## 4 Results
### Plant Available Water Capacity  
In untreated conditions, the _PAWC_ was lower at CAS than at GIN (_PAWC_ = 0.102 cm\({}^{3}\)/cm\({}^{3}\) and 0.144 cm\({}^{3}\)/cm\({}^{3}\), respectively; \(\Delta\) = \(-\)29.1%). These _PAWC_ values were lower than the minimum optimal value (_PAWC_ = 0.15 cm\({}^{3}\)/cm\({}^{3}\)). However, the difference between the actual value and the lowest optimal value of the _PAWC_ was appreciable only at CAS. At GIN, these two values were similar, since they only differed by 0.006 cm\({}^{3}\)/cm\({}^{3}\), indicating that this soil had a nearly optimal _PAWC_ value in an untreated condition. Therefore, the need to amend the soil to induce an increase of the _PAWC_ was detected at CAS but, strictly, not at GIN. However, amending the GIN soil was necessary with reference to other SPQ parameters. Therefore, the relationship between the _PAWC_ and the _ar_ was also tested for this soil to verify if, in this case, adding the amendment (i) did not have any adverse effect, since the SPQ remained good, or (ii) induced a deterioration of an initially acceptable SPQ. Adding the amendment determined an increase of the _PAWC_ with all amendments at CAS, and only with the BIO at GIN, since the COM and the VER were ineffective in this soil (Figure 5 and Table 1). Figure 4: Air capacity, _AC_, against the amendment rate for the three tested amendments (COM = compost; VER = vermicompost; BIO = biochar) at (**a**) the Castellaneta and (**b**) the Ginosa field sites. For a given site, the continuous lines are the significant linear regression lines (black for COM; red for VER; blue for BIO). The dashed grey lines define the range of values inside which the soil physical quality is considered good. At CAS, the ratio between the slopes of the \(\theta_{100}\) and \(\theta_{15,300}\) vs. the _ar_ relationships was equal to 2.90 for the COM, 2.46 for the VER, and 4.56 for the BIO (Table 2). Therefore, the _PAWC_ increased because, with more amendment, the \(\theta_{100}\) increased more than the \(\theta_{15,300}\). At GIN, the ratio between the slopes of the \(\theta_{100}\) and \(\theta_{15,300}\) vs. the _ar_ relationships was equal to 1.34 for the COM, 1.13 for the VER, and 4.15 for the BIO. With both the COM and the VER, the slope of the \(\theta_{100}\) vs. the _ar_ relationship was only a little greater than that of the \(\theta_{15,300}\) vs. the _ar_ relationship. Therefore, the increase of the _PAWC_ with the _ar_ was low and not statistically significant. Adding the BIO determined a significant increase of the _PAWC_, since the \(\theta_{15,300}\) increased a little, whereas the increase of the \(\theta_{100}\) was appreciable. At CAS, both a minimum (26 t/ha for the COM, 73 t/ha for the VER, and 8 t/ha for the BIO) and a maximum (166 t/ha for the COM, 161 t/ha for the VER, and 72 t/ha for the BIO) amendment rate were defined for all amendments (Table 1). The same improvement of the SPQ (from poor to good) was obtained with a relatively small amount of BIO as compared with the necessary amount of COM and VER. Even the maximum rate of BIO was appreciably lower than that required with the other two amendments. Therefore, the  
Figure 5: Plant available water capacity, _PAWC_, against the amendment rate for the three tested amendments (COM = compost; VER = vermicompost; BIO = biochar) at (**a**) the Castellaneta and (**b**) the Ginosa field sites. For a given site, the continuous lines are the significant linear regression lines (black for COM; red for VER; blue for BIO). The dashed grey lines define the range of values inside which the soil physical quality is considered good. BIO was more effective than both the COM and the VER, since lower amendment rates were enough in the former case to determine the same effect on the _PAWC_. The risk of inducing a soil deterioration (from a good to a poor SPQ) associated with the use of too high _ar_ values was greater with the BIO than with the COM and the VER. At GIN, the COM and the VER were ineffective, whereas the BIO induced an increase of the _PAWC_.