In view of current low corn prices, and inconsistencies of sulfur fertilizer impact on grain yields, ongoing research aims at:
- Providing evidence that corn yields can be improved with sulfur in soils that are believed to be non-responsive to S because it contains adequate soil organic matter (SOM)
- Verifying the assertion that sulfur fertilizer enhances nitrogen use efficiency (NUE)
Site 1 was at Forman, where SOM content was 4.5%, under long-term strip-till, and previous crop was soybeans. The 15 fertilizer treatments comprised of all combinations of N rates at 0, 60, 120, 180, and 200 lbs with sulfur at 0, 10, 20 lbs. The soil had 18 lbs N, and 50 lbs S before fertilizer treatments were applied. Each treatment was replicated 5 times. Because S was applied as ammonium sulfate, (which contains S and N) 18 lbs N was contributed to every treatment that received 20 lbs S. Therefore, an equivalent amount of 18 lbs N was added to all N rates. Ear leaf sulfur was analyzed to verify if it can be used to diagnose sulfur status of corn and relate to final yields.
At Carrington, SOM was 3.1%, and previous crop was field pea, N rates were 0, 73, 128, 155 lbs and S rates as ammonium sulfate, were 0, 10, 20 lbs.
At Carrington, neither N nor S fertilizer enhanced yields. SOM breakdown and N released was probably enough to satisfy the N demands by the corn crop N during the growing season
Meanwhile at Forman, application of sulfur at 10 lbs significantly improved yields by 12 bushels (a 6% increase). Yields were not different between 10 and 20 lbs S. Average yield at 10 lbs S was consistently greater than at 0 lbs S, for all levels of N (Fig 1).
Yield at 120 lbs N was not statistically different from 180 and 240 lbs N, but was greater than yields at 0 and 60 lbs N. An estimate (prediction) of 193 lbs N was calculated as a rate to produce maximum yield of 227 bushels for this site in 2017. This 193 lbs almost equaled the actual total N applied at 120 lbs N + N credit (18 lbs from soil, 40 lbs from soybeans previous crop, and 18 lbs from ammonium sulfate) for a total of 196 lbs N. This result supports the use of N credits when making N fertilizer recommendations.
Nitrogen use efficiency was calculated as the agronomic efficiency of N use (AEN), which is the unit yield (bushel) produced per unit of N fertilizer applied. Figure 2 shows that AEN declines as N fertilizer rates increased. When N rate is close to 120 lbs N or below, AEN is greater at 10 lbs S than at 0 lbs S.
Yields had a weak relationship to S content, and N:S ratio, which implies that neither ear leaf S or N:S ratio at the reproductive growth stage was reliable to tell if S application would enhance yield.
Corn yields were significantly enhanced by sulfur even though SOM level was adequate
These results highlight the magnitude of influence that S can have on yields of corn in high yielding environments, increase of12 bushel at 10 lbs sulfur
Nitrogen use efficiency was improved by sulfur but only up to the level where additional N did not improve yields significantly.
Even though the soil test levels were reported, and would be considered high, it must be emphasized that soil tests for S is unreliable and cannot be used to decide whether to apply S or not.