The effect of low and high doses of mineral and organic fertilization on the quality of top yield and root mass of alfalfa and effect of mineral and organic fertilization on formation of quality of soil organic matter in dry condition were studied in a field trial. Alfalfa was grown on leached chernozem soil without irrigation at the experimental field of Institute of Forage Crops, Pleven, Bulgaria. The doses of 70, 140 and 210 kg/ha mineral nitrogen (active ingredient) were tested as ammonium nitrate and well-rotted cattle manure. It was found dry mass yield of alfalfa fertilized with manure was to 15.9% higher as compared to dry mass yield obtained from alfalfa fertilized with mineral fertilizer. Sustainable yield index showed the alfalfa crop was more stable under organic fertilization. Alfalfa accumulated to 6027 kg/ha dry root mass for 4-year period of growing. There was from 614 to 1371 kg/ha root mass additional with manure application. Manure treated plants showed higher values for nitrogen in dry root mass/nitrogen in dry aboveground mass ratio and plant available nitrogen. In addition to nitrogen alfalfa add a considerable amount of organic matter to the soil. Humic acids content after mineral fertilization increased to 50.0% as compared to unfertilized control and threefold after manure. Humus content in the soil after manure fertilization was from 10.9 to 41.9% higher as compared to humus content after mineral one.
Key words: alfalfa, dry root mass, manure, mineral fertilization, soil fertility
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