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The difference between the maximum and minimum values of the measured heights is the highest in SP2 (2.60 m). The variation of this indicator in relation to the average height is significantly smaller (the variance has values in the range of 0.08 - 0.41). In the four case studied, the greater number of heights are centered around the arithmetic mean, with standard deviation values in the range 0.20 - 0.57 m. Values of the variation coefficient (Vh) do not exceed 11.5%. Its variation is from slight and almost equal in SP1 (7.5%); SP2 (7.8%) and SP4 (7.7%) to moderate for SP3 (11.4%). The variation of heights in the sample plots with younger plantations (SP1 and SP3) differs with negative (left) asymmetry, whereas the older plantations (SP2 and SP4) with positive (right) asymmetry. Deflection of variation curve versus normal height distribution curve is characterized by the rises and falls (due to positive and negative Kurtosis), with the highest elevation in SP3 (Kurtosis 4.97).
Essential for the development of saplings is the quality of soils. There are studies that confirm the existence of a direct relationship between the height of tree plants and soil conditions (Duhovnikov et al., 1975). Land use type can influence soil properties (Göl and Yilmaz, 2017) and in this respect is interesting to analyse soil properties under agroforestry practices.
Table 3 presents the main characteristics of soils: pH, C/N ratio and the stock of C and N - recalculated in t/ha in 1 cm of soil with an average bulk density taken as 1.5 g / m2. Stocks are calculated using the formula:
X = A * H * Q * K
A = C% or N%; H = 1 cm; Q = 1.5 g / m2; K = coefficient for relaying the soil skeleton (= 0.95 in 3% skeleton).
According to the scale of Geliaskov (Donov, 1993) the soils from the SPs fall into the range of slightly acidic soils (pH = 5.7 - 6.5). Only the soil from control (SP4) is within the range of neutral soils. It can be assumed that the planting of vegetable crops implies weakly acidifying of the soils in the area. The calculated carbon and nitrogen stocks in the soils show that the soils in the area are poorly stocked. Organic content is low, the lowest is in the control (SP4).
By studing soils from the Danube, Mihaylov (1988) also found very low and low organic carbon content in these soils (<2%), which shows that this is a characteristic feature of the Danube soils in Bulgaria. We find one exception for SP1 soils that are medium stocked with organic matter. In this sample plot, maize is currently grown, and it can be concluded that this favors the organic stock in this soil. With regard to the total nitrogen content it is very low everywhere - below 1 t / ha. High summer temperatures and low summer rainfall in the region are one of the reasons for