By John Grove

The 2014 season has been dry in many places. Corn in these areas is not growing as well, or looking as good as desired. Many fields are showing less than optimal color and there have been several calls regarding possible need for more N to "green things up a bit." Without evidence of substantial loss of fertilizer N applied earlier, additional N is not a good idea. Extra N (N above and beyond the recommendation) can do more harm than good — especially if the season stays dry.

Twenty five years of N response data from a long-term, no-till continuous corn study support this statement. The trial consists of four replications of each of four fertilizer N rates (0, 75, 150, and 300 lb N/acre) The N is applied as ammonium nitrate and is broadcast after planting but before corn emergence. The soil at this location is well-drained and the UK AGR-1 recommended N rate would be 125 to 165 lb N/acre.

The 25 years of yield data were ranked from lowest to highest and then divided into five seasonal yield groups, with five years in each group. Average seasonal yields (across all years and N rates), by group, were 56, 101, 116, 128 and 154 bu/acre, and group names were assigned (very dry, dry, moderately dry, moist and very moist, respectively). The names assume seasonal yields are strongly influenced by seasonal weather — a generally good assumption. The larger differences occur between very dry and dry (45 bu/acre) and moist and very moist (26 bu/acre).

Figure 1. Corn yield (bushels/acre) response to fertilizer N rate for the five-year seasonal groups.

The shape of the seasonal group yield response to fertilizer N rate changes as the overall yield level rises (Figure 1), and these changes in shape tell a tale. There are positive responses to 75 and 150 lb N/acre in all seasonal groups. With the driest two seasonal groups (10 out of the 25 years), the greatest N rate reduced yield by 8 to 10 bu/acre. With moderately dry to moist seasonal conditions (another 10 out of the 25 years), additional N did no harm, but did no good (-1 to +2 bu/acre), either. Only with very moist seasonal conditions (5 out of the 25 years) did the greatest N rate raise yield (7 bu/acre). Nitrogen and water do work together to make grain, and when water is predictable (irrigation), the UK AGR-1 recommended N rate rises to 175 to 200 lb N/acre.

When moisture was unpredictable (rainfall) extra fertilizer N caused lost yield 40% of the time and lost money 80% of the time. When the corn yield-limiting input is water, additional N can often do more harm than good.