Olive oil production generates high amounts of liquid and solid wastes. For a long time, such complex matrices were considered only as an environmental issue, due to their polluting properties. On the other hand, olive mill wastes (OMWs) exert a positive effect on plant growth when applied to soil due to the high content of organic matter and mineral nutrients. Moreover, OMWs also exhibit antimicrobial activity and protective properties against plant pathogens possibly due to the presence of bioactive molecules including phenols and polysaccharides. This review covers the recent advances made in the identification, isolation, and characterization of OMW-derived bioactive molecules able to influence important plant processes such as plant growth and defend against pathogens. Such studies are relevant from different points of view. First, basic research in plant biology may benefit from the isolation and characterization of new biomolecules to be potentially applied in crop growth and protection against diseases. Moreover, the valorization of waste materials is necessary for the development of a circular economy, which is foreseen to drive the future development of a more sustainable agriculture.

A field experiment was conducted on an alluvial soil with winter lettuce [Lactuca sativa (L.) cv. Bacio] plants to study, in the second year of testing and in both field and laboratory conditions, the effects of fertilizers—compost by farm residues [(FC, 2.0% nitrogen (N)], mineral (MIN, 21% N), organomineral (OM, 10% N), mycorrhizas inoculum (MYC), wine-production residue [distiller's residue (DR, 3.6% N), anaerobic digestate (AD), and mycorrhizas (ADM)—on soil physical and mechanical parameters. Application of DR plus mycorrhizas improved the available phosphorus (P) and copper (Cu) contents of the surface (0–0.20 m) soil. The soil surface macroporosity increased significantly (>20%) in treatments FC, MYC, and OM, compared to the control. Results of the oedemeter test showed that with a load of 10 daN the compression susceptibility was greater in the compost treatment compared to other fertilizers, whereas with a load of 200 daN this parameter was lower in FC, AD, MYC, and ADM treatments. The least penetration resistance in the 0- to 20-cm layer of AD and MYC treatments showed lesser values and only in treatment AD in the 20- to 40-cm layer. Hydraulic conductivity values were enhanced in AD, MYC, and ADM plots.

Organic mulching is a promising technique for sustainable weed control and soil management, as it enhances crop growth, soil quality, water retention, and erosion control. This research evaluated the effects of organic mulches—wheat straw, wood chips, spray cellulose pulp, compost, and a cover crop mixture—on the physical–mechanical properties of organic garden soil transitioning to natural farming. The controlled soil received no mulch. The soil was fertilized with mature bovine manure prior to a three-year crop rotation of tomato, lettuce, and savoy cabbage. Mulching occurred after the second harrowing and before transplanting. Soil analyses were conducted to assess changes after three years. Soil organic carbon levels increased significantly in soils treated with compost, cover crops, or chipped wood mulching (6.81, 3.17, and 2.07%, respectively) compared to other treatments (1.24% in the control plot). Different kinds of mulch had a significant impact on soil’s physical–mechanical parameters. Compost, compared to the control, decreased the bulk density (from 1.22 to 0.89 Mg m⁻³), increased the infiltration rate (from 8.53 to 21.07 L m⁻²), and reduced compressive deformation (from 37.08 to 18.23%). The composition of mulch materials, specifically their nitrogen and carbon concentrations, C/N ratio, and moisture content, plays a significant role in influencing changes in soil properties.

As soil phosphorus (P) availability changes with soil type and time after fertilizer application, use of a P availability index, “F”; (fraction of added P remaining available after a given time), was evaluated for 28 Portuguese soils as a function of soil P extractant (Egner, Fe‐oxide strip, Mehlich 3, and Olsen). The F index values, based on a short‐term P recovery after 2‐h and 24‐h soil‐P equilibration periods, were related to longer‐term F values determined after a 90 day soil‐P equilibration of soil and P (r=0.87 to 0.98; P<0.001). Soil sorption was related to F values (r=0.73 to 0.88; P<0.001). The results suggest that the 2‐h and 24‐h equilibration procedures could be used as a rapid prediction of P fertilizer availability in Portuguese soils widely ranging in P sorptivity.

The current guidelines of waste management are aimed at the recovery and recycling of biowaste while respecting the protection of human health and the environment. The recent European legislation on fertilizers provides for the use of digestates derived from the organic fraction of municipal solid waste (OFMSW). The objectives of this study were to verify the fertilizing effect of three types of OFMSW digestates on the ryegrass culture comparing mineral fertilization and to evaluate the nitrogen lost to leaching in soil diversely fertilized following simulated rainfall. The ryegrass was grown in pots. The soil was fertilized with mineral fertilizer or OFMSW digestates. For each treatment, five mowing procedures were performed on the crop, and the ryegrass biomass production and nitrogen concentration were determined from the ryegrass samples. During the experiment, six rains were simulated, and the leached nitrogen was analyzed. The results showed that: (i) the fertilizing effect of OFMSW digestates on nitrogen nutrition of ryegrass was similar to ammonium sulphate fertilization, confirming the agronomic validity of these by-products to reduce the use of synthetic fertilizers; (ii) soil fertilization with OFMSW digestates had a positive effect limiting nitrogen loss due to leaching compared to mineral fertilization, highlighting the soil-improving properties of these by-products, in particular of the composted digestate.