**4. Conclusions**

The application of the Verdiberia-ASP-NP (Plateau-ASP-ActiSolids©) biosolid, which origi‐ nates from the sludge of urban treatment plants, was demonstrated to be suitable for maize production (*Zea mays* L., Automat-Advanta variety). This suitability can significantly increase the yield compared with equivalent mineral fertiliser.

This product does not present a risk of increasing the heavy metal concentration; considering the elemental concentration of the biosolid in total quantities (kg ha−1), the resulting values were substantially lower than the established normative limits.

From the analysis of temporal variation, the values measured for the C/N, C/P and N/P ratios during the evaluation time period indicate that sludge rapidly incorporated into the soil with a high rate of mineralisation.

Although the application of this biosolid produced a slight decrease in the pH value at the end of the harvest period, this tendency is not present for the percentage of Al saturation. The availability of nutrients is not negatively affected.

The doses that were employed in this study administered a level of nutrients (NPK) that was adequate for crop development; at the end of the harvest, an excess of available P was detected in the soil. This result may represent a potential problem and the cause of eutrophication of the ground water layers.

Significant relationships between the contribution of nitrogen and phosphorous and the production indicators have not been observed. The rate of mineralisation of the biosolid should be established prior to use in field applications to adequately administer the correct dose depending on the production crop.
