NUTRIENT SURPLUSES ON INTEGRATED ARABLE FARMS

From 1990 to 1993 nutrient fluxes were monitored on 38 private arable farms that had adopted farming strategies aiming at reduced nutrient i nputs and substitution of mineral fertilizers by organic fertilizers. The nutrient surplus was defined as the difference between inputs (inc luding inputs through deposition, seeds and biological fixation) and o utputs in crop products, and amounted to 117 kg nitrogen (N), 14 kg ph osphorus (P) and 21 kg potash (K) ha(-1) year(-1) on average. Potato a nd sugar beet had relatively high nutrient surpluses resulting both fr om crop characteristics and the use of organic manure. The surplus var ied markedly among farms due to differences in cropping frequency, fer tilizer inputs and crop outputs. Averaged over the years, ca. 70% of t he participants achieved surpluses below 150 kg N, 20 kg P and 50 kg K ha(-1) year(-1). The amounts of residual soil mineral N (RSMN) exceed ed those normally found in field experiments except for data collected after the wet summer of 1993. Distinct differences between crops were observed. Only in the case of potato a significant relationship was o bserved between the effective N input and RSMN. On a whole-farm level: RSMN amounted to more than 70 kg ha(-1) N on 77, 74, 87 and 18% of the farms in the consecutive years.