Yield and protein response to fertilizer nitrogen in different parts of a cereal field: potential of site-specific fertilization

Crop response to fertilizer nitrogen (N) may vary within a field due to variations in soil N and potential yield. To study this, a 3-year field investigation was started in 1998 on a 15 ha arable field with large texture variability in south-west Sweden. Winter wheat was grown in 1998 and 2000 and spring barley in 1999. Using annual field trials with seven N application levels at five sites, curves for yield and crude protein (CP) response to N application were created and economic optimum N fertilization rates calculated. Plant-available soil N (Np), soil moisture, frequency of weeds and diseases, N accumulation in the crop and residual soil mineral N were measured at each site. Differences in potential yield were small between sites, obviously due to sufficient soil moisture. Observed differences between yield response curves could instead be explained by variations in Np, local infections of fungal diseases or probable losses of fertilizer N due to excess soil moisture. The fertilization had no clear effect on CP in 1998 and 1999, probably due to unfavourable weather, bad timing of the fertilization and low N efficiency. In conclusion, N efficiency and yield but not always CP can be improved and fertilizer N saved with site-specific fertilization, but in practice uncertainty in predicting is an obstacle.