Olive cultivation is considered as one of the most significant agricultural activities in Greece, from a financial, social, and ecological point of view. Intensive cultivation practices in combination with the Mediterranean climate, lead to depletion of soil organic matter, erosion, desertification, and degradation of water resources. This paper describes sustainable olive crop management practices that were comparatively applied in 120 olive groves in Greece for 5 years with the participation of three farmers groups. Organic materials recycled in the olive groves during the present study were valuable sources of carbon, nitrogen, phosphorus, and potassium. Carbon content was highest in pruning residue (53.8–54.2%) while all materials studied were considered rich in C ranging between 41.9–46.2% (compost) and 34.9–42.5% (three-phase olive mill waste-OMW). The highest content in nitrogen was detected in compost (2–2.45%) followed by pruning residue (0.93–0.99%) and OMW (0.03–0.1%). Compost was considered a good source of phosphorus (0.3–0.6%) followed by pruning residue (0.08–0.13%) and OMW (0.01–0.3%). Potassium was also considerable in the organic materials recycled ranging 0.5–1.5% in compost followed by pruning residue (0.5–0.7%) and OMW (0.3–1.1%). Adoption of modified pruning also had important contribution toward sustainable management of olive trees. Sustainable pruning resulted in a well-balanced ratio between vegetative growth and fruiting (balanced, every year, in order to eradicate biennial bearing). Significant fluctuation in olive yields was observed in the first years of the project while yields were gradually stabilised by applying sustainable crop management. In parallel, yield increase without additional inputs, lowers the carbon—environmental footprint of the product regarding several environmental impact categories. Results can be integrated in the national agricultural and environmental policy in Mediterranean countries toward the achievement of a circular economy.

Organic fertilization has been proposed as an alternative approach to supply nutrients for crops, in the frame of organic and sustainable agriculture, with the aim to decrease high inorganic fertilization rates, protect the environment and decrease production costs for farmers. Since different types of organic fertilizers, such as manures, olive mill wastewater (OMW), sewage sludge (SS), crushed pruning wastes, composts and cover crops, exist as soil amendments to improve soil fertility, enhance plant nutrition and sustain the productivity of tree crops, their role as biofertilizers has been fully analyzed under the most important published papers. In addition, the benefits and drawbacks of organic fertilization, in a comparative approach with inorganic fertilization, are presented and discussed. Within the most important advantages of organic fertilizers, the enhancement of beneficial soil microorganisms and the improvement in soil physical properties and fertility should be included, while their most important disadvantage is their inability to directly satisfy the prompt N nutritional needs of tree crops, due to slow N mineralization rates. Finally, some novel aspects on the interrelation among innovative organic fertilizers for tree crops, sustainable field management, crop productivity and fruit quality are also included in this review, under the light of the most important and recent research data existing in the literature, with the aim to provide recommendations and future directions for organic fertilizers by tree growers.

The proper management of treated agricultural wastes (e.g., composts) contributes to the protection of water and soil quality by reducing the use of chemical fertilizers, lowering leachate, and protecting renewable and nonrenewable resources. Natural zeolites, particularly clinoptilolite, can be used in agriculture to improve soil quality and increase yields due to their unique properties. The objective of the study was to test the effects of the co-addition of compost, zeolite and ammonium-based fertilizer on the Above-ground Fresh Weight (AFW) of lettuce (Lactuca sativa L.), leaf nutrients and soil fertility. To this aim, a soil pot experiment was carried out at the Department of Soil Science of Athens, which is located in the region of Attica, in the area of central Greece. Two levels of olive compost originated from olive leaves (0% and 10% v/v), three levels of zeolite (0%, 2%, and 5% w/w) and two levels of chemical fertilization (no fertilization and NPK fertilization) were combined. Furthermore, two different soils were introduced, one moderately acidic (pH = 5.6) and sandy loam in texture (Ac-LT), and the other slightly alkaline (pH = 7.7) and sandy clay in texture (Al-HT). Results showed that the response of lettuce yield to chemical fertilization and zeolite application is soil type-dependent, whereas compost application significantly improved AFW in both soil types. The availability of macronutrients (P, K, and Na) in the soil, as well as their concentration in leaves, were significantly increased by NPK fertilization in most cases. Conversely, the impact of inorganic fertilization on DTPA extractable micronutrients and leaf micronutrient contents was found to be associated with the type of soil. The study recorded a significant reduction in available Fe, Cu, and Mn in AL-HT soil, whereas DTPA-Mn and -Zn were significantly enhanced in Ac-LT soil. Comparable patterns were also documented for the micronutrient concentration in leaves. In most cases, compost application had significant and beneficial effects on plant nutrients. On the contrary, different responses of soil properties to compost addition were registered. The main effect of compost treatment on soil pH, EC, SOM, total N, and available P was significant and positive in both soil types, except for pH and EC in Al-HT soil. On the other hand, exchangeable K and Na were significantly reduced by compost. Zeolite substantially increased the availability of P, K, and Na in soil and plants, whereas the concentrations of DTPA-extractable micronutrients and leaf macronutrients were largely unaffected. In addition, the results of our study indicated that co-additions of organic and inorganic amendments did not yield any significant impact on the lettuce yield, leaf nutrient content and soil fertility. It is suggested that the degree of changes in main soil properties (e.g., pH, EC, SOM) as a result of amendment application as well as the interaction of the amendments with nutrient availability are strongly related to soil type.

Agricultural residues are generated during the production and processing of agricultural crops. Under modern date palm plantation practices, field operations generate huge quantities of residues, which are discarded with little valorization. The date palm agro-industry produces significant amounts of waste. The accumulation of these residues can cause ecological damage to the oasis ecosystems. There is a lack of comprehensive data on long-term research studies that aim to assess the impact of date palm waste management practices. Composting and/or pyrolysis of date palm residues showed benefits for improving soil physical and chemical properties, particularly in sandy soils. This claim holds particular significance for arid and semi-arid regions, which are characterized by low fertility and are susceptible to soil degradation, accentuated by ongoing climate change. This review summarizes the existing literature concerning the valorization of date palm residues with regards to compost and pyrolysis processes, as well as the impact of their application on soil quality. Further research is required to assess the effects of using date palm residues for better soil amendment management. Research should focus on composting and biochar technologies for date palm residues and their application in arid and semi-arid regions to combat soil erosion and degradation. Increasing the beneficial uses of date palm residues could lead to sustainable and economic growth in dry areas.