## 4 Discussion
Finally, it was found that a significantly higher tree height was recorded in apple trees fertilized with cow manure and compost compared to cases when they were grown on other organic mulches (barley straw, chipped pine bark, forest humus, commercial peat moss, and commercial mycorrhizal substrate) [10]. Although no direct comparison of our data with those of [PERSON] et al. (2018) [10] can be performed due to the different experimental conditions (a pot experiment vs. a field study, respectively), it cannot be neglected that these results are partially in disagreement with our data. The leaf N and K concentrations were significantly higher either in the SM or in the SM + EBLS treatments compared to the control (Table 4), which shows the beneficial effect of these organic amendments on enhancing the N and K uptakes by olive plants. [PERSON] et al. (2021) [37] found that the N content of kernels in almond trees did not significantly differ among the organic treatments (composts elaborated from different kinds of waste, such as sheep manure) and the control, while the K content was significantly lower in the control compared to three out of the four compposts used as biofertilizers. This partially agrees with our data for the K uptake, although the experimental conditions were different (a field experiment in the study of [PERSON] et al. (2021) and a pot experiment in our case). Similarly, for Zn, the SM and EBLS applications positively influenced the Zn uptake by the olive plants, as indicated by the data shown in Table 5, which also agrees with the results of [PERSON] et al. (2021) for the Zn kernel content among the organic treatments and the control trees. In the study of [PERSON] et al. (2018) [10], it was found that cow manure, compost, and forest humus applications had similar beneficial effects on the foliar nutrient contents of apple trees [10]. With regard to the total plant nutrient contents, significant differences among the treatments were recorded only in the cases for the P, Mn, Zn, and Cu contents (Figures 1 and 2). These differences may be explained as follows: (i) differences in the dry weight of the plant tissues (Table 3); (ii) differences in the nutrient uptake (Tables 4 and 5) among the treatments; or (iii) differences in both of them. Although non-significant differences in the stomata opening were determined among the treatments, the photosynthetic rate was significantly lower in the SM compared to the SM + EBLS (Figure 3A,B). This could probably be attributed to the significantly lower intercellular CO\({}_{2}\) concentration determined in the SM (Figure 3C). Despite the fact that the differences in the stomatal conductance were not significant among the four treatments (Figure 3B), the similar tendency in the stomata opening and intercellular CO\({}_{2}\) concentration (i.e., in the SM and the control, slightly lower stomatal conductance values were determined) (Figure 3B,C) may explain the differences recorded in the intercellular CO\({}_{2}\) between the following treatments: (i) SM and control and (ii) EBLS and SM + EBLS (Figure 3C). Cow manure supplies significantly influenced the stomata opening, intercellular CO\({}_{2}\) concentration, and photosynthetic rates of apple trees compared to the control [39], which partially confirms our data (only for CO\({}_{2}\) concentration, but not for stomata opening or photosynthetic rate, where the differences were insignificant). In our case, the most beneficial strategy to boost the stomata opening, intercellular CO\({}_{2}\) concentration, and photosynthetic rate of olive plants was the combinational supply of SM and EBLS (Figure 3A-C). Finally, a study on cow manure (CM) and goat manure (GM) applications found that the optimum strategy to boost the photosynthetic rate of olive plants was GM application, while the most beneficial effect for the increase in stomata opening derived from the combinational supply of CM and GM [5], which means that CM failed to increase stomatal conductance.