To ensure soil quality and soil health, it is necessary to improve fertilization practices while minimizing environmental impacts. The aim of this study was to record the state of the art on soil fertility related to fertilization management (organic and/or mineral) and to detect environmental challenges in highly productive fields. A soil survey was set up in a new irrigated area (c. 20 years old), in the north-eastern part of Spain, which is mainly devoted to double annual crop rotations of cereals and maize. The area also supports an important animal rearing activity. The survey covered 733 ha of calcareous soils, owned by 35 farmers. At each farm, fertilization management was recorded, and soil was analyzed for nutrients and heavy metals. Multivariate analyses were performed. Total N, P, Cu and Zn, and available P, Cu, Zn and Mn soil concentrations were associated to the use of organic amendments. Heavy metals concentrations were below established thresholds. Available P (Olsen-P) was identified as an indicator of the previously adopted fertilization management and of the potential of P leaching towards deeper soil layers. Regression analyses were performed. A displacement of available P from the uppermost layer (0–0.3 m) occurs in the breakpoint of 86 mg P kg⁻¹ soil. Preventative actions might be established from 53 mg P kg⁻¹ soil due to the slowdown in P immobilization. Our results reinforce the importance of setting up P threshold soil levels for best practices of fertilization, as a basis for sustainable agriculture intensification.

Organic fertilization is a key issue in European Union (EU) regulations, particularly in the context of promoting a circular nutrient economy, maintaining soil quality, and sequestering carbon to face climate change. In a rainfed system in Northeastern Spain, an experiment was set up (split-plot design). It included five pre-sowing N fertilization treatments: control, mineral, pig slurry, and composted sewage sludge (two rates). The average N rates were 0, 30, 141, 176, and 351 kg N ha⁻¹, respectively. They were combined with mineral N topdressings (0, 50, and 100 kg N ha⁻¹). Three crops were grown: barley (nine years), wheat (three years), and rapeseed (one year). In the driest years (c. 350 mm rainfall), the yields averaged 2.5, 2.0, and 1.9 Mg ha⁻¹, respectively. The maximum yields were for barley (6.5 Mg ha⁻¹) and wheat (5.5 Mg ha⁻¹). The avoidance of a significant increase in soil residual NO₃⁻-N, plus the control of soil build up of available P, micronutrients, and Cd, defines the fertilization strategies. (i) With a previous spring drought season, no fertilization is needed in the following year, if devoted to winter cereals. (ii) In rainier seasons, pig slurry or composted sewage sludge (lowest rate) applied at sowing is sufficient; however, 50 kg of mineral-N ha⁻¹ at the topdressing can be applied. The study found that pig slurry favors K, Mg, Cu, and Zn availability, while composted sewage sludge enhances Fe availability. Although it is possible to reduce N inputs from organic fertilizers, organic C build-up will be constrained.

Crop diversification and the reduction of nitrogen (N) inputs are key issues in the EU for more sustainable agriculture. An experiment was set up in a semiarid rainfed Mediterranean system. Our hypothesis was that these challenges could be addressed by introducing new crops and using pig slurries (PSs). The experimental factors were N fertilization at sowing (with or without PS) combined (according to a split-block design) with N fertilization as topdressing (the control, two N mineral rates, and two N rates from PS). Barley, rapeseed, and pea performances were evaluated in two different crop sequences: (i) barley–rapeseed or rapeseed–barley after a fallow season, and (ii) barley–pea or pea–barley after a fallow season followed by a non-fertilized barley crop. The results of the four-year study demonstrated that under a spring drought risk, barley performed better than peas in terms of relative crop yield maintenance. After fallow, N can be saved while maintaining the yields and total biomass of barley and rapeseed. In the second crop sequence, maximum pea and barley yields were associated with a minimum topdressing of 60 or 120 kg mineral N ha⁻¹, respectively. However, slurry fertilization at sowing also allowed the highest yields for barley. Rapeseed and peas can be introduced to reduce N fertilization inputs. However, the obtained yield plateau for pea and rapeseed (3 and 4 Mg ha⁻¹, respectively) and the effect of a yield spring drought on pea yields (50% reduction) might be a constraint for the success of EU policies on crop diversification.

An increase in nitrogen (N) use efficiency (NUE) in agriculture is a requirement for sustainable development. The reduction in nitrogen inputs might benefit from the residual N of former organic fertilization procedures. A 10 year experiment was established in a rainfed Mediterranean system (barley–wheat rotation). The objective of this experiment was to quantify the N residual effects of a single pig slurry (PS) application at sowing (20, 40, and 80 m³ ha⁻¹) and up to three years later. The mineral N equivalence method was used to compare the grain yield (GY) and the whole-plant N uptake (WPNU) between the slurry fertilized plots (slurries applied in previous years) and annual mineral N fertilized ones (30, 60, 90, 120, and 150 kg N ha⁻¹). From the total N applied with PS, a fraction of, ca., 21% accounted for the residual equivalent mineral N for a total period of three years after the PS application. For the 20 m³ ha⁻¹ rate, the relative residual N based on the GY and WPNU equaled to 89–90% of the applied organic N, respectively. This rate also allowed for an increase in NUE values to above 33%. In semiarid areas, the introduction of residual N when scheduling fertilization is important to reduce N inputs and to increase NUE.