1. Isermann K 1991 Nitrogen and Phosphorus balances in Agriculture — A Comparison of several Western European Countries. Proceedings International Conference on Nitrogen, Phosphorus and Organic Matter, May, 13–15, Helsingor, Denmark. pp 1–20.
2. Sapek A and Sapek B 1993 Assumed non-point water pollution based on the nitrogen budget in Polish Agriculture. Water Sci. Technol. 28, 483–488.