\upmu\)m. The amount of pores with \(D=9.09\)\(\upmu\)m was classified as storage pores (0.5-50 \(\upmu\)m), and the amount of pores with \(D=0.2\)\(\upmu\)m, called residual pores (\(D<0.5\)\(\upmu\)m) [52] increased after BC application in the loamy sand, but decreased in the loam + BC as well as in the clay + BC. The amount of pores corresponding to \(\theta_{s}\), indicated as macropores, significantly increased in the loamy sand + BC and in the loam + BC, but decreased in the clay + BC. Plant available water, PAW (cm\({}^{3}\)/cm\({}^{3}\)), calculated on the basis of the \(\theta\) values reported in significantly increased from 0.046 to 0.076 for the loamy sand + BC, non-significantly increased from 0.078 to 0.079 in the loam + BC, and significantly increased from 0.069 to 0.104 in the clay + BC (Table 4). The increase in PAW was equal to 66.1% and to 45.7% for the loamy sand and for the clay, respectively. These results indicated that BC positively affected the PAW of the loamy sand and of the clay but did not determine any significant increase in the PAW of the loam. PAW values, calculated based on the \(\theta\) values, were lower than those obtained on the basis of the \(U\) values, PAW\({}_{\text{U}}\) (Table 4), as a consequence of the reduced \(\rho_{b}\) in the soils + BC mixtures, but the trend was the same, with the highest increment in the PAW values obtained for loamy sand + BC, followed by the increment observed on the clay + BC.