## 5 Discussion
According to this investigation, there were several reasons why the BIO appeared preferable to improve the SPQ as compared to both the COM and the VER: (i) there were always significant relationships between the _BD_, _AC_, _PAWC_, and _RFC_ and the amendment rate. The same was not true with the other two amendments; (ii) there were higher \(R^{2}\) values with the former amendment (BIO) than the latter ones (COM and VER); (iii) it was possible to define both a minimum and a maximum amendment rate for the four considered parameters. This possibility was not detected with the COM and the VER; (iv) the objective of the treatment, that is passage from a poor to a good SPQ, was reached with lower amendment rates. Figure 7: Relationship between the relative field capacity, _RFC_, and the air capacity, _AC_, for the three tested amendments (COM = compost; VER = vermicompost; BIO = biochar) at Castellaneta (CAS) and Ginosa (GIN) and fitted regression line to the whole dataset. A practical problem that emerges is related to the fact that the dose to be applied in practice is one, but the parameters to be improved are four. In light of this investigation, it is possible to define an optimal dose of soil amendment, which is the highest value of the minimum amendment rates determined for the different parameters. The validity of this definition presupposes that this value is not higher than the lowest value of the maximum dose. From the minimum and maximum _ar_ values reported in Table 1, an optimal dose of BIO can be defined for both soils. At CAS, this dose was equal to 54 t/ha and, at GIN, it was 52 t/ha. Therefore, the optimal dose was nearly identical with reference to the two considered sandy-loam soils. About 50 t/ha of BIO were enough to improve the physical quality of the tested soils to acceptable levels with reference to the most agronomically important air and water storage parameters. Adding more BIO, up to nearly 110-160 t/ha, can be expected not to induce a deterioration of the SPQ with reference to the _BD_, _AC_, and _RFC_. However, such high doses are appreciably higher than the _ar\({}_{max}\)_ with reference to the _PAWC_, equal to 56-72 t/ha, depending on the soil (Table 1). Therefore, according to this investigation, applying a quantity of BIO appreciably greater than the optimal dose can be expected, at first, to determine a decrease in the available water for the crop. The results obtained in this investigation could suggest that BIO doses of 50 t/ha, corresponding to an amendment/soil ratio, _ar_, of nearly 9-10%, could generally be used for soils like those tested in this investigation. Evidently, this suggestion should be considered with caution and subjected to specific experimental checks. One reason is that the dosage could appear too high in practice. However, other authors have also concluded that a high dosage of BIO can be expected to improve the water retention of coarse-textured soil with limited water storage capacity [48]. Moreover, an advantage of using the BIO is that a single application of the amendment could be effective for a long time, since the BIO has a very slow decomposition rate [49]. This investigation has contributed to making a step forward with reference to one of the research needs listed by [PERSON] [23], since it has allowed us to obtain an experimental information on both the minimum and the maximum doses of biochar to be used in coarse-textured soils. One of the possible limitations of the applied approach is that, for the considered SPQ parameters, the values that define the optimal conditions were taken from the literature. Although this is a very common practice, it is necessary to acquire further certainties on the validity of these optimal values in a wider range of situations, different from those considered by [PERSON] et al. [3]. BIO addition effects on the SPQ parameters can generally be expected to vary with the tested soil.