## 4 Discussion  
Soil is considered saline if it exceeds 4 dS m\({}^{-1}\) as EC (approximately 40 mM NaCl) at 25 \({}^{\circ}\)C, as already illustrated in the Section 1. In our experiment we reached mean values between 11 and 15, and 18 and 21 dS m\({}^{-1}\) in the case of the experimental groups CC150 and CC300, respectively. These values are very far above the limit of 4 dS m\({}^{-1}\) over which, plants and many crops feel the stress caused by salt excess. In particular, maize plants could not tolerate the saline concentrations reached by the soil irrigated with the solution 300 mM NaCl (around 21 dS m\({}^{-1}\) mean value, Table 2), confirming that maize, being a glycopyte plant, was the most sensitive one among those tested. The effects of salt stress on crop growth, and especially in the case of maize, are mainly due to high osmotic imbalance caused by low external water potential, ion toxicity induced by Na\({}^{+}\) and/or Cl\({}^{-}\), photosynthesis inhibition, but also by an altered nutrition status which determines a reduction of essential element up-take [29]. Moreover, the presence of high level of these two ions in the plant tissues affects cellular and organelle membranes, primarily due to production of reactive oxygen species (ROS), limiting the plant growth and causing evident signs of phenotypic alterations before mortality, as we observed in the case of maize treated with 300 mM NaCl. However, plants/crops show a unique capacity to adapt to soil salinization, such as stomatal regulation, variation of hormone balance, activation of the antioxidant defense system, etc. [30]. Therefore, it can be hypothesized that the higher resilience observed for all assayed crops of our experiment, whenever grown on compost saline amended soils, is due to an improved adaptation of their physiology, metabolism, and morphology to the stress imposed by high soil EC, as mentioned above. Compost is renewable resource which can also be obtained from separate collection of organic waste and it is employed to return organic matter to agricultural soils [31]. Its use as organic amendment dates back to the beginning of human civilization, in fact, animal manure was used when agriculture and cattle breeding were born, about 10-11,000 years ago. Thousands of scientific publications, in the last three decades, deal with compost, illustrating the beneficial effects on soils depleted of its organic matter due to the reckless use of inorganic fertilizers [32]. The benefits of compost on soil chemical-physical-biological properties, and on crop productivity have been widely demonstrated [33; 34; 35]. In our pot experiment the positive effect on the seedlings growth and biomass was confirmed once more, in almost all the treated crops and groups. Similar results were obtained by [PERSON] et al. [36], showing that \"compost improved soil fertility and microbial activity, \begin{table}
\begin{tabular}{c c c c c} \hline
**Strain** & **TCP** & **NH4** & **IAA** & **Siderophores** \\ \hline MD6 & +++ & 408 \(\pm\) 49 & 0.57 \(\pm\) 0.17 & +++ \\ \hline QB21 & - & 186 \(\pm\) 12 & 1.35 \(\pm\) 0.09 & - \\ \hline MA1 & - & 175 \(\pm\) 53 & 0.40 \(\pm\) 0.14 & - \\ \hline QH11 & - & 257 \(\pm\) 53 & 0.41 \(\pm\) 0.11 & - \\ \hline QB13 & + & 306 \(\pm\) 43 & 2.23 \(\pm\) 0.15 & + \\ \hline QH8 & - & 427 \(\pm\) 58 & 0.44 \(\pm\) 0.05 & - \\ \hline \end{tabular} (TCP= capacity to solubilize tricalcium phosphate, NH4 = ammonium production (mg mg\({}^{-1}\)), IAA= indole acetic-3-acid production (mg mg\({}^{-1}\)), siderophores production). +, +++ indicate low and high production (in the case of TCP the capacity of its solubilization), respectively.  
\end{table} 
Table 5: PGP featuresincluding nodulation in soybean roots. Biomass production was strongly increased; content of chlorophyll, carotenes, sugars, and protein also increased\". Soil salinity has always a detrimental effect on almost all vegetables and crops, as well illustrated by [PERSON] and [PERSON] [7].