Identification of suitable management units for the variable application of fertilizer N is an essential component of a site-specific N management program. Topography and/or soil N test results are examples of two parameters that could potentially delineate N management units for improved grain yield or protein concentration. Field studies were conducted on soft red winter wheat (Triticum aestivum L.) to examine the effects of mineral soil N, soil organic carbon (SOC), and N fertilization on grain yields and protein concentrations in two variable landscapes in southwestern Ontario, Canada. Six N rates (0 to 145 kg N ha-1) were applied to plots (3 × 400 m long), arranged in a randomized complete split block design with four replicates. Sampling on a 3 × 20 m grid indicated slope position affected soil nitrate (NO3-N) and SOC levels at site 1, but not at site 2. The range of spatial correlation for NO3-N was 11.5 m at both sites and for SOC was 16.0 m and 32.4 m at site 1 and 2, respectively. Yields decreased and protein concentrations increased while moving upslope at both sites. Yield response to applied fertilizer N was characterized using a quadratic model. The most economic rate of N (MERN) for site 1 was 105 kg ha-1, and did not vary with topography. At site 2, yield response to applied fertilizer N varied with topography and MERN increased while moving downslope (91, 104, 120 kg N ha-1 for upper, mid, and lower, respectively). Protein concentration responses to applied N were fitted to cubic models, and the nature of the response to applied N did not vary with slope position. Based simply on grain yield, the potential to use topography and/or mineral N in the soil to identify management units for variable application of N was limited at these two sites; however, variably applying N may reduce the risk of exceeding the protein limits for export markets on the upper slope positions without sacrificing yield. Key words: Site-specific crop management, nitrogen application, N fertilizer, variogram, topography
The Development of Management Units for Site-Specific Farming