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Michael Raviv, Ido Aviani and Yael Laor

Fruit growers apply fertilizer at high rates with soil organic matter (SOM) below 2.0%. As organically certified fertilizers are costly, our objective was to compare the effects of two modes of organic nitrogen nutrition to conventional control on plum tree yield and soil properties. The orchard is located on a Vertisol soil, and planted to plums in 1998. The tested treatments were: A) conventional control, receiving an average of 350 kg of N/ha per year; B) fertilization using cattle manure compost (15 tons/ha per year) + feather meal (1 ton/ha per year); and C) a combination of the same amount of compost + 500 kg of feather meal/ha per year + leguminous cover crop (alfalfa, Medicago sativa cv. Gilboa). By 2003, the SOM of treatment A remained stable and that of treatments B and C increased by 36% and 91%, respectively. As a result, soil bulk density of treatments B and C declined with no change in A. During the first year, levels of soil nutrients were lower in treatments B and C than those in A, but they became higher after 2 to 3 years. Nitrification capacity of the soils of treatments B and C was higher than that of treatment A. This enabled a drastic reduction of the application rates of organic amendments without resulting a decline in the soil's nutrient content in the organic treatments over the next 3 years, due to continued mineralization of the SOM pool. Various soil microbial characteristics (microbial counts, total hydrolytic activity, functional richness, and diversity) were determined. In all these parameters the organic treatments showed higher levels than treatment A. Stem circumferences and yields were similar for all treatments. It can be concluded that soil productivity is affected by SOM, so that after achieving a threshold level of SOM, fertilization needs are reduced considerably.