## 4 Discussion
As recommended by many Authors [5; 17; 34; 35], a volume that is adequate enough to reach the root area should be applied to protect roots from potential colonization of pathogens and promote the growth of healthy plants. In addition, repeated applications are necessary in order to constantly supply the soil system with nutrients and beneficial microorganisms [17; 34; 35]. On the other hand, the repeated distribution of CT with a sodium imbalance could cause sodium accumulation into the soil (solcity phenomenon), which has negative impacts on soil structure (swelling and dispersion of clays, soil surface crusting with consequent soil pore sealing). Therefore, society can significantly affect water infiltration into the soil causing runoff and soil erosion especially on steep lands. In case of CT dilution, such detrimental agronomic/environmental effects can be reduced. A certain increase in N content was observed in CTs produced with the addition of both whey and molasses. Such concentrations, although they are not exhaustive to fully satisfy nutritional requirements of horticultural or fruit crops to be treated, has to be considered complementary for nitrogenous nutrition of crops. By simulating the soil distribution of 300 L year\({}^{-1}\), as suggested by [PERSON] [17], the experimental CTs could apply total nitrogen inputs ranging from 29.1 to 86.4 g ha\({}^{-1}\). These amounts should be increased by fertilizers, taking into account other agronomical principles, such as the real nutrient needs of crops along the different stages of plant life cycle; soil nutrient availability and crop nutritional status; synchronization between nutrient requirements by the crops and their availability in soil volume where roots are present; fertilization techniques and their efficiency; soil management techniques and water availability linked to natural conditions(rainfall) or irrigation practice. Probably, a longer brewing period could allow a greater amount of nutrients to be extracted from the compost [36]. Generally, additives combined to the starting compost allowed to increase TOC and humic and fulvic acid concentrations within the final CTs. These organic matter forms, distributed in the soil by means of frequent CT applications, have important agronomic implications: they improve soil fertility, provide for labile nutrients and create an environment useful for microbial proliferation and activity, increase soil water retention [37; 38]. The latter is a positive effect, especially in arid climates where irrigation water may be limited and high air temperatures quickens soil mineralization processes. In addition, humic substances can incite plant biostimulations by affecting both nutrient uptake and plant metabolism [39]. All such conditions create balanced and high-performance crop systems, able to cut plant chemical needs and reduce external inputs (pesticides, fertilizers). The high capacity to extract metals from compost by the used additives suggests to take them into account in CT field application scheduling. The amount of elements with nutritional functions for crops, such as Ca, Mg and K, applied by means of CTs are low (data not shown) and, therefore, should be integrated according to the rules of sustainable fertilization. Monitoring of Ca/Mg and K/Mg rations in compost extracts is recommended to avoid possible equilibrium changes of these elements into the soil, which could affect crop nutrition. In order to determine whether heavy metal concentrations were of concern in terms of negative impacts on the soil system, they were compared with the chemical limits, where available, imposed by Italian legislation, which regulates the reuse of wastewater for irrigation purposes (Decree No. 185, 12/06/2003) [40]. With the exception of Cu, Fe and Mn, the contents were found to be below the limits permitted by law (Table 4).