195 CONTENT AND RESERVES OF TOTAL NITROGEN AND PHOSPHORUS FORMS IN THE UPPER PART OF THE ROOT-DEEP LAYER UNDER 40-YEAR SYSTEMATIC MINERAL FERTILIZATION IN TWO-FIELD CROP ROTATION Margarita Nankova 1 , Peter Yankov 2 , Genoveva Bankova-Atanasova 1 1 Dobrudzha Agricultural Institute — General Toshevo 2 Technical University — Varna Abstract The investigation was carried out in a stationary field trial initiated in 1967 at two-field rotation of wheat and maize. The influence of forty-years application of the following fertilizer combinations: N 0 P 0 K 0 , N 6 P 0 K 0 , N 12 P 0 K 0 , N 18 P 0 K 0 , N 0 P 18 K 0 , N 6 P 18 K 0 , N 12 P 12 K 12 and N 18 P 6 K 6 on the content and reserves of total nitrogen and phosphorus and on the bulk density of soil have been tested. The soil samples were taken down the soil profile in the 0-10, 10-20, 20-40 and 40-60 cm layers. The systematic mineral fertilization caused significant differentiation in the content of the total forms of nitrogen and phosphorus and the reserves from them in the upper part of the root-deep layer (0-60 cm). Highest concentration of total nitrogen was found in the variants with independent nitrogen fertilization at low (6 kg N/da) and moderate (12 kg N/ da) norms, which exceeded the check variant with 22.2% and 19.7%, respectively. They formed also higher sum reserves till depth 60 cm. The concentration and reserved of total phosphorus were highest in the variants involving the phosphorus from norms 18 kg P 2 O 5 /da. In the 20-40 cm layer, 36.2% of nitrogen reserves and 35.8% of phosphorus reserves were concentrated, which made this layer richer in nutrients in comparison to the other investigated layers. The systematic mineral fertilization had significant effect on the bulk density of soil. Till depth 60 cm, the bulk density was highest under independent fertilization with 18 kg P 2 O 5 /da, followed by independent nitrogen fertilization with 18 kg. N/da. The 20-40 cm layer had highest density, followed by the 10- 20 cm layer. The concentration and reserves of total phosphorus were in better expressed correlation with soil bulk density in comparison to total nitrogen. Key words: 40 — years mineral fertilization, total N, total Ð, NP — reserves, bulk density. INTRODUCTION Transportation, transformation and redistribution of nutrients down the soil profile is influenced by a number of factors, such as soil reaction, aeration, structure, mineral fertilization, productive moisture and other agro physical and agro chemical soil characteristics (Goldbi et al., 1995; Karlen et al., 1998). The investigations on the ecological status of the slightly leached chernozem in South Dobrudzha region encompass primarily the systematic and long-term effect of some agronomy practices, mainly mineral fertilization, which has been an object of the research program of the Agro Technology Department of ÀÃÐÎÒÅÕÍÈ×ÅÑÊÈ ÈÇÑËÅÄÂÀÍÈß ÇÀ ÓÑÒÎÉ×ÈÂÎ ÏÐÎÈÇÂÎÄÑÒÂÎ ÍÀ ÏÎËÑÊÈ ÊÓËÒÓÐÈ