T. K. TONEV1, H. KIRCHEV2 and V. KOTEVA3
1 Dobroudja Agricultural Institute, BG - 9520 General Toshevo, Bulgaria
2 Agricultural University, BG - 4000 Plovdiv, Bulgaria
3 Institute of Agriculture, BG - 8400 Karnobat, Bulgaria
TONEV, T. K., H. KIRCHEV and V. KOTEVA, 2005. Dynamics of winter wheat yield after reduction of NPK rates in crop rotation. Bulg. J. Agric. Sci., 11: 539-550
Analyses are presented on the results from a 6-year investigation under the conditions of North-East Bulgaria. Winter wheat cv. Enola was grown in 7-field crop rotation after 3 predecessors: dry bean, sunflower and grain maize. Two systems of empirical reduction of mineral fertilization were studied: (1) 25 % of the optimal nitrogen rate, 50 % of the optimal phosphorus rate and not using potassium fertilization; (2) 50 % of the optimal nitrogen rate and not using PK-fertilization. The two systems were investigated in two sub-variants: (A) Proportional reduction of the fertilization rates in all crops, and (B) Specific reduction of mineral fertilization, applying to wheat nitrogen rates close to the optimal nitrogen rates. The systems of reduced mineral nutrition were compared to two check variants - of optimal NPK fertilization and N0P0K0. It was established that when growing winter wheat under conditions of the slightly leached chernozem soil in the region of North-east Bulgaria and under the necessity to reduce mineral fertilization, an acceptable reduction could be considered 25 % of the mean optimal nitrogen rate for the crop rotation and 50 % of the optimal phosphate rate, with the exception of potassium fertilization. It is justifiable from an agronomic point of view to reduce mineral fertilization in accordance with the responsiveness of the crops to macro elements. This implies using the optimal nitrogen rate, while excluding phosphorus fertilization in wheat and applying it to the previous crops. Reduced fertilization had a similar effect on wheat yield and its components. A negative independent effect of nitrogen fertilization was established on absolute grain weight, which was a balancing reaction to the increased number of grains per spike.