As already observed for the above discussed parameters, peaks of nitrate concentration were also detected for soil S1, especially for treatments T1, T3 and T4 (Fig. 3, lower right), whereas for soil S2 no significant changes were found throughout the test for all treatments. The sudden decrease of concentration near the end of the test may also be explained by the rain water input after five months. From data in Fig. 4 it is evident that the average concentration of nitrate in the leachate from the control lysimeters (i.e., those fed with freshwater) was not negligible, and it further increased in lysimeters fed with olive mill wastes. In soil S1 this effect was especially evident, as the outcoming nitrate was twice over the control in all treatments, except for T2. These results may be ascribed to the effect of slow mineralization of organic nitrogen, both originally present in the soils and supplied by treatments. It may be supposed that the degradation of organic nitrogen was promoted in soil S1, due to the slow leaching of water already discussed in previous sections, whereas in soil S2 the short time required by water to pass through the soil column did not allow complete biodegradation. Based on the above observations, the amount of nitrate potentially reaching groundwater was highly dependent on the hydraulic characteristics of the specific soil, and it was only slightly influenced by the amount and composition of OMW. On the contrary, the application of exhausted pomace compost resulted safe for both the investigated soils.