## 1 Introduction  
Organic fertilization is an alternative approach to supplying the critical quantities of nutrients for crops in the framework of organic and sustainable agriculture without negatively influencing crop yields or the environment [1]. In many cases, the overuse of inorganic fertilizers has led to the following: (i) soil quality deterioration; (ii) surface and groundwater pollution; (iii) increased greenhouse gas emissions [2]. Thus, as a result of the recent energy crisis, which has increased the prices of inorganic fertilizers and the cost of production for farmers, it is expected that organic fertilization will play a crucial role in the near future in decreasing inorganic fertilization inputs and improving farmer incomes. There is a strong interest in organic fertilization [3]. Apart from the beneficial impact of organic fertilizers at reducing production costs, other beneficial roles of organic fertilizers are as follows: (i) increasing soil organic C and matter [4; 5; 6]; (ii) enhancing soil microbial activities and improving soil quality [7; 8]; (iii) boosting yields [8; 9; 10]; (iv) ameliorating fruit quality [8; 11]; (v) increasing root production [12]. Different kinds of organic materials and agricultural/food industry bio-products (e.g., olive mill waste compots and pomace, other kinds of composets, municipal wastes, and recycled pruning materials) are used as soil amendments and bio-fertilizers for crops [13; 14; 15; 16; 17; 18]. In other cases, in order to support organic farming and ecosystem services, decrease fertilizer inputs, and enhance the sustainable management of agricultural ecosystems, the integration of laying hens with orchards and the use of cover crops were suggested [15; 19].  
Animal manures are included among the commonest, cheapest, and easily used organic fertilizers in tree orchards. Manures from different animal origins (e.g., cow, goat, sheep, horse, poultry, and pig) may be used as soil amendments to improve organic C, enhance nutrient uptake, and support plant growth [5]. According to [PERSON] (1996) [20], animal manures differ in nutrient content. These differences in nutrient content may be owed either to the kind of animal, animal feeding, or manure preservation [20]. [PERSON] et al. (2020) [5] realized a comparative study among cow and goat manure supplies and the inorganic fertilization of olive plants and showed the beneficial role of both manures to improve organic C, soil fertility, and influence plants' nutrition and physiology (including WUEi and translocated sugars). These beneficial effects of manure applications depend not only on the kind of animal but also on the rate at which the manure is applied. According to [PERSON] and [PERSON] (2016) [21], both the rate of sheep manure application and the application method had a significant impact on the growth, fruit setting, and fruit qualitative characteristics of guava (cv. \"Balady\") trees. According to our knowledge, no further investigations were realized on the effects of sheep manure on the nutrition and physiological performance of olive trees, while no studies at all were realized on the influence of well-decomposed litter from indigenous field vegetation of evergreen broadleaf species (such as that of _Quercus_ sp.) on the soil's nutrient availability and nutrient absorption by crops. Based on the above published information, our study was based on the premise that SM and EBLS could influence the nutrient uptake, photosynthetic rate, and WUEi of olive plants.  
The aim of our study was to investigate the influence of the following: (i) sheep manure and (ii) well-decomposed litter from indigenous field _Quercus_ sp. vegetation (as well as their combinational application) on the growth, nutrition, photosynthetic rate, and leaf sugar concentrations in potted olives (_Olea europaea_ L., cv. \"Koroneiki\"). \"Koroneiki\" was selected for the investigation since it is the most well-known Greek olive cultivar producing high-quality oil (rich in oleuropein).