Dear Colleagues,

This Special Issue focuses on improving the sustainability of organic fertilization for tree crops by improving soil quality characteristics, as well as on soil management techniques that could promote the conservation of tree crop systems and thus contribute to the sustainability of natural resources. In the context of the variability of climate-related risks across productive regions, soil management strategies and processes need to be adjusted to ensure we are using the best available practices, policies, and tools, including financial instruments. Soil quality is an ideal indicator of sustainability in the overall functioning of an ecosystem, and organic matter plays a key role in determining soil quality, thus its preservation is critical. Soil quality is essential for carbon sequestration, leading to further positive effects such as soil protection from runoff on groundwater and aquatic bodies, the improvement of soil water content, a lower bulk density in surface soil, and reduced soil erosion. However, the limited implementation of practices to enhance soil organic carbon (e.g., the application of organic materials, reduced tillage, no tillage) has been recorded in many regions, particularly in those subjected to land degradation. Soil conservation practices, when combined with the reduced use of chemical fertilizers or organic inputs, cause a reduction in soil erosion and an increase in soil fertility.

Compared to chemical fertilizers, organic fertilization preserves soil quality since the addition of organic materials to the soil is of paramount importance for improving soil properties because it increases water infiltration in the soil, nutrients absorption by the crops, and C assimilation. Organic fertilization has been proposed as an alternative method of providing nutrients to crops in the context of organic and sustainable agriculture, with the objectives of reducing chemical fertilization rates, preserving the environment, improving soil fertility and the biodiversity of soil microbes, enhancing plant nutrition, and maintaining the productivity of tree crops while reducing production costs. Composts, vermicomposts, biochars, humic compounds, and microorganisms in biofertilizers are all examples of organic fertilizers. In recent years, the principles of sustainable development and the circular economy have incorporated the quality and health of soil fertilized with organic matter from treated wastes (e.g., originated from olive mill wastewater, manures) as well as the addition of crop residues and the introduction of cover crops. When such alternative techniques are combined with conservation tillage practices or even with conventional tillage practices, sustainable soil management can be achieved. Different fertilization regimes differentially impact tree yields, soil attributes, soil biodiversity, and the ecosystem functioning of soil microbial communities. Recent research confirms the viability of using multiple fertilizer mixes to enhance soil quality and productivity and reveals the fundamental role soil microbiological diversity plays in maintaining the economic viability of intensively farmed tree crops. The growth of beneficial soil microorganisms and the improvement of soil physical properties and fertility are two of the most significant advantages of organic fertilizers. However, one of the most significant disadvantages of organic fertilizer is its inability to directly satisfy the immediate nutritional requirements of tree crops, particularly those of nitrogen, due to its slow mineralization rates. To reduce the utilization rate of inorganic fertilizers and to clarify the complex ways in which shifts in fertilization practices may affect soil quality and nutrient dynamics, it is vital to employ sustainable soil management approaches and a variety of potentially available organic materials. The proposed approaches can be used as guidelines to assist stakeholders in sensitive areas in adapting to climate change and enhancing tree crop ecosystems.

The following are some topics which contributions are encouraged to focus on:

• Benefits and drawbacks of inorganic fertilization and organic fertilization in tree crops and conventional and alternative soil management practices. Factors influencing the shift from conventional fertilization to organic fertilization. Combined fertilization as an alternative to conventional fertilization.  Regional and local socio-economic and biophysical features and policy opportunities and constraints.
• Innovative or targeted organic fertilization strategies and soil management practices. Approaches and technologies. Innovative and alternative organic soil amendments utilized as biofertilizers in intensively managed tree crops in the context of fertilization practices, environmental change, and decision making.
• Effects of organic and mineral fertilization on soil quality, tree yield, and nutrition. Synergies of organic fertilization with other sustainable agricultural practices. Organic and mineral fertilization interactions that substantially improve soil fertility and the growth and yield of commercially or ecologically important trees. Organic fertilizer sources that promote tree–soil–microbial interactions. The relationship between soil microorganisms and tree yield under distinct fertilization regimes.
• Adoption of sustainable organic fertilization and soil management practices. Critical factors and future requirements. Technology transfer and research priorities for different regions. Agronomic, socio-economic, cultural, and policy conditions necessary for technology adoption by stakeholders. Proposals for appropriate local and regional policies. 

Dr. Victor Kavvadias
Guest Editor

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• organic fertilization
• soil quality
• soil management techniques
• soil conservation practices