5-year-fixed experiments indicated that spontaneous nutrient-suppling capacities of typical black soil in Song Nen Plain were: nitrogen, around 90%; phosphorus, over 95%; and potassium, 100%. The amounts of N, P and K released from farmland were 55-99 kg/ha·year, 14-26 kg/ha·year and 48-93 kg/ha·year seperately. Although black soil farmland has rather high potential fertility, its dependence on fertilization is rising gradually. The spontaneous supplying capacity of nitrogen decreased from 90% to 60% in the treatment with no nitrogen appling during the 5 years, and that of phosphorus declined from 95% to 80-90% in the treatment without phosphorus application during the 5 years.