## 4 Results
### Air Capacity  
In the absence of an amendment, the CAS soil was more aerated than the GIN soil (\(AC\) = 0.267 cm\({}^{3}\)/cm\({}^{3}\) and 0.240 cm\({}^{3}\)/cm\({}^{3}\), respectively; \(\Delta\) = +11.5%). Aeration was excessive (\(AC\) > 0.22 cm\({}^{3}\)/cm\({}^{3}\)) in both soils, hence the SPQ was poor with reference to this soil parameter. The expectation was to detect a decrease of the \(AC\) in both soils as a consequence of the treatment.  
Adding the COM at CAS and the VER at GIN did not influence the \(AC\) (Figure 4 and Table 1). In the other cases (VER and BIO at CAS, and COM and BIO at GIN), the \(AC\) decreased as the _ar_ increased, in accordance with the objective of the treatment. The statistically significant relationships were rather weak for the COM and the VER (\(R^{2}\) = 0.24-0.28) and stronger for the BIO (\(R^{2}\) = 0.62-0.80).  
At CAS, the ratio between the slopes of the \(\theta_{100}\) and \(\theta_{s}\) vs. the _ar_ relationships was equal to 1.10 for the COM, 1.71 for the VER, and 1.46 for the BIO (Table 2). Therefore, the \(\theta_{100}\) increased more than the \(\theta_{s}\) with all amendments. However, with the COM, the two relationships were nearly parallel, since the slopes did not differ much. Therefore, the \(AC\) remained nearly constant as the _ar_ changed. With the other two amendments, the slope of the \(\theta_{100}\) vs. the _ar_ relationship was greater enough than that of the \(\theta_{s}\) vs. the _ar_ relationship as to induce a decrease of the \(AC\). At GIN, the ratio between the slopes of the \(\theta_{100}\) and \(\theta_{s}\) vs. the _ar_ relationships was equal to 1.47 for the COM, 1.26 for the VER, and 1.67 for the BIO. Even in this case, a relatively large difference between the two slopes induced the detected decrease of the \(AC\) that instead did not change when these slopes were similar.  
In the range of the _ar_ values considered for the experiment, using the VER at CAS and the COM at GIN allowed us only to define an \(ar_{min}\) value, that is the amount of the amendment inducing a decrease of the \(AC\) from a too high value to \(AC\) = 0.22 cm\({}^{3}\)/cm\({}^{3}\). These \(ar_{min}\) values were equal to 118 t/ha for the VER at CAS and 111 t/ha for the COM at GIN. It was not possible to define an \(ar_{max}\) value beyond which the \(AC\) became too low (<0.15 cm\({}^{3}\)/cm\({}^{3}\)). Instead, both the \(ar_{min}\) and the \(ar_{max}\) were defined for the BIO, and they were equal to 9 t/ha and 150 t/ha, respectively, at CAS, and to 45 t/ha and 162 t/ha, respectively, at GIN. The minimum amendment rate was five times lower at the CAS than at the GIN soil even if, in the absence of an amendment, the former soil was more aerated than the latter. The maximum amendment rate was similar for the two soils.  
Therefore, the COM and the VER were generally ineffective or not very effective since the \(AC\) did not change by adding the amendment, or it decreased a little, and according to the rather scattered \(AC\) vs. the _ar_ relationship (Figure 4). Consequently, only a minimum amendment rate was detectable. The BIO induced a clearer decrease of the \(AC\), and both a minimum and a maximum amendment rate were identified for the two soils with this amendment.