Investigations on the change of soil chemical and biological characteristics as influenced by crop rotation and long-term fertilization in the Moldavian Plain

Investigations conducted in long-term (39 years) stationary experiments, under non-irrigated, followed the influence of different fertilizer rates on the evolution of soil agrochemical characteristics and yield obtained in main crops, placed in 2, 3 and 4 year rotations. In maize crop, placed in 5 year rotations, the mean yield obtained during 1980-2005 was of 5920 kg /ha. Rates of N100P100 have determined yield increases by 94% (3030 kg/ha) and rates of N40P40 + 30 t/ha manure resulted in a double yield, in hybrid Oana (9 %; 3180 kg/ha) compared to unfertilized. The long-term use of 3 and 4 year rotation with ameliorative plants resulted in getting yield increases by 46 - 48% (1120-1150 kg/ha) in wheat and 27 - 35% (1180 – 1530) in maize, compared to continuous crop. Annual application of rates of 80 kg/ha P2O5 determined the accumulation of a reserve of mobile phosphates in soil, comprised, according to applied nitrogen, between 47 and 66 ppm. The humus and nutrient content from soil was maintained at a supply level proper to the requirements of plant nutrition, only under 3 or 4 year rotation and in case of annual use of rates of at least N100P100 or of mean rates of mineral elements with 30 t/ha manure