Accounting for N Supply and Fertilizer Efficiency in N Response Trials.

The impact of soil type on N fertilizer response is critical to fine-tuning N recommendations. Soil N supply and fertilizer efficiency are critical components affecting response to fertilizer N. Fertilizer N response trials were conducted with the same corn variety at seven Indiana locations varying considerably in soil type across three seasons and two crop rotations (corn after corn or soybean). Corn yield varied 5-fold with little fertilizer N (0 or <30 kg N/ha). Fertilizer N needed to optimize yield varied 2-fold. Estimates of fertilizer-equivalent soil N supply determined from the response curve explained most of the variation in N response. Fertilizer-equivalent soil N plus fertilizer N totaling 300 kg N/ha was required to maximize yield at 5 of the 7 locations.