**3. Conclusion**

The systematic mineral fertilization for a period of 40 years with different norms and at different ratios between nitrogen, phosphorus and potassium had high effect on the agrochemical condition of slightly leached chernozem (Haplic Chernozems) down the soil profile.

The soil acidity forms, averaged for the investigated depth of the 0-400 cm profile, were significantly affected by the type of fertilizer combination. The depth was the factor with decisive effect in all forms of soil acidity. Influence of mineral fertilization was higher on exchangeable Al3+, Ca2+ and the ∑Са+Mg, and significantly lower - on the values of residual hydrolytic acidity and the rate of alkali saturation. The amount of exchangeable Mg2+ had a clear tendency toward increasing down the soil profile

Independent long-term mineral fertilization with 180 kg N/ha and with N180P60K60 caused the occurrence of exchangeable Al3+ in the soil absorption complex in the surface layers 0-10 and 10-20 cm. It was not present further down the profile.

The variant with independent nitrogen fertilization with 180 kg/ha has also the lowest ∑Са+Mg and the lowest value of sorption capacity. Intensive mineral fertilization with N180P60K60 caused decreasing of degree of saturation with bases.

The value of the pH, sorption capacity, acidity on strongly acid positions and degree of saturation with bases showed a tendency of decreasing at the end of 40th year of trail beginning comparing the end of 30th year. In the same way independently of fertilizer variant the acidity on the slightly acid positions is strongly increased.

The mineralization ability down the soil profile was affected to a maximum degree of significance by the mineral fertilization and the incubation. Depth had decisive effect on the value of the index. The maximum effect of this factor was registered after 28-day incubation – 92.3 %. The role of mineral fertilization on nitrogen mineralization according to the incubation period was significantly less expressed – 9.2 %, 3.2 5 and 4.0 %, respectively. The amount of nitrified nitrogen increased with the longer incubation periods with 52.7 % (28 days) and with 118.8 % (56 days), respectively, in comparison to 14-day incubation. The long-term mineral fertilization with N180P60K60 had the highest values of mineralization ability for all three incubation periods. The established strong effect of systematic mineral fertilization regardless of the norm and ratios of nitrogen, phosphorus and potassium on the mineralization ability of soil in comparison to the check variant was highest at depths 0-100 cm and 300 – 400 cm.

Systematic mineral fertilization carried out for 40 years in two-field crop rotation (wheat – maize) affected the content of Ctotal deep down the profile of the slightly leached chernozem

Soil depth, cm N0P0K0 N60P0K0 N120P0K0 N180P0K0 N0P180K0 N60P180K0 N120P120K120 N180P60K60 0 – 10 1,659 1,762 1,955 2,262 2,250 2,162 2,069 1,801 10 - 20 2,323 2,104 2,301 2,397 2,446 2,080 2,127 2,240 20 - 40 3,694 4,233 4,593 4,384 4,754 4,891 4,548 4,645 40 - 60 3,161 3,957 2,981 3,161 3,807 3,677 4,333 4,005

The systematic mineral fertilization for a period of 40 years with different norms and at different ratios between nitrogen, phosphorus and potassium had high effect on the agrochemical condition of slightly leached chernozem (Haplic Chernozems) down the soil

The soil acidity forms, averaged for the investigated depth of the 0-400 cm profile, were significantly affected by the type of fertilizer combination. The depth was the factor with decisive effect in all forms of soil acidity. Influence of mineral fertilization was higher on exchangeable Al3+, Ca2+ and the ∑Са+Mg, and significantly lower - on the values of residual hydrolytic acidity and the rate of alkali saturation. The amount of exchangeable Mg2+ had a

Independent long-term mineral fertilization with 180 kg N/ha and with N180P60K60 caused the occurrence of exchangeable Al3+ in the soil absorption complex in the surface layers 0-10

The variant with independent nitrogen fertilization with 180 kg/ha has also the lowest ∑Са+Mg and the lowest value of sorption capacity. Intensive mineral fertilization with

The value of the pH, sorption capacity, acidity on strongly acid positions and degree of saturation with bases showed a tendency of decreasing at the end of 40th year of trail beginning comparing the end of 30th year. In the same way independently of fertilizer

The mineralization ability down the soil profile was affected to a maximum degree of significance by the mineral fertilization and the incubation. Depth had decisive effect on the value of the index. The maximum effect of this factor was registered after 28-day incubation – 92.3 %. The role of mineral fertilization on nitrogen mineralization according to the incubation period was significantly less expressed – 9.2 %, 3.2 5 and 4.0 %, respectively. The amount of nitrified nitrogen increased with the longer incubation periods with 52.7 % (28 days) and with 118.8 % (56 days), respectively, in comparison to 14-day incubation. The long-term mineral fertilization with N180P60K60 had the highest values of mineralization ability for all three incubation periods. The established strong effect of systematic mineral fertilization regardless of the norm and ratios of nitrogen, phosphorus and potassium on the mineralization ability of soil in comparison to the check variant was highest at depths 0-100

Systematic mineral fertilization carried out for 40 years in two-field crop rotation (wheat – maize) affected the content of Ctotal deep down the profile of the slightly leached chernozem

Table 15. Reserves by depth up to 60 cm according to fertilization, C – kg/m2

clear tendency toward increasing down the soil profile

and 10-20 cm. It was not present further down the profile.

N180P60K60 caused decreasing of degree of saturation with bases.

variant the acidity on the slightly acid positions is strongly increased.

**3. Conclusion** 

cm and 300 – 400 cm.

profile.

soil. Systematic use of N180P60K60 contributed most for the increase of its content at average depth 0-400 cm. Fertilizer variants N180P60K60 and N120P120K120 led to lower amount of HA-Ca, below the level of the check and all other investigated fertilization variants. There was a well expressed tendency towards increase of carbon in HA-Ca as a result from the independent phosphorus and nitrogen fertilization, regardless of the nitrogen norm.

The ratio CHA/CFA putted under average for the 400 cm profile to distinct differentiation between the fertilization variants.Variation was within a wide range: from 1.72 to 3.75. The long-term systematic nitrogen fertilization with 180 kg N/ha in combination with low fertilization norms of phosphorus and potassium determined the type of humus as fulvichumic, averaged for the investigated depth 0-400 cm. In all other fertilization variants, regardless of the norms and ratios between the macro elements and in the check variant, the values of this ratio were above 2, which determined the type of humus as humic. Independent nitrogen fertilization, especially with annually applied high norms, had a strong negative effect on the mobility of the organic substance and caused serious decrease of the percent of carbon in the insoluble residue.

As a result from the systematic mineral fertilization in the 20-40 cm layer, higher reserves were formed by the layers lying above and below. Triple NPK combinations (N120P120K120 and N180P60K60) enriched organic mater reserves in 40-60 cm layer.

These results showed that regardless of the intensive agricultural activities and changes of some agrochemical characteristics, slightly leached chernozem soil (Haplic chernozems) in Sough Dobrudzha region in Bulgaria preserved its main genetic characteristics at the lower depths of the soil profile.

The effect of long-term fertilizer treatments on detail nutrient balances, technological quality of crops, concentration of available forms of macro elements and trace elements in soil and plant biomass dynamics and many other aspects of this experiment await detailed analysis.
