## 2 Soil Physical Quality Parameters and Their Optimal Ranges  
The physical quality of agricultural soils refers primarily to the soil's strength and fluid transmission, and the storage characteristics in the crop root zone. According to [PERSON] et al. [24] and [PERSON] et al. [25], an agricultural soil has a good physical quality if it is strong enough to maintain good structure, hold crops upright, and resist erosion and compaction, but also weak enough to allow for unrestricted root growth and the proliferation of soil flora and fauna. Moreover, in a physically good soil, fluid transmission and storage characteristics permit the correct proportions of water, dissolved nutrients, and air for both maximum crop performance and minimum environmental degradation. The most frequently considered SPQ parameters in agricultural contexts include dry soil bulk density, macroporosity, air capacity, plant available water capacity, and relative field capacity (e.g., [3; 25; 26; 27]). Dry soil bulk density, \(BD\) (g/cm\({}^{3}\)), is an indirect indicator of aeration, strength, and ability to store and transmit water. The other four parameters are obtained from the volumetric soil water content, the \(\theta_{h}\) (L\({}^{3}\)/L\({}^{3}\)), values corresponding to pre-established pressure heads, \(h\) (L). In particular, macroporosity, \(P_{MAC}\) (cm\({}^{3}\)/cm\({}^{3}\)), is defined as follows:  
\[P_{MAC}=\theta_{s}-\theta_{10} \tag{1}\]  
in which \(\theta_{s}\) (cm\({}^{3}\)/cm\({}^{3}\)) is the saturated soil water content and \(\theta_{10}\) (cm\({}^{3}\)/cm\({}^{3}\)) is the soil water content at a pressure head of \(-10\) cm. This parameter is expressive of the soil's ability to quickly drain excess water and facilitate root proliferation. Air capacity, \(AC\) (cm\({}^{3}\)/cm\({}^{3}\)), is defined as follows:  
\[AC=\theta_{s}-\theta_{100} \tag{2}\]  
in which \(\theta_{100}\) (cm\({}^{3}\)/cm\({}^{3}\)) is the soil water content at a pressure head of \(-100\) cm, also defined as the field capacity water content. The ability to store root-zone air is expressed by \(AC\). The plant available water capacity, \(PAWC\) (cm\({}^{3}\)/cm\({}^{3}\)), is given by the following:\[PAWC=\theta_{100}-\theta_{15,300} \tag{3}\]  
where \(\theta_{15,300}\) (cm\({}^{3}\)/cm\({}^{3}\)) is the soil water content at a pressure head of \(-15\),\(300\) cm, also defined as the permanent wilting point water content. This parameter describes the soil's ability to store plant-available water. Finally, the relative field capacity, \(RFC\), is defined as follows:  
\[RFC=\frac{\theta_{100}}{\theta_{s}} \tag{4}\]  
and it indicates the soil's primary limitation with respect to water and air storage. With reference to some parameters, the physical quality of a given soil is considered acceptable if the parameter falls within a range of values, and is unacceptable otherwise. This is the case for the \(BD\) and \(RFC\), for which the optimal ranges are \(0.9\leq BD\leq 1.2\) g/cm\({}^{3}\) and \(0.6\)\(RFC\leq 0.7\), respectively [3]. Consequently, both too low (less than the lower limit of the optimal range) and too high (higher than the upper limit of this range) values of the \(BD\) and \(RFC\) denote an unsatisfactory SPQ. For other parameters, only a lower limit above which the SPQ is considered ideal or optimal has been suggested. In other terms, there is not any information for a possible upper limit above which the SPQ could be more or less poor. For example, according to [PERSON] et al. [3], optimal conditions occur if the \(P_{MAC}\geq 0.07\) cm\({}^{3}\)/cm\({}^{3}\), the \(AC\geq 0.14\) cm\({}^{3}\)/cm\({}^{3}\), and the \(PAWC\geq 0.20\) cm\({}^{3}\)/cm\({}^{3}\). There are at least four reasons why defining an upper limit for the \(P_{MAC}\), \(AC\), and \(PAWC\) is advisable and even necessary: (i) methodological consistency, i.e., a range of optimal values is associated with each individual parameter; (ii) it cannot be believed that very high \(P_{MAC}\), \(AC\), and \(PAWC\) values unquestionably indicate good SPQ conditions.