Soil supply of nitrogen is the most intensively studied of all nutrient cycles. Prior to the widespread use of fertilizers, economically viable agriculture was dependent solely on the soils ability to supply N. From 1945 to 1970, relatively cheap and abundant N fertilizer forced the issue of soil N supply to the background. More recently, however, concern about environmental degradation caused by overuse of fertilizer and manure has renewed interest in predicting the soil supply of N. A Canada-Saskatchewan Greenplan study was undertaken to screen some promising methods of assessing soil N supply. This paper describes the methods and evaluates their performance as compared to plant uptake in the growth chamber. We also
discuss the ability of these indices to improve our predictions of optimal N fertilizer requirements.