Conclusions
The application of PS as a fertilizer (45 m3 ha1) in a sprinkler-irrigated maize crop did not improve yields compared to plots treated with mineral fertilizer containing a similar dose of N (300 kg ha1). A better strategy is therefore required to maximize the benefits of PS in the Ebro Valley, e.g. novel application methods such as immediate ploughing to prevent volatilization of the N content. The positive effects of organic fertilizers may take several years to realize, possibly because the nutrients are initially immobilized (as organic compounds and microbial biomass) but are later released causing the soil quality to improve. The application of PS resulted in marginally higher soil quality parameters in both test years, but the most significant impact was on earthworm abundance, a key soil quality indicator that is linked to other parameters such as soil moisture and resistance to penetration. The most important findings were that the repeated application of PS to soil under irrigated semiarid conditions has a beneficial impact on the soil quality without increasing maize yields, and that earthworm abundance is an inexpensive and early indicator of soil quality.