Wes Wood, Brenda Wood, Emmett Carden, and Malcomb Pegues

Anecdotal and experimental evidence over many years indicates that row crops in the Gulf Coast region respond less to nitrogen (N) fertilization than crops in other parts of Alabama. Limited crop response to N suggests that N is entering soil systems along the Gulf Coast from some avenue other than fertilizer application. One possible source of N input to soils plowed and managed for row crops, other than fertilization, is N deposition in rainwater. AAES research indicates that rainwater may contribute to the natural N available to crops, but is not the sole source.

Two forms of N received by the soil in rainwater are ammonia, an atmospheric gas that dissolves in rainwater, and nitrate, a compound that is produced during lightning strikes. Each year more thunderstorms occur in the Gulf Coast region than in other parts of Alabama, so greater nitrate-N deposition in that part of the state would be expected. Upon entering soil systems, nitrate and ammonia dissolved in rainwater are available for uptake by crops.

AAES researchers collected rainwater between November 1994 and November 1998 at the Gulf Coast Research and Extension Center near Fairhope to determine nitrate and ammonia concentrations. Separate samples were collected for each rainfall, providing a total of 358 sampling events during the measurement period. Measurement of rainfall amounts coupled with N concentration data for each sampling event allowed calculation of N received by the soil.

On average, only about 18 pounds N per acre per year as ammonia and nitrate were received in an average of 77.5 inches of rainwater at the Gulf Coast Research and Extension Center between November of 1994 and 1998. Although this amount is slightly higher than typical reported values (five to 10 pounds N per acre per year) for most of the Southeast, it is not enough to diminish response of row crops to N fertilizer. Recommended N rates for corn (120 pounds of N per acre) and cotton (90 pounds N per acre) are much greater than N received in rainfall on the Gulf Coast. As is often the case in research, these results lead to further questions, e.g., if only part of the “missing” N is in rainwater, where is the rest? AAES researchers will have to examine other components of the N cycle to answer this question.