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Abstract:A field trial at the Institute of Forage Crops, Pleven, Bulgaria (2000-2003 was conducted to study the effect of mineral and organic fertilization on seed yield of alfalfa, root biomass accumulation and soil humus content. Alfalfa variety 'Victoria' was grown on a leached chernozem soil without irrigation. Rates of 70, 140 and 210 kg ha -1 mineral nitrogen (active ingredient) were tested as ammonium nitrate and well-matured cattle manure. It was found that mineral and manure fertilization at 140 and 210 kg ha -1 increased seed yield by 9.9% and 20.9% for mineral, and by 30.3% and 40.6% for manure. Seed yield was more stable under manure fertilization compared with mineral fertilization. Alfalfa accumulated between 2669 and 3098 kg ha -1 dry root mass after mineral fertilization and between 3310 and 3570 kg ha -1 after manure application. Additional root mass of 482 to 698 kg ha -1 was found to be accumulated for manure treatments compared to mineral fertilization. High ratios of nitrogen yields (192 -216 kg ha -1 ) and plant available nitrogen (77 -86 kg ha -1 ) have been obtained from the treatments with manure. The highest amount of humus remained in the soil after fertilization of alfalfa with manure at the rate of 210 kg ha -1 .

Residual effect of crops on the performance of succeeding crops is well documented, but their comparisons through inorganic fertilization versus organic manuring is conspicuously missing. In this background, grain and straw oats (Avena sativa L.) yield in response to mineral versus organic manures on alfalfa grown soil was studied in field trial in the Institute of Forage Crops, Pleven, Bulgaria. Alfalfa (Medicago sativa L.) was used for forage (4 years) and seeds (3 years), and fertilized with mineral nitrogen fertilizer in form of ammonium nitrate and well matured cattle manure. Productivity of oats was influenced by both, type of fertilization and whether or not grown alfalfa grown as predecessor. When oats was cultivated after alfalfa for forage, grain yield of oats was higher by 18.68% and 9.84% for straw as compared to alfalfa for seeds. Alfalfa for forage appeared to be better predecessor, and more tolerant with regard to productivity of the next crop such as oats in comparison to alfalfa for seeds. The applied manure as organic fertilizer have long lasting (4 years) positive effect on both yields of experimented crops and soil fertility.

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