In the paper the economic and environmental effects of conversion from conventional into organic farming system was analyzed. The research was conducted in 2004-2008 at the Experimental Station of IUNG-PIB in Grabów. Cropping pattern, crop yielding and productivity of the whole crop rotations were evaluated in the economic part of the research. Whereas in the environmental part of research soil fertility in P and K, soil pH, nutrient and organic matter balance and concentration of Nmin in a soil profile were analyzed. Conversion of a farm from conventional into organic farming system decreased plant productivity by around 24%. In a consequence of transforming the forage base and at the same milking cows density a slightly greater milk production was achieved. Analysis of soil nutrient fertility revealed on some fields a tendency to drop a potassium and phosphorus content in soil, and therefore there will be a need for application of appropriate fertilizers. Monitoring of Nmin content in soil and organic carbon indicated a lack of environmental risks and a positive balance of organic matter.