The possibility of different background levels of soil preservation created as a result of systematic mineral and organo-mineral fertilizing for the realization of the biological potential of corn, wheat and barley in the period of conversion from conventional to organic production in the region of Plovdiv was studied. It was found that the productivity of wheat and barley grain was not considerably different from the obtained yields of conventional agriculture in crop rotation. They were highest in the variants after a systematic application of nitrogen fertilization (moderate and high rates) and an organic-mineral fertilization. The values for treatments with excluded phosphorus nutrition were similar to those for the unfertilized control. The maize grain yield was 644 kg/da in average for the study period. Proven difference among the yields was observed only between the control and fertilized variants. The verage total uptake as a result of crop cultivation without fertilization was 27.9 kg NPK for barley and 33.7 kg NPK for wheat or annual balance of nutrients was strongly negative.