Submitted to: National Workshop on Constructed Wetlands/BMPS for Nutrient Reduction and ...
Publication Type: Proceedings
Publication Acceptance Date: 9/8/1999
Publication Date: N/A
Technical Abstract: The appearance of hypoxic zones in the Gulf of Mexico has caused concern about the efficiency of nitrogen (N) fertilizer use in the upper Midwest. New application technologies and management strategies have given farmers a promising tool to fine-tune application of fertilizers according to crop need within fields. However, site-specific N fertilizer management has lagged behind other nutrients because there is no clear method for making recommendations. To evaluate the potential of variable-rate N, field studies were initiated on Missouri production fields where uniform-rate N strips ranging from 0 to 224 kg/ha were applied across the field. Crops were harvested using a yield monitor-equipped combine. Response curves for harvested areas of about 0.3 ha were determined with each area containing a full set of N rates. Optimum N fertilizer rates by location within the field were determined using current fertilizer and crop costs. Within fields, crop yield and optimum N rate varied between 60 and 300%, reflect- ing major differences in growing conditions within fields. Optimum N rates varied within fields for apparently different reasons. In some situations N losses due to leaching and denitrification were measured. In other cases, sandy, coarse-textured soils and high rainfall rates resulted in nitrate leaching as the primary N loss pathway. Uniform-rate strips provided good information for the previous crop year but alone are not predictive of optimum N rate in following years.